No-insulation (NI) rare-earth barium copper oxide (REBCO) pancake coils have attracted much attention because of their high thermal stability. It is, therefore, expected to apply NI REBCO pancake coils to various applications. In recent years, there have been discussions on the possibility of applying NI REBCO pancake coils to field coils of synchronous motors for ship and aircraft propulsion. NI REBCO field coils would be exposed to an AC magnetic field of similar to 1 Hz. The AC losses must be discussed because a current is induced in an NI REBCO coil under externa AC field. AC losses in NI REBCO pancake coils exposed to changing magnetic fields should be investigated in detail. In this paper, the AC Joule loss of the NI-REBCO pancake coils is investigated by simulation using a partial element equivalent circuit (PEEC) model. The magnetization loss is also investigated based on the Bean model when a low-frequency AC magnetic field is applied to the NI REBCO pancake coil. From the simulation results, we concluded a high contact resistivity can sufficiently reduce the Joule loss.

We have been developing a no-insulation (NI) REBCO coil for applications in medical cyclotrons for cancer therapy and high-magnetic-field whole-body MRIs. NI-REBCO pancake coil is expected to realize both high current density and thermal stability, which are essentially trade-off relationships. Because electrical insulation is absent in an NI coil, the operating current can bypass toward adjacent turns to avoid the defect area when a local defect occurs. Using this feature, we investigated the possibility of continuous operation even if there is a local defect in a part of the coil windings. However, the contact condition between turns in actual coil windings is not uniform, and there is a possibility of an area that is not completely in contact. Moreover, when winding noncircular coils, e.g., coils for an accelerator, the noncontact area between turns can be large since the straight area is not tensioned. In such a case, even if a local defect occurs, it cannot bypass toward adjacent turns, compromising thermal stability. In this study, we examined the thermal stability of NI-REBCO pancake coils when a local defect occurred using the length, number, and location of noncontact areas between turns as parameters. We also examined the possibility of continuous operation and explored conditions that allow for continuous operation.

We have been developing a no-insulation (NI) REBCO pancake coil technology for applications in medical cyclotrons and high-field whole-body MRI. The thermal and electromagnetic behaviors of multi-stacked NI-REBCO double pancake coils are numerically analyzed in this study when a local defect occurs in one coil using coil diameter and operating temperature parameters. Based on changes in voltage at both ends of the coil and the magnetic field at the center, we present a numerical study of the possibility of continuous operation after the occurrence of a local defect. In the numerical analysis, a new simplified model based on the PEEC model (Partial Element Equivalent Circuit model) is suggested and used for current distribution analysis, and the results are compared.

We have been developing a cyclotron accelerator for radioisotope production of medical use. The features of developed cyclotron system are compactness, light weight, and multi energy outputs. To achieve such features, coils are wound with rare-earth barium copper oxide (REBCO) tapes and the magnet has no iron core to generate an azimuthal varying field (AVF). To install the developed cyclotron accelerator into hospitals, an iron shield is needed for protection of leakage radiation and magnetic field. In this paper, to optimally design main coils with iron shield, a fast field computation method called "Model Order Reduction (MOR)" is adopted. Using MOR technique, the field computation is accelerated similar to 50 times. The error of MOR field computation is sufficiently small. Hence, the MOR is effective in the optimal configuration design of coils with iron shield. The optimized configuration of a miniaturized cyclotron accelerator magnet is also shown.

We have been developing a new-generation accelerator named high-temperature superconducting skeleton cyclotron (HTS-SC), an extremely compact and high-intensity accelerator capable of producing At-211 for targeted alpha-particle therapy, which is expected to be effective in treating advanced cancer. The coil system for HTS-SC consists of multiple air-core REBCO coils that create a magnetic field of high intensity and precision required for particle acceleration. The HTS-SC needs high-precision magnetic field distribution and temporal stability in the particle acceleration plane. However, the magnetic field induced by the screening current in the REBCO tape (SCIF: Screening Current-Induced magnetic Field) deteriorates the quality of the generated magnetic field spatially and changes it temporally. Therefore, in this paper, the SCIF is evaluated based on numerical analysis for a full-scale REBCO coil system designed for targeted alpha-particle therapy. In addition, the effect of the overshoot method, which is one of the current waveform control methods used to reduce the effect of SCIF on temporal stability, is investigated.

We have been developing large-diameter (m-class) no-insulation (NI) REBCO coil systems for applications in areas, such as medical cyclotrons and high-magnetic-field whole-body magnetic resonance imaging (MRI). NI coil technology can achieve both high thermal stability and current density, which have an in-verse relationship. Our previous studies revealed that the application of NI winding technology can allow a coil to continuously operate even when some parts of the coil windings are degraded. Accordingly, by applying the NI winding technology, the use of REBCO-coated conductors with defects or degradation, that is, coil production that allows for variations in conductor characteristics, can result in reduced costs. In this study, the transient thermal stability of NI pancake coils wound with a REBCO-coated conductor with one or more defects was analyzed and evaluated numerically. The conditions for stable operations in a coil with defects were clarified using turn-to-turn contact electrical resistance and I-OP/I-C ratios as parameters. In addition, we evaluated the possibility of establishing a monitoring technology to detect the state of health of the coil by observing changes in the voltage at both ends of the coil to see if any new deterioration has occurred.

In order to achieve a high intensity and compact multifunctional air-core cyclotron, the behavior of non-circular coils in a high magnetic field at 4.2 K were investigated. Two types of isosceles triangle shaped double-pancake coils: a coil with reinforcement by Yoroi-coil (Y-based oxide superconductor and reinforcing outer integrated coil) structuring and a coil without reinforcement, were manufactured and their behaviors were observed when current was applied to each coil in a high magnetic field. The non-reinforcement coil showed in 10 T external magnetic field that the superconducting properties of the coil were partially lost and a voltage generated in the coil winding was observed when the applied current exceeded 160 A. After disassembling this coil, it was found that the superconducting tape in the coil winding was plastically deformed and permanently bent at the corners. The reinforced coil due to Yoroi-coil structuring was powered up to the coil current of 300 A in a magnetic field of 14 T. A voltage of 0.3 mV was observed when the coil current was 277 A, and the excitation was terminated when the voltage reached 0.6 mV at 300 A. The coil winding, after the current flowing up to 300 A in 14 T field, indicated that the deformation of it was significantly limited. It was confirmed that the Yoroi-coil type structural reinforcement, even for triangular coils, has sufficient mechanical strength under the conditions of our air-core cyclotron.

We have been developing a no-insulation (NI) coil technology to achieve high thermal stability and high current density. NI coils can continue to operate even if a part of the coils degrades; however, a charging delay occurs in the coils. Moreover, the turn-to-turn contact electrical resistance of the NI coils must be increased to shorten the charging delay; however, a large turn-to-turn contact electrical resistance reduces thermal stability. In this study, using turn-to-turn contact electrical resistance and IOP/IC ratios as parameters, we analyze thermal stability and consider the possibility that NI coils can continue to operate even if they have local defects. Furthermore, to establish an evaluation criterion for determining turn-to-turn contact electrical resistance that can shorten charging delay and ensure high thermal stability, we analyze and evaluate the relationship between turn-to-turn contact electrical resistance and thermal stability, focusing on three types of heat generation.

We aim to develop REBCO coil systems for high-magnetic-field whole-body magnetic resonance imaging and medical cyclotron for cancer therapy. These applications require high temporal stability and spatial uniformity of the generated magnetic field. Although the magnetic field generated during excitation changes very slowly, a screening-current-induced magnetic field (SCIF) is generated, which affects the required accuracy of the magnetic field. To reduce the SCIF, we focus on two methods: 1) a current control method, and 2) the use of striated coated conductors. When adopting a striated coated conductor, it is necessary to plate copper onto it in order to maintain its thermal stability and mechanical strength. However, in the copper-plated layer, changes in the magnetic field induce a coupling current, which flows between the filaments of the REBCO coated conductor. In this study, we apply the overshoot method with a plateau as a current control to eliminate the effect of coupling currents in the copper plating layer. We numerically analyzed the effectiveness of this overshoot method by visualizing the changes in the current distributions in a model pancake coil wound with a REBCO-coated conductor separated into two filaments. For that purpose, we used a three-dimensional nonlinear transient electromagnetic field computation in which the spiral arrangement of the pancake coil windings is appropriately considered.

An technique for optimal design of magnetic field systems that utilizes a hybrid finite-element-boundary-element HFB method and mathematical programming is proposed. This technique is applied to the design of a high-homogeneous-field superconducting magnet system with an environmental shield for whole-body NMR imaging. Design results for three 1. 0-Tesla magnet systems with different types of shields (an iron shield, a shield magnet, and a combination of the iron shield and the shield magnet) are shown.

No-insulation (NI) coils are an important technique for high-field magnetic resonance imaging (MRI). One of the tech-nical issues with NI coils is their thermal and electromagnetic be-havior when a local normal transition occurs. Given that multiple stacked coils are used in MRI, the interaction between coils must also be investigated, because a local normal transition may affect the other NI coils through magnetic coupling. In fact, when a local normal transition occurs, there is a possibility that large electro-magnetic forces are applied to the other coils, or quench may be induced in some of them. Even though overcurrent tests of stacked NI coils have been conducted, the results are not helpful in ac-tual MRI operation. In this study, we investigate the effects of the occurrence of a local normal transition in multi-stacked NI coil systems under conditions similar to those of actual operation, with a constant transport current below the coil’s Ic . The occurrence of a local normal transition is simulated by applying current to a heater inserted in manufactured small NI double pancake coils. To evaluate the interaction between NI coils, the voltage and changes in the generated magnetic field are measured at each coil. We also show the current distribution in the coils, as obtained by the partial element equivalent circuit method. The obtained results allow us to evaluate the interaction of the NI coils and evaluate coil protection methods.

In this study, a computer program was developed to simulate the temperature and pressure distributions of a liquid ni-trogen (LN) coolant in a high-temperature superconducting (HTS) power cable. This program is important for realizing a practical HTS power cable. According to the Japanese criterion for a 66-kV power-transmission system, in a worst case short-circuit accident, a fault current of 31.5 kA may flow in a 66-kV cable system for 2 s. In addition, when a short-circuit accident occurs, the temperature and pressure of an LN coolant increase rapidly The temperature behaviors of the cable cores and coolant were analyzed by solving the nonlinear partial differential heat-conduction equations in the cable cores by considering the energy balance of the heat flow by using the finite-difference method. Moreover, the pressure behav-iors of the coolant in a cooling system were analyzed, considering the density and volume behaviors of the coolant. The simulation results for the 66-kV 40-m model cable qualitatively reproduced the experimental results satisfactorily.

We previously developed a high-magnetic-field RE-BCO coil for use in medical devices such as medical accelerators and high-magnetic-field MRI. The use of high-temperature supercon-ducting (HTS) coils in such applications necessitates their minia-turization while providing high magnetic fields. However, when the current density is improved by reducing the copper-stabilization-layer thickness in the conventional insulated HTS coil, the thermal stability is lowered, which leads to a tradeoff between high cur-rent density and high thermal stability. Therefore, we examine no-insulation (NI) coils that allow compatibility between high current density and thermal stability. Recent research has indicated that NI coils with multiple defects can afford a stable and high critical current (Ic) and operate without serious problems, which makes it possible to produce coils wound with commercial REBCO tapes containing several defects along the tape, which in turn makes fab-rication economically viable. However, there are many unknown aspects of the current, magnetic field, and heat behaviors of NI coils with defects, particularly when large currents flow through them. Here, in order to examine the behavior of a multiple-defect NI coil subjected to a large current at the temperature of ∼30 K via application of the conduction cooling method, we use a com-bination of the partial element equivalent circuit method and the finite-element method to investigate the detailed current and ther-mal behaviors. Our results indicate that defective NI coils can af-ford a critical current Ic value of ∼80% of that of defect-free NI coils, which makes our findings valuable in terms of contributing to further developments in HTS coil applications.

In autumn 2017, the project "Quantum Innovation for Safe and Smart Society (QiSS)" was started in Program on Open Innovation Platform with Enterprises, Research Institute and Academia (OPERA) by Japan Science and Technology Agency (JST). One of the key issues of this project is the advancement of compact cyclotron and beam irradiation device for stable supply of short-lived radioisotope (RI). We proposed the air-core cyclotron using high-temperature superconducting (HTS) technology, named Skeleton Cyclotron, as high intensity compact cyclotron. Currently, we are carrying out a feasibility study on a variable-energy multi-particle Skeleton Cyclotron for RI production. In this paper, key issues in HTS magnet technologies for Skeleton Cyclotron and the preliminary conceptual design of variable-energy multi-particle Skeleton Cyclotron are reported.

No-insulation (NI) and metal-insulation (MI) winding techniques are promising for enhancing the thermal stability of REBCO pancake coils, toward the practical use of nuclear magnetic resonance and magnetic resonance imaging systems. The NI and MI REBCO pancake coils showed high thermal stability in many over-current tests. However, a disadvantage is a charging delay due to low turn-to-turn contact resistance. Therefore, many researchers have tried to increase turn-to-turn contact resistance to overcome the charging delay problem. Meanwhile, a partial element equivalent circuit (PEEC) model can accurately simulate the over-current and charging/discharging tests of NI REBCO
pancake coils in detail. Using the PEEC model coupling with thermal finite element analysis (FEA), the thermal stability was evaluated during a local normal-state transition. From the simulation results, it was found that a best range of turn-to-turn contact resistance existed to keep a high thermal stability
during a local normal-state transition. However, the PEEC and thermal FEA method requires a substantial amount of time with complicated simulation approach. In this paper, we propose a simple mathematical formulation to obtain an appropriate range of turn-to-turn contact resistance during a local normal-state transition, supposing a worst cooling condition. The simple computation is easy to use and timesaving to roughly estimate the operation reliability of NI REBCO pancake coils.

A simulation code that analyzes the temperature and pressure in a high-temperature superconducting (HTS) power cable cooled by a forced flow of subcooled liquid nitrogen (LN) has been developed. Analysis of the HTS cable is useful for comprehending the normal operating condition and the generation of heat during the transient state. In Japan, an excessive current of 63 kA may flow in an HTS cable for 0.6 s in the worst case when a short-circuit current accident occurs in a 275-kV class cable. To assess the safety under this condition, a simulation of a 275-kV class 20-m model cable was performed, and the simulation results of the temperature profiles qualitatively agreed with the experimental results. In this study, a finite-difference method was used to solve the nonlinear partial differential equations of the heat-transfer phenomenon through heat conduction. By solving these equations, the temperature profiles of the LN coolant and the cable cores were analyzed. The GASPAK software package (Cryodata) was used to evaluate the fluid properties when the LN temperature in the cable was calculated. According to this result, the simulations focused on the analysis of a real-scale HTS cable in this work.

This paper presents the thermal and electromagnetic behaviors of multistacked no-insulation (NI) REBa2Cu3O7- x (REBCO, RE = rare earth) pancake coils. The NI winding technique gives a high enough thermal stability not to damage REBCO coils even though a normal-state transition occurs. The high thermal stability has been verified through overcurrent tests. Moreover, the numerical simulation of multistacked NI REBCO pancake coils has been performed to investigate the electromagnetic behaviors in detail. The simulation results also confirmed the high thermal stability of NI REBCO pancake coils. However, it does not mean that an NI REBCO magnet never quenches. In experiments, it was observed that a quench in one of the pancake coils propagated to the other pancake coils sequentially. The above sequential quench from pancake to pancake has not been confirmed in the numerical simulation since the thermal behavior is not considered. For the more reliable verification, we have developed a numerical simulation to investigate the thermal and electromagnetic behaviors by the partial element equivalent circuit method and the two-dimensional thermal finite-element method. In this paper, the six-stacked NI REBCO pancake coil is simulated with several operating temperatures, and the sequential quench is reproduced. The velocity of the sequential quench is also shown.

Superconducting magnets wound with second-generation high-temperature superconductors, i.e., REBa2Cu3O7-x (REBCO, RE = Rare Earth) tapes, are desired to apply high magnetic field nuclear magnetic resonance, MRI, and accelerators. However, a major problem for practical application is an undesirable irregular magnetic field caused by screening currents induced in REBCO tapes. To investigate the screening current-induced magnetic field, a few simulation methods have been proposed. One of the effective simulation methods employs a finite-element method with a thin approximation method. Although the thin approximation method was developed to simulate eddy currents in magnetic steel sheets, and it is not applicable to REBCO tapes carrying a transport current. Therefore, the thin approximation method is extended to simulate screening currents in REBCO tapes considering a carrying transport current. To show the validity of the proposed extended thin approximation method, the screening currents of an REBCO magnet are computed, and the results are compared with the measurement and simulation results of the conventional thin approximation method. The more accurate solution is available by using the proposed methods.

The type of syntactic operations that increase neuronal activation in humans as a result of syntactically erroneous, unexpected lexical items in hearing sentences has remained unclear. In the present study, we used recordings of magnetoencephalographic (MEG) activity to compare bare infinitive and full infinitive constructions in English. This research aims to identify the type of syntactic deviance that may trigger an early syntax-related mismatch field (MMF) component when unexpected words appear in sentences. Six speakers of English as a first language were presented with auditory stimuli of sentences or words in a passive odd-ball paradigm while watching a silent movie. The experimental protocol included four sessions, specifically investigating the sentential (structural) versions of full (with the ‘to’ infinitival particle) and bare infinitival structures (without the particle) and the lexical (non-structure) versions of the verb either with or without the particle to determine whether the structure processing of sentences was a more crucial factor in the detection of the MMF than the simple processing of lexical items in verb-only conditions. The amplitude analysis of the resulting evoked fields showed that the presence of the syntactic category error of bare infinitival structures against syntactic predictions evoked a significantly larger MMF activation with a peak latency of approximately 200 ms in the anterior superior temporal sulci in the left hemisphere, compared with the lexical items that did not have any syntactic status. These results clearly demonstrate that syntactically unexpected, illegal input in the bare infinitival structure is likely to be noticed more robustly in the brain while processing the structural information of the entire sentence than the corresponding verb-only items.

An extremely high field homogeneity is required for applications of NMR and MRI magnets.

No-insulation (NI) REBCO pancake coils show high thermal stability, however the charging delay is an issue. Hence, to improve the charging delay, it was proposed that a stainless steel (SUS) tape was cowound between turns, called "metal-as-insulation" or "metal-insulation (MI)," to increase the turn-to-turn contact resistivity. The MI technique could make the turn-to-turn contact resistivity approximately 50 times large compared to a conventional NI REBCO pancake coil. This paper reports a bypass current behavior on the cross section of MI REBCO pancake coil using two-dimensional finite element method. The influence of the SUS tape thickness, the MI material, and the number of SUS tapes on the turn-to-turn contact resistivity is investigated. As the result, it is clarified that the turn-to-turn contact resistivity strongly depends on the surface condition and number of SUS tapes.

Inmagnetic resonance imaging and nuclear magnetic resonance applications, it is important that the high-temperature superconducting coils provide a high magnetic field in a miniaturized form factor. Our research group considers these requirements to be compatiblewith no-insulation (NI) coils in which the thickness of the Cu stabilization layer and the turn-to-turn contact electrical resistance are key parameters that enable both high current densities and high thermal stabilities. When the Cu stabilization layer is thin, the current density is impaired, but the thermal stability is improved. If the turn-to-turn contact electrical resistance is very large, then the characteristics of the NI REBCO coil approach the characteristics of conventional insulated coils, and the thermal stability may be degraded. However, when the turn-to-turn contact electrical resistance is large, the excitation delay time is reduced. In this study, we used a combination of the partial element equivalent circuit method and the finite element method to perform a current distribution and thermal analysis of the behavior of NI REBCO coils when a local normal-state transition occurs. We also considered the appropriate turn-to-turn contact electrical resistance that could shorten the excitation delay time as much as possible while still suppressing the formation of hotspots, which is a disadvantage of the NI REBCO coil.

REBCO coils applied to superconducting magnetic energy storage system, magnetic resonance imaging, medical accelerator, and so on, are expected to result in high efficiency and compactness due to the realization of higher magnetic field and current density. Because these devices require large coil radius, huge hoop stress "BJR" (= magnetic field x current density x coil radius) is produced in the coil winding and the damage to these devices is likely to cause. In our previous study, we proposed a novel coil structure called the Y-based oxide superconductor and reinforcing outer integrated (YOROI) structure to achieve a highstrength REBCO pancake coil. The YOROI structure comprises upper and lower plates and the outer frame of the winding outer periphery. The frames are forcibly expanded by the superconducting coil winding situated inside when the coil winding is subjected to an electromagnetic force. The outer frame is connected to the upper and lower plates and transfers a part of the electromagnetic force from the coil winding to the reinforcing outer plates. The reinforcing outer plates then support the coil to suppress the coil deformation against the electromagnetic forces. The YOROI structure achieved the resistance to a maximum hoop stress of 2 GPa, which was calculated from the BJR in an excitation test at 4.2 K in 8-T backup fields. In this study, we investigated the effect of the stress control structure to realize the HTS coil with a mechanical strength of over 2 GPa using a three-dimensional finite-element structural analysis.

A computer program has been developed by numerically solving the nonlinear heat conduction and heat transfer equations, which are discretized using the finite difference method, to calculate the temperature and pressure distributions in high-temperature superconducting (HTS) power cables cooled using liquid nitrogen (LN2). The LN2 coolant properties are estimated using Cryodata's GASPAK software package. This evaluation of the temperature and pressure distributions is important to understand how we can make practical use of HTS power cables and to assess the effects of short-circuit accidents. To incorporate HTS power cables into real grids, it is necessary to evaluate HTS power cables that are a few kilometers long. In this study, 3-km-long HTS cables were evaluated using our proposed computer program. This program was used to model the cooling system, ac loss, dielectric loss, and intermediate-joint losses. The results revealed that the temperature of the LN2 coolant at the cable outlet could reach the saturation temperature when a fault current of 31.5 kA flowed for 2 s (worst case for the 66-kV-class transmission lines in Japan). From the results, it was also clear that the effects of the ac loss, dielectric loss, and intermediate-joint losses on the increase in the temperature of the LN2 coolant could not be ignored.

We started developing REBa2Cu3O7-delta (REBCO) magnets for ultrahigh-field magnetic resonance imaging (MRI) systems in 2013. Our final targets are 9.4 TMRI systems for whole-body and brain imaging. In this paper, a conduction-cooled 1.5 T REBCO MRI magnet having a room-temperature bore of 396 mm was fabricated and tested in order to evaluate the magnetic field homogeneity and stability. The magnet was composed of 60 single pancakes whose inner diameter was 500 mm. The total conductor length was 10.3 km, and the total inductance was 12.4 H. The size of the homogeneous magnetic field region was 200 mm diameter spherical volume. The central magnetic field was as high as 1.5 T at 192.7 A, and the current density of single pancakes was 301 A/mm(2). The magnet was cooled from room temperature to 4.7 K in 55 hours, and the temperature difference among the coils was 0.1 K or less during both initial cooling and excitation. The magnetic field inhomogeneity was 249.7 parts per million (ppm), and the Z(2) coefficient was largest at 579.6 ppm. The main reason for the error magnetic field was dimensional errors in the positions on the z-axis. The magnetic field inhomogeneity was improved to 4.1 ppm by passive shimming using iron pieces. The magnetic field stability was about 2 ppm/h because of a reduction in screening-current induced in the REBCO-coated conductors. Current sweep reversal improved the magnetic field stability from 2 ppm/h to 0.8 ppm/h.

Superconducting magnets for magnetic resonance imaging systems require homogeneous, stable magnetic fields. The homogeneity of the magnetic field is highly dependent on the coils shape and positions. Furthermore, in REBa2Cu3O7-delta (REBCO) magnets, the screening-current-induced magnetic field, which changes the magnetic field distribution of the magnet, is one of the most critical issues. In the work described here, a small REBCO coil that can generate a magnetic field of over 10 T was fabricated and tested. The small REBCO coil was composed of a stack of 22 single pancake coils whose inner and outer diameters were 50 mm and 130 mm, respectively. The total conductor length was 1.5 km. The small REBCO coil could generate a central magnetic field of 8.27 T at 10 K. A magnetic field of over 10 T was achieved by replacing 12 of the single pancake coils with new ones wound with the latest REBCO-coated conductors. The measured value of the central magnetic field at 10 K was 13.53 T, which was lower than the calculated value of 13.94 T obtained without taking account of the screening-current-induced magnetic field. Furthermore, it was examined whether the current sweep reversal would be effective for 10 T-class magnetic field conditions.

In recent years, high-field magnets wound with RE-BCO tape and their applications to magnetic resonance imaging (MRI) have undergone considerable research and development. MRI coil systems require a highly accurate field in the imaging space. However, in REBCO coils, the irregular magnetic field generated by the screening current is induced by the radial component of the magnetic field in the REBCO tape, and may deteriorate the field quality both spatially and temporally. Therefore, a design process with consideration for the effect of the screening-current field is required for the reduction and correction of this field for a high-accuracy-field REBCO coil. In this study, the screening-current distribution in the REBCO tape and the irregular magnetic field were analyzed by using the simulation we developed for a REBCO model coil system for whole-body MRI. We additionally discuss the effect of the screening-current field in the REBCO coil based on the coil shape and arrangement to use it as feedback to the design stage.

Yoroi-coil (Y-based oxide superconductor and reinforcing outer integrated coil) structure has been developed to fabricate very strong pancake coils against electromagnetic force. In Yoroi-coil structure, the outer supporting structure is effective to make a mechanically robust coil having a high current density in the case that the hoop stress must exceed the yield strength of a REBCO-coated conductor without this reinforcement. By the experiments and analysis, we pointed out that the remarkably weak delamination stress can cause the delamination of the coated conductor in coils. So that Yoroi-coil structure has advantages to delamination because it could maintain a coil shape with reinforcing materials, without impregnation. Since higher current density is required by various applications, the Yoroi-coil using austenitic stainless steel reinforcing frames and plates, has been verified by the hoop stress test with high current density. Then, high coil current density of 446 A/mm2 was achieved by the Yoroi-coil-structured double pancake (DP) coil, with the total number of the winding turns of 574, at 4.2 K in the back-up magnetic field of 11 T. The inner and outer diameters of the DP coil were 190 and 250.1 mm, respectively. Although the hoop stress evaluated on the assumption of excluding the supporting structure exceeds 1 GPa, Yoroi-coil structure restricted the strain of coil winding ranging 0.2% to 0.3%. The stainless steel reinforcing member well supported a part of electromagnetic force in Yoroi-coil structure, even with high coil current density.

A numerical analysis program has been developed to simulate the transient temperature and pressure distribution in an high-temperature superconducting (HTS) power cable cooled by a forced flow of subcooled LN2 during a short-circuit current accident. This computer simulation program includes computations of temperature and pressure profiles not only in the HTS power cable itself, but also in the cooling system, including the circulation pump. In 2015, Sumitomo Electric Industries demonstrated a 66 kV 40 m model cable with a fault current test, during which the void between the copper conductors of the copper former was filled by insulation material. In this study, simulations of themodel cable were performed by considering that LN2 is not absorbed in the copper former. This simulation was found to more precisely match experimental data by considering the rise in LN2 temperature at the inlet and transient value of the thermal conductivity of the dielectric layer.

This paper presents the current behavior and axial Lorenz force of stacked no-insulation (NI) REBa2Cu3O7-x (REBCO, RE = Rare Earth) double-pancake coils during local normal-state transition. The NI winding technique enhances the thermal stability and increases the engineering current density in REBCO pancake coils. The high thermal stability of NI REBCO coil has been verified in experiments of overcurrent test. In addition, the numerical simulations of NI REBCO single-pancake coils have been performed to confirm the high stability. However, REBCO magnets usually consist of some stacked double-pancake coils to generate a high magnetic field for practical applications. When a normal-state transition occurs in one of the stacked pancake coils, it may affect the other pancake coils. For the more reliable verification, the current and electromagnetic force behavior of stacked NI REBCO pancake coils during a normal-state transition should be simulated. To examine the stacked NI REBCO pancake coils, we extend the previously proposed partial element equivalent circuit (PEEC) model to deal with a few double-pancake coils. In this paper, an NI REBCO double-pancake coil and their three stacked coils are simulated by using the extended PEEC model.

In this paper, we have proposed a detection method for detecting and locating a local normal-state transition in a meter-class no-insulation (NI) coil that is deployed in next-generation high-field magnetic resonance imaging scanners (MRIs), using a copper pickup coil. There are two schemes for detecting and locating the normal zone: one is a ring pickup coil assembled on the midplane of a central bore and the other consists of several sector pickup coils deployed on the coil winding, namely "scanning pickup coil." To verify the effectiveness of these detection methods, we performed a numerical simulation coupling consisting of a partial element equivalent circuit, a thermal analysis, and a magnetic field analysis. Based on the results of analyses, we reproduced the distributions of the temporal variations of the magnetic flux in the central bore and on the coil winding of the meter-class NI coil, and discussed the effectiveness of these pickup coil schemes for detecting and locating a local normal-state transition in a meter-class NI coil used in next-generation high-field MRIs.

High-temperature superconducting (HTS) cables can transmit large amounts of electricity in a compact size with minimal losses. Therefore, it expects to save the construction cost of underground lines in urban areas and decrease transmission losses. The stability and reliability of HTS cable systems in normal operation has been verified by real grid interconnection tests around the world. While general practical use of HTS cable systems requires the verification of their stability not only in normal operation but also in the event of various accidents (ground fault, short-circuit current, liquid nitrogen leakage, etc.) and understanding these phenomena and their impact. However, a few studies have been conducted to investigate the safety of HTS cable systems in the event of the accidents so far. Regarding the short-circuit current, the maximum current depends on the voltage class and grid conditions, and it was set as 31.5 kA for the 66 kV systems in Japan. In order to verify the phenomena and impact to the cable systems, a 40-m cable system was constructed and cooled by circulated subcooled liquid nitrogen (LN2). After the initial critical current (Ic) measurement of the cable system, the short-circuit current tests were conducted with ac 28.5 kA for 0.6 s and 16 kA for 8.4 s. The former testing condition is almost simulated as the current of 31.5 kA and the latter one is as equivalent heat generation in the HTS cable to 31.5 kA for 2 s. As the initial state of LN2, temperature and pressure of the cable inlet were around 68 K and 0.26 MPaG. The measured temperature and pressure rises of the coolant were not so high to make some damage to the HTS cable. After these tests, the cable Ic was measured again and no damage was found.

A simulation has been developed for estimating the transient temperature and pressure distributions in a high-temperature superconducting (HTS) power cable with a fault current accident cooled by a forced flow of subcooled liquid nitrogen (LN2). This simulation is an essential tool for realizing a practical HTS power cable in order to assess the effects of shortcircuit accidents. When a fault occurs in a 275 kV class power transmission system in Japan, an excessive current of 63 kA may flow in the cable for 0.6 s. When faults occur, it is important to estimate the temperature and pressure profiles in a cable cooled by the forced flow of subcooled LN2. The temperature profiles of the LN2 coolant and the cable cores were analyzed by solving nonlinear partial differential equations of the heat transfer phenomenon through heat conduction using finite-differencemethod. The GASPAK software package (Cryodata) was used to evaluate the fluid properties. The simulation results for 275 kV class 20 m model cable show fairly good agreement with the experimental results obtained by Fukukawa Electric.

<p>The number of patients with internet addiction disorder (IAD), especially among school-age children, is on the rise. Development of objective examination technique to assist the current diagnostic methods using medical interview and inquiry tests is desirable for detection of IAD at its early stage. In this study, we extracted the values of functional connectivity (FC) that correlated with a tendency of IAD, using resting-state functional magnetic resonance imaging (rs-fMRI) data. We recruited 40 males [mean age (SD): 21.9 (0.9) years] with no neurological disorders, conducted rs-fMRI recordings for 7 min 30 s, and administered five questionnaires including internet addiction test (IAT), to assess their mental states. The IAT scores of all participants were in the range of healthy to a tendency of IAD. The FC values were calculated using the cross-correlation of time-series signals in a low frequency band (0.017 to 0.09Hz) between all possible connections of brain region pairs defined by Automated Anatomical Labeling (AAL). For each region pair, we calculated Pearson's correlation coefficients "γ" between the FC values and the scores of IAT as well as other inventories for psychological states, in all subjects. The "γ" were subjects-cross-validated, and the FC of the survived region pairs were statistically assessed by correction of multiple comparison. Finally, we obtained some region pairs in which FC specifically correlated with the IAT scores, but not with other psychological scores. These pairs had mostly negative "γ" over long-distance connections across right and left brain hemispheres. The results suggested that the functional connectivity between specific brain regions was significantly degraded already at the stage prior to the onset of IAD. We expect that our connectivity method can be an objective tool for detecting a tendency of IAD to assist the present diagnostic methods.</p>

A project on the development of REBa2Cu3O7-delta (REBCO) magnets for ultra-high-field magnetic resonance imaging (MRI) was started in 2013. Since REBCO-coated conductors feature high mechanical strength under tensile stress and high critical current density, use of REBCO coils would allow superconducting magnets to be made smaller and lighter than conventional ones. In addition, a conduction-cooled superconducting magnet is simpler to use than one cooled by a liquid helium bath because the operation and maintenance of the cryogenic system become simpler, without the need to handle cryogenic fluid. Superconducting magnets for MRI require homogeneous, stable magnetic fields. The homogeneity of the magnetic field is highly dependent on the coil shape and position. Moreover, in REBCO magnets, the screening-current- induced magnetic field, which changes the magnetic field distribution of the magnet, is one of the critical issues. In order to evaluate the magnetic field homogeneity and the screening-current- induced magnetic field, a 1 T model magnet and some test coils were fabricated. From an evaluation of the 1 T model magnet, it was found that the main reason for magnetic field inhomogeneity was the tolerances in the z-axis positions of the coils, and therefore, it is important to control the gap between the single pancakes. In addition, we have already demonstrated the generation of an 8.27 T central magnetic field at 10 K with a small test coil. The screening-current-induced magnetic field was 0.43 T and was predictable by using an electromagnetic field simulation program. These results were reflected in the design of a conduction-cooled 9.4 T REBCO magnet for whole-body MRI systems. The magnet was composed of six main coils and two active shield coils. The total conductor length was 581 km, and the stored energy was 293 kJ. The field inhomogeneity was 24 ppm peak to peak and 3 ppm volume-root-mean-square (VRMS) for a 500 mm diameter spherical volume (DSV). The axial and radial 5 gauss line locations were less than 5 m and 4.2 m respectively.

© 2002-2011 IEEE.We have been developing a no-insulation (NI) winding technique to be used in both high thermal stability and high current density, which have a tradeoff relationship. This paper presents numerical and experimental studies on the thermal stability of an NI REBCO pancake coil by focusing on a quench protection scheme. In the case of an accident or a local normal-state transition, the power supply for the superconducting coil must be immediately shut down in order to ensure its safety. During the sudden discharging of a conventional insulated winding coil, most of the stored electromagnetic energy is dissipated by an external dump resistor connected across the coil terminals as a quench protection system. However, in an NI pancake coil, current flows in the radial direction in order to bypass the local normal-state area to the adjacent turns. The stored energy in the coil may be dissipated as Joule heat owing to the turn-to-turn contact electrical resistance. In this study, by using experiments and numerical analyses, we evaluated two transient behaviors during sudden discharging: the distribution of energy dissipation and the temperature increase in model NI REBCO pancake coils. We also evaluated the thermal behavior of the NI coil during a local normal transition.

© 2016 IEEE.To achieve a high current density and a compact REBCO coil, a reinforcing structure is essential to prevent coil degradation and deformation caused by the large Lorentz force. A new reinforcing structure for the REBCO coil called the Y-based oxide superconductor and reinforcing outer integrated (YOROI) coil was proposed in the previous study. The YOROI coil, which exhibited no degradation after an excitation test with a maximum hoop stress of 1.7 GPa at 4.2 K in 8-T backup fields, has great ability to reduce the stress and strain acting on the coil winding to maintain the coil shape and prevent degradation in the REBCO wire. In this study, the hoop stress in the winding and the stress shared by the reinforcing structure of a YOROI model coil were measured at 40 K in 10-T backup fields to clarify the reinforcing mechanism of the YOROI coil. A three-dimensional structural analysis, which simulated the experimental condition, was performed on the model coil to determine the distribution of stress and strain in the winding and reinforcing structure.

© 2015 IEEE.Recently, no-insulation (NI) winding techniques have been proposed for achieving high current density and high thermal stability. To verify the excellent characteristics achieved using these techniques, we investigated the transient behaviors of NI REBCO pancake coils. In this paper, we evaluate transient electromagnetic and thermal behaviors during a local normal-state transition, based on a partial element equivalent circuit and thermal coupled numerical analysis. Moreover, we investigate temporal and spatial current distributions in the NI coil winding after a local normal transition occurs. We also clarify the reason for the suppression of temperature increase caused by the local normal-state transition in the NI winding. Subsequently, we evaluate the influence of the thickness of a Cu stabilizer on the thermal stability of the NI and conventional insulation windings. Finally, we discuss the mechanism of high thermal stability attained in the NI coil winding technique.

A REBCO-coated conductor has a tape shape and is suitable for forming a stack of single pancakes for generating a high magnetic field. However, the magnetic flux going through the tape surface causes a screening current, which reduces the magnetic field in conventional coil designs. Magnetic resonance imaging systems need high uniformity of the magnetic field, both spatially and temporally, and thus, the screening field resulting from the screening current is one of the critical issues. In this paper, a 10-T class small test coil using REBCO-coated conductors was fabricated and tested in a conduction-cooled configuration from 10 to 60 K for investigating the effect of the screening field on the generated magnetic field in the inner space of the coil. The coil was composed of a stack of 22 single pancakes using 4-mm-wide REBCO tapes with an inner diameter of 50 mm, an outer diameter of 129 mm, and a height of 104 mm. For different stacking orders based on the coil critical currents, the measured values of the central magnetic field at 10 K were 7.66 and 8.27 T, which were lower than the calculated values of 8.07 and 8.70 T obtained without taking account of the screening-current fields.

It is important to investigate the stability and behavior of an epoxy-resin-impregnated no-insulation (NI) REBCO pancake coil to implement high-field applications, such as ultra-high-field magnetic resonance imaging. We have performed sudden discharging and overcurrent tests for the impregnated NI REBCO pancake coil. From the discharging test, the contact resistivity is estimated, and it changes depending on the initial current. From the overcurrent test, the high thermal stability of the impregnated NI REBCO pancake coil is confirmed. The REBCO pancake coil is charged up to 67 A though the critical current is only 46 A, and no degradation has been found. To investigate in detail the electromagnetic behavior of an impregnated NI REBCO pancake coil, the simulation is performed by means of the partial element equivalent circuit (PEEC) model. In the sudden discharging test, the simulation results for the case of lower initial current are in good agreement with the experimental data. As can be inferred from the simulation results, the current drastically decreases from the inside of the impregnated NI REBCO pancake coil. The result of the overcurrent simulation is almost identical to the experimental one. However, since the contact resistivity is presumed to be constant in the simulation, the difference is observed in the high current region.

In recent years, research and development of applications for a high field magnet wound with a REBCO tape into nuclear magnetic resonance and magnetic resonance imaging have been carried out. These applications require a very high accuracy field, that is, the spatial uniform and temporally stable field. However, in the REBCO coils, the irregular magnetic field is generated by the screening current, which is remarkably induced by the radial component of the magnetic field in a REBCO tape. Therefore, a nonconventional approach is required for the reduction or correction of the irregular magnetic field generated by the screening current in the stage of design for a high-accuracy-field REBCO coil. The prediction and improvement of the irregular magnetic field generated by the screening current is a serious issue for the development and design of REBCO coils and correction shims. In this study, we numerically investigated the screening current distribution in the winding and the irregular magnetic field generated by the screening current. Moreover, we discuss the irregular effect of the magnetic field generated by the screening current in the REBCO coil from the arrangement of the coil windings and excitation pattern.

Coil systems are fabricated with various combinations of multiple coils in order to generate homogenous fields for NMR and MRI applications. In multiple coil systems using REBCO coils, the behavior of the screening current, which deteriorates field homogeneity, depends on the patterns of the transport current, the charging sequence of the coils, and the various resulting magnetic fields. It is therefore important to investigate and clarify the behavior of the irregular magnetic field and screening current in multiple coil systems under different operating environments. In a previous study, we experimentally and numerically clarified the different behaviors of irregular magnetic fields based on the charging sequence of coils in multiple-model REBCO coils [1], [2]. In this paper, we constructed the multiple-model REBCO coils in order to obtain the fundamental data for reducing or improving the resulting irregular magnetic field. Furthermore, the distribution of the irregular magnetic field in terms of the screening current was measured by using the reversal current method [3], [4]. We also calculated the spatial and temporal behavior of the irregular magnetic field by using our developed 3-D numerical simulation. Based on the experimental and numerical results, we discuss the behavior of the magnetic field in terms of the screening current when using the current sweep reversal method in multiple-model REBCO coils.

In a REBCO superconducting magnet for magnetic resonance imaging (MRI) applications, the large screening current induced by a magnetic field perpendicular to the winding tape is a serious problem: it generates irregular magnetic field that deteriorates the spatial field homogeneity and temporal field stability in MRI applications. Therefore, we need to estimate the magnetic field generated by the screening current using electromagnetic field analysis to design and develop a REBCO magnet that generates highly accurate and stable magnetic field. Therefore, to examine the influence of the magnetic field generated by the screening current, we constructed cryocooler-cooled small bore-coil stacked pancake REBCO coils with an inner diameter of 50 mm. This coil generates a magnetic field of 10 T at the center. In this study, we report the evaluation of the current distribution and magnetic field in the REBCO coil using the developed 3-D numerical simulation code for electromagnetic field analysis in a REBCO tape using the finite-element method and fast multipole method. The numerical results agree well with the experimental results. Furthermore, we discuss the current distribution in the REBCO tape and the spatial and temporal behavior of the magnetic field on the basis of the results of the experiments and numerical simulation.

An R&D project on high-temperature superconducting (HTS) magnets using (RE) Ba2Cu3O7 (REBCO; RE = rare earth) wires for ultrahigh-field (UHF) magnetic resonance imaging (MRI) systems is described. Our targets are 9.4-T MRI systems for whole-body imaging and brain imaging. REBCO wires are promising components for UHF-MRI because REBCO wires have high critical current density in high magnetic fields and high strength against hoop stresses, which allows MRI magnets to be smaller and lighter than conventional ones. The aim of the project is to establish basic magnet technologies for adapting REBCO coils for UHF-MRI. The project term is three years, and this year is the final year. We have already demonstrated the generation of an 8.27-T magnetic field with a small test coil composed of 22 REBCO pancake coils. A magnetic field spatial distribution with inhomogeneity of several hundreds of parts permillion within 100-mm diameter spherical volume (DSV) was demonstrated with a 1-T model magnet. A stable magnetic field of a few parts per million per hour was also demonstrated with the 1-T model magnet. The targets of the project, to be achieved by March 2016, are to demonstrate the generation of a 9.4-T field with the small REBCO coil, and to demonstrate a homogeneous magnetic field in 200-mm DSV with a 1.5-T magnet having three pairs of split coils. Imaging will be performed with the 1.5-T magnet.

© 2002-2011 IEEE.Waseda University has developed computer programs to estimate the transient temperature and pressure distributions in a high-temperature superconducting (HTS) power cable cooled by a forced flow of subcooled LN2. This simulation is crucial for realizing a practical HTS power cable to assess the effects of short-circuit accidents. When a short-circuit accident occurs, a fault current of 31.5 kA with a duration of 2 s may flow in a cable in the worst case, which is the Japanese criterion for a 66-kV transmission line. The temperature profiles of the LN2 coolant and cable cores were analyzed by solving the heat conduction and heat transfer equations using a finite-difference method. The pressure profiles of the LN2 coolant were calculated using Fanning's equation. For practical use, it was assumed that HTS cables that are a few kilometers long will be adopted. In this study, an evaluation of the stability of a 1.5-km HTS cable with a fault current of 31.5 kA over 2.0 s was performed using our computer program. According to the results, the temperature of the LN2 coolant at the outlet reached the saturation temperature after the fault, and vaporization suddenly occurred. The evaporated gas was released from the outlet to the terminal.

© 2002-2011 IEEE.The no-insulation (NI) winding technique for an NI ReBCO pancake coil is expected to improve dynamic and thermal stability and enhance current density. The investigations on electromagnetic and thermal behaviors are important for the development of NI ReBCO coils. Many stability investigations of the NI ReBCO coil itself have been carried out by experiments and simulations. However, the detailed behavior of the bypass current between turns has not been shown. Although contact resistivity was obtained through prior experiment, it included the resistivity of not only contact surface but also the components, i.e., the copper stabilizer and the Hastelloy substrate, of a ReBCO tape. To investigate the detailed bypass current behavior in this paper, the true contact surface resistivity is taken into account in the simulation. The bypass current on the cross section of the NI ReBCO tape is simulated using the two-dimensional finite-element method. From the simulation results, the influence of the coil-radial resistivity between turns on the turn-to-turn contact surface resistivity is clarified. In addition, the heat loss is also reported, and a simple equivalent circuit of the turn-to-turn contact is proposed.

A cable-in-conduit (CIC) conductor using an Nb3Sn strand is applied to the ITER TF coil. The performance of CIC conductor was degraded due to electromagnetic force. This is because a strand in the conductor is subjected to periodically bending strain by electromagnetic force, which causes the performance to become degraded. Strands in the conductor touch each other in a complicated manner due to twisting and the electromagnetic force is transferred via another strand, which leads to complication of bending strain distribution in the conductor. The complicated bending strain distribution makes it difficult to simulate the conductor performance analytically. The author thus developed a new model to evaluate conductor performance using a periodically bent strand model developed in a past study. However, the calculated result showed some error when compared to actual measured results. This may be due to the difference of physical phenomenon between a single freely bent strand and a strand constricted by an adjacent strand in the conductor. The author thus redeveloped a calculation model taking this effect into account. As a result, conductor performance can be successfully simulated. This indicates that a more practical solution can be achieved in quantitative evaluation of conductor performance.

Development of a high-temperature superconducting magnet wound with REBa2Cu3O7-delta (REBCO)-coated conductor for ultrahigh-field magnetic resonance imaging (MRI) is in progress. Our final targets are 9.4-T MRI systems for whole-body and brain imaging. Since REBCO-coated conductors feature high mechanical strength under a tensile stress and high critical current density, superconducting magnets could be made smaller by using REBCO coils. Superconducting magnets for MRI require homogeneous stable magnetic fields. The homogeneity of the magnetic field is highly dependent on the size and current density of the coils. Furthermore, in REBCO magnets, the screening-currentinduced magnetic field that changes the magnetic field distribution of the magnet is one of the critical issues. In order to evaluate the magnetic field homogeneity and the screening-current-induced magnetic field of REBCO magnets, a conduction-cooled REBCO magnet with a room-temperature bore of 200 mm was fabricated and tested. The REBCO coils were composed of 12 single pancakes, and the size of the homogeneous magnetic field region was 100-mm diameter spherical volume (DSV). The central magnetic field was as high as 1 T at 285 A. The magnetic field distribution on the z-axis was measured by using an NMR probe. The maximum error magnetic field was 470 parts per million (ppm) in the range from -50 to +50 mm, as well as in the coefficients of the spherical harmonic expansion for a 100-mm DSV. The error magnetic fields due to the screening-current-induced magnetic field were less than 5 ppm, because there was a sufficient distance between the coil and the homogeneous magnetic field region. The main reason for the error magnetic field was dimensional errors in the outer diameters and positions on the z-axis.

A cable-in-conduit (CIC) conductor using Nb3Sn strand is applied to an ITER TF coil. The Nb3Sn strand in the conductor is periodically bent due to electromagnetic force, which causes degradation of performance. This degradation should be evaluated to predict conductor critical current performance. In a past study, a numerical simulation model was developed to evaluate the superconductivity of a periodically bent single strand. However, this model is not suitable for application to strands in the conductor because of the extensive calculation time required. The author thus developed a new analytical model with a much shorter calculation time to evaluate the performance of periodically bent strand. This new model uses the classical model concept of a high transverse resistance model (HTRM). The calculated results show good agreement with the test results of a periodically bent Nb3Sn strand in a ends fixed beam model while there was little error in the uniformly bent model under high transverse load due to the difference in bending moment distribution between the two models. However, for both cases, the newly developed model has a much shorter calculation time than that required for numerical simulation. This indicates that a more practical solution can be achieved when evaluating the performance of periodically bent strands. Thus, the model developed in this study can be applied to evaluation of the performance of conductor incorporating many strands.

The development of ultra-high-field (similar to 10 T magnetic resonance imaging (MRI) device is underway. It is, therefore, required to achieve the extremely high field homogeneity at a magnet design stage, where the field homogeneity is evaluated using the Legendre polynomials. Commonly, it is supposed that the current uniformly flows on the cross section of coils. However, for REBCO magnet, the no-current-flowing region has to be taken into account, such as a copper stabilizer. In addition, the REBCO layer inside REBCO tape is very thin, 1 - 2 mu m, so the current-flowing region is 1/100 of the REBCO tape area. Therefore, it is necessary to consider that the current flows only in the REBCO layer, for accurate evaluation of field homogeneity.

High temperature superconducting (HTS) wires are high aspect ratio cross-section tapes. Therefore, the applications of HTS tapes lead to various problems such as AC loss, nonuniform current, and screening current. In practice, we must investigate the detail of current distribution in HTS tapes using the electromagnetic analysis technique dealing with high aspect ratio tape and nonlinearity of superconductor. We developed the numerical simulation code of electromagnetic field analysis in HTS tape using a finite element method (FEM) and fast multipole method (FMM).

An R&D project on high-temperature superconducting (HTS) magnets using rare-earth Ba2Cu3O7 (REBCO) wires was started in 2013. The project objective is to investigate the feasibility of adapting REBCO magnets to ultrahigh field (UHF) magnetic resonance imaging (MRI) systems. REBCO wires are promising components for UHF-MRI magnets because of their superior superconducting and mechanical properties, which make them smaller and lighter than conventional ones. Moreover, REBCO magnets can be cooled by the conduction-cooling method, making liquid helium unnecessary. In the past two years, some test coils and model magnets have been fabricated and tested. This year is the final year of the project. The goals of the project are: (1) to generate a 9.4 T magnetic field with a small test coil, (2) to generate a homogeneous magnetic field in a 200 mm diameter spherical volume with a 1.5 T model magnet, and (3) to perform imaging with the 1.5 T model magnet. In this paper, the progress of this R&D is described. The knowledge gained through these R&D results will be reflected in the design of 9.4 T MRI magnets for brain and whole body imaging. (C) 2016 Published by Elsevier B.V.

© 2015 IEEE.Significant correlation exists in the blood-oxygen-level-dependent (BOLD) signals of resting-state fMRI across different regions in the brain. These regions form the default mode network (DMN), salience network (SN), sensory networks, and others. Among these, the DMN is widely investigated in relation to various mental diseases. Several analytic methods are available for obtaining the DMN activity from individuals' fMRI time-series signals, but a fully effective method has not yet been established. In the present study, we examined a functional connectivity analysis and three algorithms of blind source separation including independent component analysis, second-order blind identification, and non-negative matrix factorization using a set of resting-state fMRI data measured for twelve young participants. Results showed that the second-order blind identification yielded superior performance for the DMN detection, indicating significant activation in all DMN regions based on statistical parametric maps.

© 2015 IEEE.Utilizing small animal magnetocardiograms (MCG), we have developed a diagnostic method to detect the development of pulmonary hypertension (PH) in a rat heart. We obtained multiple MCG of rats with monocrotaline-induced PH and monitored the development of pathophysiological conditions. Current dipole estimation was then applied to determine the association between abnormal propagation of the cardiac excited wavefront and disease states. The progress of right ventricular hypertrophy correlated with a decrease in the angles of the current dipoles during R and S waves. In addition, clear changes in the current dipole angles during S waves were observed 9-19 days before the availability of echocardiographic diagnosis of the PH. Our results showed, using a rat PH model, that continuous monitoring of myocardial conditions allows PH to be detected at an earlier stage than echocardiographic screening.

© 2002-2011 IEEE. A computer simulation has been developed to estimate the transient temperature and pressure distributions in a high-temperature superconducting (HTS) power cable cooled by a forced flow of subcooled LN2. This simulation is critical for assessing the effects of short-circuit accidents in practical HTS power cables. When a fault occurs, an excessive current of 31.5 kA may flow in a cable for 2 s, and it is important to understand the temperature and pressure profiles in a cable cooled by the forced flow of LN2 when these faults occur until the disturbance flows out from a cable. The temperature profile of the LN2 coolant and the cable cores was analyzed by solving heat conduction and heat transfer equations using the finite-difference method. The Cryodata GASPAK software package was used to estimate the fluid properties. The simulation results show a fairly good agreement with the experimental results. By employing a new model of the induction refrigeration system and the circulation pump, a small discrepancy is resolved in pressures between simulation and experiment. The analysis results show that the pressure in the cable significantly changes depending on the initial gas volumes in the terminals and the volume of the LN2 gas that evaporates from the copper former in the cable.

© 2002-2011 IEEE. This paper presents analytical studies on the charging characteristics of a no-insulation (NI) REBCO pancake coil. A partial element equivalent circuit (PEEC) model has been developed to evaluate the local current paths in NI REBCO pancake coils. The PEEC model is able to consider the asymmetry of the coil winding, the partial turn-to-turn contact electrical resistance, and the partial self and mutual inductances. The analysis results are compared with previous charging experimental results and simulations using a simplified parallel equivalent circuit (SPEC) model to validate the proposed approach with the PEEC model. To investigate the charging characteristics of a meter-class NI REBCO pancake coil, numerical simulations were performed by using the PEEC and SPEC models as a parameter of the turn-to-turn contact electrical resistance. In addition, the magnetic field in a 3-T NI REBCO coil was calculated by the SPEC model to clarify the charging delay of a practical scale magnet.

© 2002-2011 IEEE. The strengthening of Yoroi-coil (Y-based oxide superconductor and reinforcing outer integrated coil) structure at stronger electromagnetic force was investigated using the Yoroi-coil structured double-pancake coil with higher strength outer plates of carbon-fiber reinforced plastic (C-FRP) than those of glass-FRP. The coil achieved 1.14 kA transporting at 4.2 K in 14-T back-up magnetic field and the superconducting wire was burnt by thermal runaway after that. The calculated maximum hoop stress, based on the BJR (magnetic field × current density × coil radius) calculations, at the hoop stress test reached 2.0 GPa. This value was far exceeding the tensile strength of superconducting wire ranging from 1 to 1.3 GPa; however, the maximum strain of superconducting wire in the test coil was held about 0.5%. Yoroi-coil structure is useful for the applications, which need high magnetic field precision. The strengthening effect of C-FRP outer plates was observed at outer part of the coil, but it did not cover the whole coil body, so that arrangement of reinforcing outer plates could admit of some improvement.

Stability margin of a high-temperature superconducting (HTS) coil is two or three orders of magnitude greater than that of a low-temperature superconducting coil. In recent years, many papers have reported test results of turn-to-turn no-insulation (NI) HTS coils having extremely enhanced thermal stability, such that burnout never occurs in an NI coil, even at an operating current exceeding 2.5 times the critical current. Thus, The main goal of this paper is to clarify transient electromagnetic and thermal behaviors and mechanism of the high thermal stability in an NI REBCO coil. A partial element equivalent circuit (PEEC) model is proposed for the numerical simulation of an NI REBCO coil, which considers a local electrical contact resistance between turns, an I-V characteristic of an REBCO tape, and local self and mutual inductances of the NI REBCO coil. Using the PEEC model, we investigate the influence of the turn-to-turn contact resistance on the transient behavior of the NI REBCO coil during sudden discharging. We also perform thermal conduction analyses with the PEEC model to clarify the transient behavior of an NI REBCO coil during an overcurrent operation.

A<tt> </tt>cable-in-conduit (CIC) conductor using Nb₃Sn strand is applied to an ITER TF coil. The Nb₃Sn strand in the conductor is periodically bent due to electromagnetic force, which causes degradation of performance. This degradation should be evaluated to predict conductor critical current performance. In a past study, a numerical simulation model was developed to evaluate the superconductivity of a periodically bent single strand. However, this model is not suitable for application to strands in the conductor because of the extensive calculation time required. The author thus developed a new analytical model with a much shorter calculation time to evaluate the performance of periodically bent strand. This new model uses the classical model concept of a high transverse resistance model (HTRM). The calculated results show good agreement with the test results of a periodically bent Nb₃Sn strand in a ends fixed beam model while there was little error in the uniformly bent model under high transverse load due to the difference in bending moment distribution between the two models. However, for both cases, the newly developed model has a much shorter calculation time than that required for numerical simulation. This indicates that a more practical solution can be achieved when evaluating the performance of periodically bent strands. Thus, the model developed in this study can be applied to evaluation of the performance of conductor incorporating many strands.

This paper introduces a novel method to automatically remove electro-oculogram (EOG) from electroencephalogram (EEG) using second-order blind identification (SOBI) and nonnegative matrix factorization (NMF). To demonstrate the effectiveness of the proposed method, we applied SOBI, NMF, or combined SOBI and NMF (SOBINMF) to the EEG data during motor imagery task in which participants imagined moving their left or right hand. Ocular artifacts were removed more effectively in our proposed SOBINMF method compared to SOBI or NMF alone.

A project to develop an ultra-high-field magnetic resonance imaging (MRI) system based on HTS magnets using (RE)Ba2Cu3O7 (REBCO; RE=rear earth) coils is underway. The project is supported by the Japanese Ministry of Economy, Trade and Industry and aims to establish magnet technologies for a whole-body 9.4 T MRI system. REBCO wires have high critical current density in high magnetic fields and high strength against hoop stresses, and therefore, MRI magnets using REBCO coils are expected to have cryogenic systems that are smaller, lighter, and simpler than the conventional ones. A major problem in using REBCO coils for MRI magnets is the huge irregular magnetic field generated by the screening current in REBCO tapes. Thus, the main purpose of this project is to make the influence of this screening current predictable and controllable. Fundamental technologies, including treatment of the screening currents, were studied via experiments and numerical simulations using small coils. Two types of model magnets are planned to be manufactured, and the knowledge gained in the development of the model magnets will be reflected in the magnet design of a whole-body 9.4 T MRI system. (C) 2015 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

In the real application of high-temperature superconducting (HTS) coils to a superconducting magnetic energy storage (SMES) system, coated conductors are cyclically subjected to tensile strain due to electrical charging and discharging. A quench is not only caused by failure in the power supply and cooling system but also could be induced by local deterioration of the superconducting characteristics because of the cyclic strain during operation. In such a local deterioration case, the conventional detection method using voltage signal for low-temperature superconducting (ITS) coils is not applicable for the HTS coils because of the significantly slow velocity of normal-zone propagation. Furthermore, the voltage detection method is considered to be extremely difficult because the noise of the converters and other equipment is much larger than the local normal-zone voltage of the HTS coils. Therefore, a new quench-detection method for HTS coils is required. In our previous studies, a current detection method was developed for a cryocooler-cooled SMES coil wound with a kA-class laminated bundle conductor composed of four electrically insulated coated conductors. In the present study, experiments and numerical simulations were carried out on a double pancake model coil that assumes real SMES operation to verify the validity of the current detection method.

In recent years, YBCO high-temperature superconducting (HTS) tapes have been expected to be applied to devices in power systems or in other industrial applications, owing to their rapidly improving quality and productivity. Our objective is to develop an HTS cyclotron with a compact body, high efficiency of output power, and high performance in heavy particle radiotherapy. In such an application, extremely high precision is required in the range 0.01% to 0.1% of magnetic field, both spatially and temporally. An isochronous field along with an azimuthally varying field is generated by an air-core pancake coil system during construction. Therefore, developing a technology for extremely high coil-winding accuracy is very important. In this study, we constructed an experimental model of a YBCO pancake coil using a high-precision coil winding machine. Then, the winding error in the radial and axial directions was experimentally evaluated using a pair of laser displacement meters and a surface roughness tester. We also developed a numerical simulation model to analyze the influence of winding error on the magnetic field distribution. The relationship between the winding error and the magnetic field of the meter-class YBCO coil assuming a real system was also evaluated by numerical simulation.

In coils wound with high-temperature superconducting (HTS) tapes, large screening currents are induced in the tape by the radial component of the magnetic field. Furthermore, when a transport current is supplied to the HTS tape under a magnetic field, the total current (which consists of the transport and screening currents) flows inhomogeneously. The magnetic field generated by the screening currents affects the magnetic field distribution in cyclotron applications. Investigating the effects of the screening currents on the reduction, hysteresis, and drift of the magnetic field in the HTS coil is therefore necessary. In our previous studies, we have developed a novel simulation code for a three-dimensional electromagnetic field with the aim of clarifying these effects. In the present study, we measured the reduction and hysteresis of the magnetic field generated by the screening currents for a small model of HTS pancake coil. Additionally, we numerically investigated the influence of the screening current in the Furs coil on the spatial and temporal behavior of the magnetic field using our developed simulation code, which considers the nonlinear voltage-current characteristics.

A series of feasibility studies were carried out on the applications of a high-temperature superconducting (HTS) coil system for a medical accelerator system for particle cancer therapy by our group. A new HTS cyclotron is designed to be more compact and have higher efficiency than the conventional medical accelerator. High strength against electromagnetic forces is required for the development of compact HTS coils during the use of an HTS cyclotron. A new stress control structure for a high-strength REBCO pancake coil was proposed in the previous study. No deterioration was observed in the double pancake coil after a charging test of 1.5 kA at 4.2 K in 8-T backup fields. The maximum hoop stress was determined to be greater than 1.7 GPa because of the Lorentz force. The developed coil structure was able to share the hoop stress with not only the coil winding but also the reinforcing structures. In this study, numerical structural analysis based on the finite element method was performed on the new coil structure to clarify the mechanism of the stress sharing between the coil winding and reinforcing structures. In addition, the stress sharing effect of the meter-class REBCO coil assuming an HTS cyclotron system was determined.

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In this paper, we describe a numerical method for an electromagnetic phenomenon in type-II superconductors containing pins. Models of type-II superconductors containing pins use the eddy current problem of Maxwells equations together with nonlinear constitutive relationships between the electric field (E) and the current density (J). The E-J relationship must lead to basic phenomena of superconductors such as the zero-resistance, flux flow, flux creep, hysteresis and magnetization. As typical models of the E-J relationship in superconductors, the critical state model, the power law "n-value model" and the percolation model are presented. Additionally, as an example, we introduce a superconducting slab, a magnetic shielding type of superconducting fault current limiter and a screening field in a HTS pancake coil.

The REBCO high-temperature superconducting cable, rated at 275 kV and 3 kA, was developed in the Materials and Power Applications of Coated Conductors project in 2008. The cable is expected to be put to practical use as a backbone power line in the future because the transmission capacity is about the same as the UHV overhead transmission line. Transmission loss is significantly reduced by using superconducting wires instead of Cu or Al conductors. Furukawa Electric plans a demonstration of the 275 kV high-temperature superconducting cable. The cable system consists of a 30-m cable, terminations, a cable joint, and a cooling system. The 30-m cable has already been manufactured and installed on a test site. In this demonstration, various electrical evaluations will be conducted, such as a long-term test under a current of 3 kA, and the test voltage will be determined assuming 30 years of insulation degradation.

Coated conductors are subjected to short-circuit currents 10 to 30 times greater than the operating current in the use of superconducting power cables. The I-c and thickness of the stabilizer are considered to be nonuniformly distributed in the real manufacturing process. Thus, the coated conductor may be damaged locally by the hot spot due to the overcurrent. Therefore, it is important to clarify the local degradation characteristics of the coated conductor and determine its tolerance against the fault current in the actual operation. In a previous study, overcurrent experiments and numerical simulations on thermal stress-strain were carried out on a coated conductor with a nonuniformly distributed I-c. In this study, two samples with nonuniformly distributed copper plating thickness were tested. Numerical simulations based on the finite element method were performed to simulate the temperature and thermal stress-strain distribution caused by the overcurrent. The nonuniform thickness of the copper plating and the longitudinal length of the nonuniform area were considered as the simulation parameters.

The REBCO high-temperature superconducting cable, rated at 275 kV and 3 kA, was developed in the Materials and Power Applications of Coated Conductors project in 2008. The cable is expected to be put to practical use as a backbone power line in the future because the transmission capacity is about the same as the UHV overhead transmission line. Transmission loss is significantly reduced by using superconducting wires instead of Cu or Al conductors. Furukawa Electric plans a demonstration of the 275 kV high-temperature superconducting cable. The cable system consists of a 30-m cable, terminations, a cable joint, and a cooling system. The 30-m cable has already been manufactured and installed on a test site. In this demonstration, various electrical evaluations will be conducted, such as a long-term test under a current of 3 kA, and the test voltage will be determined assuming 30 years of insulation degradation.

In this study, we focus on a quench detection method for a cryocooler-cooled double pancake coil wound with an YBCO bundle conductor for superconducting magnetic energy storage. Because the local normal-zone propagation velocity is significantly slower in a high-temperature superconducting coil than that in a low-temperature superconducting coil, it is difficult to detect the local normal transition using a voltage between both coil ends. In the application of an YBCO coil for superconducting magnetic energy storage, quench detection is considered to be more difficult because of the noise of the converter or other equipment. Therefore, the normal transition and quench must be detected using some other method. A current detection method, which measures the current redistribution in the YBCO coil with hall probes, was proposed and verified to be able to determine the normal transition by the numerical simulations in the previous paper. In this study, on the basis of the current detection method, the experiments and numerical simulation were performed on a cryocooler-cooled small model double-pancake coil wound with an YBCO bundle conductor composed of four electrically insulated YBCO tapes. As the results, the current detection method was verified to be effective in detecting the local normal transition; Hall probes were used to measure the magnetic field and to estimate the current in individual YBCO tapes making up the bundle conductor.

High strength against electromagnetic force is required for high magnetic field and large capacity coil in order to develop large-capacity superconducting magnetic energy storage (SMES) systems for electric power system control. Also, suppression of delaminating of Yttrium (Y) based coated conductor in a coil is required to manufacture the highly reliable and durable superconducting coil. Insulating coating, using liquid resin of low-temperature-curable-polyamide, was developed and showed durability at very low temperature without deterioration of transport properties of superconducting wire in a coil. Combining paraffin molding, delaminating, and deterioration of transport properties of superconducting wire were not also observed in a coil. These insulating techniques were applied to the pancake coil, in which superconducting wire and the reinforcing outer plates of the coil withstand electromagnetic force. The double pancake coil of this coil structure, called "Yoroi-coil; Y-based oxide superconductor and reinforcing outer integrated coil" was prepared and the durability against electromagnetic force by hoop stress test was verified. The coil achieved 1.5 kA transporting at 4.2 K in 8 T backup magnetic field without the degradation of transport properties. Maximum hoop stress at the hoop stress test reached 1.7 GPa, based on the calculations. This result confirmed that Yoroi-coil structure has a capability to withstand the large hoop stress, which exceeded the tensile strength of Y-based coated conductors, and bring out highly durable and reliable superconducting coils.

A Japanese national project called "Materials & Power Applications of Coated Conductors (M-PACC)" started in FY2008. In this project, we are developing a 66 kV/5 kA large-current high-temperature superconducting (HTS) cable and 275 kV/3 kA high-voltage HTS cable, using rare-earth barium copper oxide (REBCO) tapes. These HTS cables are expected to offer a compact cable with a large capacity and low power transmission loss. After the cable design has been studied and elemental technologies for each component of the cable system, such as ac loss reduction, protection against over-current, and high-voltage electrical insulation have been developed, two cable systems will be constructed and verified to meet the required specifications in FY2012. This paper describes the progress and status of these HTS cable developments in the M-PACC project.

We intend to develop and obtain a compact, high-efficiency and high-performance cyclotron using the high-temperature superconducting (HTS) magnet technology. In cyclotron applications, a high-accuracy magnetic field distribution with an isochronous field and an azimuthally varying field is spatially and temporally required for an HTS coil. In coils wound with HTS tapes, large shielding currents are induced in the HTS tape by the radial component of the magnetic field. The magnetic field generated by the shielding current affects the magnetic field distribution in the cyclotron application in the following issues: (1) field reduction by the shielding current, (2) accuracy of the field distribution, (3) temporal stability of the time-dependent shielding currents, and (4) field repeatability after charge and discharge repetitions. These effects are very critical in cyclotron applications. We developed a novel numerical simulation based on the finite element method, boundary integral equation, and fast multipole method. We investigated the spatial and temporal behavior of the magnetic field attributed to the shielding current in the HTS coil for the HTS cyclotron application.

In recent years, the quality of high-temperature superconducting (HTS) tapes has been improving rapidly, becoming available in sufficient quantity and length, and can be applied to devices for power systems and industrial applications. Therefore, we are carrying out a feasibility study on the application of HTS coil to a medical accelerator system for particle cancer therapy, and designing a new HTS cyclotron that is to be compact, highly efficient, is able to extract high-energy beam and consume less power. In this paper, we report our effort to develop a conceptual magnetic design of a "next generation HTS cyclotron" using high-temperature superconducting technology.

Using Magnetoencephalography (MEG) we studied functional connectivity of cortical areas during phonological working memory task. Six subjects participated in the experiment and their neuronal activity was measured by a 306-channel MEG system. We used a modified version of the visual Sternberg paradigm, which required subjects to memorize 8 alphabet letters in 2s for a late recall period. We estimated functional connectivity of oscillatory regional brain activities during the encoding session for each trial of each subject using beamformer source reconstruction and Granger causality analysis. Regional brain activities were mostly found in the bilateral premotor cortex (Brodmann area (BA) 6: PMC), the right dorsolateral prefrontal cortex (BA 9: DLPFC), and the right frontal eye field (BA 8). Considering that the left and right PMCs participate in the functions of phonological loop (PL) and the visuospatial sketchpad (VS) in the Baddeley's model of working memory, respectively, our result suggests that subjects utilized either single function or both functions of working memory circuitry to execute the task. Interestingly, the accuracy of the task was significantly higher in the trials where the alpha band oscillatory activities in the bilateral PMCs established functional connectivity compared to those where the PMC was not working in conjunction with its counterpart. Similar relationship was found in the theta band oscillatory activities between the right PMC and the right DLPFC, however in this case the establishment of functional connectivity significantly decreased the accuracy of the task. These results suggest that sharing the memory load with both PL- and VS- type memory storage circuitries contributed to better performance in the highly-demanding cognitive task.

A 275 kV 3 kA high temperature superconducting (HTS) cable has been developed in the Materials & Power Applications of Coated Conductors (M-PACC) project. The cable is expected to be put to practical use as the backbone power line in the future because the capacity of 1.5 GW is about the same as overhead transmission lines. The 30 m cable has been designed on the basis of design values that had been obtained by various voltage tests, AC loss measurement tests, short circuit tests, and other elementary tests. Cable insulation was determined by the design stresses and test conditions based on IEC, JEC (Japan electrical standards), and other HTS demonstrations. This cable was also designed to withstand the short circuit test of 63 kA for 0.6 seconds and to have low losses, including AC loss and dielectric loss of 0.8 W/m at 3kA, 275 kV. Based on the design, a 30 m cable was manufactured, and short samples during this manufacturing process were confirmed to have the designed characteristics. Furukawa Electric prepared a demonstration of the 30 m cable with two terminations and a cable joint. The long-term test under a current of 3 kA, and test voltage determined from 30 years of insulation degradation has been conducted since November 2012 at Shenyang in China. © 2013 The Authors.

In recent years, the quality of high-temperature superconductors (HTS) has been improving. Our goal is to apply an HTS coil to superconducting magnetic energy storage, because an HTS coil is more thermally stable than a low-temperature superconductor coil owing to high thermal margin during its transition to the normal state and its high thermal capacity at a high operational temperature. On the other hand, to enhance the reliability and safety of an HTS coil, it is necessary to establish a stability criterion to prevent thermal and mechanical damages during a quench. Therefore, we have to clarify the thermal behavior of a cryocooler-cooled HTS coil assuming practical applications. In this study, we evaluated the cooling effect using a numerical simulation and thermal conduction experiments. The numerical simulation was based on the finite element analysis, and the thermal conduction experiments were carried out on a model coil wound with electrically insulated copper and stainless steel laminated tapes. These had the same shape and dimensions as YBCO tape, assuming an application to an superconducting magnetic energy storage coil. We focused especially on the cooling effect of a winding with paraffin impregnation compared with that of a dry winding. © 2002-2011 IEEE.

In the present study, we evaluated activated areas of the cerebral cortex with regard to the mirror neuron system during swallowing. To identify the activated areas, we used magnetoencephalography. Subjects were ten consenting volunteers. Swallowing-related stimuli comprised an animated image of the left profile of a person swallowing water with laryngeal elevation as a visual swallowing trigger stimulus and a swallowing sound as an auditory swallowing trigger stimulus. As control stimuli, a still frame image of the left profile without an additional trigger was shown, and an artificial sound as a false auditory trigger was provided. Triggers were presented at 3,000 ms after the start of image presentation. The stimuli were combined and presented and the areas activated were identified for each stimulus. With animation and still-frame stimuli, the visual association area (Brodmann area (BA) 18) was activated at the start of image presentation, while with the swallowing sound and artificial sound stimuli, the auditory areas BA 41 and BA 42 were activated at the time of trigger presentation. However, with animation stimuli (animation stimulus, animation + swallowing sound stimuli, and animation + artificial sound stimuli), activation in BA 6 and BA 40, corresponding to mirror neurons, was observed between 620 and 720 ms before the trigger. Besides, there were also significant differences in latency time and peak intensity between animation stimulus and animation + swallowing sound stimuli. Our results suggest that mirror neurons are activated by swallowing-related visual and auditory stimuli.

In practical applications, high temperature superconducting (HTS) power cables can be subjected to short-circuit fault currents. Further, these cables are assumed to operate over a period of 30 years. Therefore, it is important to investigate the current margin and aging degradation against the fault current. In order to ensure the over-current characteristics of 66 kV classHTS power cables against the fault current, preliminary experiments were carried out on some HTS model cables. Concurrently, numerical simulations were performed to clarify the electromagnetic and thermal behaviors of the HTS model cables under fault conditions. Moreover, the validity of our computer simulation was confirmed by comparing the experimental results with the simulation results. In this study, the maximum fault current in one GdBCO coated conductor assembled in the HTS power cable was numerically simulated. AC over-current experiments were carried out on an HTS model cable by using the maximum fault current of the simulation result to evaluate the current margin against the fault current under conduction cooling and liquid nitrogen bath cooling condition.

In Japan, the development of a 66-kV-class superconducting power cable was started in 2008 as a national project. A high-temperature superconducting (HTS) power cable typically consists of a copper former, HTS conductor layers, electrical insulation layers, HTS shield layers, and copper shield layers. A 66-kV-class superconducting power cable may be subjected to a fault current of 31.5 kA(rms) for 2 s. Therefore, in order to ensure the stability and feasibility of the cable, the thermal characteristics and current distributions of the cable need to be investigated under fault conditions. In this study, overcurrent experiments were performed on a 2-m-long HTS model cable. Numerical simulations were also performed on the model cable by using a computer program on the basis of a 3D finite element method and an electrical circuit model.

In practical applications, high temperature superconducting (HTS) power cables can be subjected to short-circuit fault currents. Further, these cables are assumed to operate over a period of 30 years. Therefore, it is important to investigate the current margin and aging degradation against the fault current. In order to ensure the over-current characteristics of 66 kV classHTS power cables against the fault current, preliminary experiments were carried out on some HTS model cables. Concurrently, numerical simulations were performed to clarify the electromagnetic and thermal behaviors of the HTS model cables under fault conditions. Moreover, the validity of our computer simulation was confirmed by comparing the experimental results with the simulation results. In this study, the maximum fault current in one GdBCO coated conductor assembled in the HTS power cable was numerically simulated. AC over-current experiments were carried out on an HTS model cable by using the maximum fault current of the simulation result to evaluate the current margin against the fault current under conduction cooling and liquid nitrogen bath cooling condition.

In this paper, we focus on the quench detection and protection of a cryocooler-cooled pancake coil for SMES wound with a YBCO bundle conductor composed of laminated electrically insulated YBCO-coated conductors. Because the normal-zone propagation velocity is much slower in a high-temperature superconducting (HTS) coil than in a low-temperature superconducting (LTS) coil, the detection of the non-recovering normal zone using a voltage signal is quite difficult. Quench detection is considered to be more difficult during SMES operation because of the noise of the converter or other equipment. Therefore, a quench must be detected using some other method. In the previous paper, we showed that a quench in the coil could be detected by observing the transposition of the current in the bundle conductor caused by local normal transition. In this paper, based on this quench detection method, we investigate the transposition of current and the thermal behavior during the quench detection and the dumping of the stored energy by an external resistance in coils wound with a YBCO laminated bundle conductor for SMES and determine the appropriate stabilizer thickness of the YBCO-coated conductors.

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In Japan, the development of the 66 kV class REB2C3O7-x (RE123; RE stands for rare earth) high-temperature superconducting (HTS) power cable was begun in 2008 as a national project. 66 kV class RE123 HTS power cables may be subjected to a fault current of 31.5 kA(rms) for 2 s. Therefore, the electromagnetic and thermal characteristics of HTS power cables have to be determined under fault conditions to ensure stability and feasibility. In this study, numerical analyses were performed using a computer program on the basis of the finite element method and an equivalent circuit model to evaluate the electromagnetic and thermal behaviors of a 66 kV class HTS model cable resulting from the fault current. The electromagnetic forces acting on coated conductors that are assembled in the HTS model cable were also numerically simulated under the fault condition. The result found was that the maximum electromagnetic force acting on the coated conductor in the peeling and compression direction was less than 20 kPa. However, the irreversible I-c degradations caused by the peeling and compression stress were above several MPa in previous studies. Thus, the results of this study indicate a low probability of I-c degradation of the 66 kV class HTS power cable being caused by the electromagnetic force due to the fault current.

Sumitomo Electric (SEI) has been involved in the development of 66 kV-class HTS cables using REBCO wires. One of the technical targets is to reduce the AC loss to less than 2 W/m/phase at 5 kA. SEI has developed a clad-type textured metal substrate with lower magnetization loss than NiW substrate. REBCO tapes 30 mm-wide were slit into 4 mm-wide strips, and these strips were wound spirally on a former with small gaps. The measured AC loss of the manufactured cable was 1.8 W/m/phase at 5 kA, achieving the AC loss goal. (C) 2012 Published by Elsevier B. V. Selection and/or peer-review under responsibility of the Guest Editors.

Sumitomo Electric Industries (SEI) has been involved in the development of 66 kV/5 kA-class HTS cables using REBCO wires. One of the technical targets was to reduce the AC loss to less than 2 W/m/phase at 5 kA. SEI developed a clad-type textured metal substrate with lower magnetization loss than NiW substrates. REBCO wires of 30 mm wide were slit into 4 mm-wide strips, and these strips were wound spirally on a former with small gaps. The measured AC loss of the manufactured cable was 1.8 W/m/phase at 5 kA, achieving the AC loss goal. Another important target was to manage fault current. The copper protection layers were designed based on simulation findings. Fault current tests (max. 31.5 kA, 2 sec) showed that the designed HTS cable has the required withstanding performance. The development of the elemental technologies was finished on schedule, and a 15 m-long HTS cable system will be constructed to demonstrate that it meets all the required specifications. (C) 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of ISS Program Committee.

A Japanese national project, called "Materials & Power applications of Coated Conductors (M-PACC)", started in FY2008. In this project, we are developing 66 kV/5 kA large current high temperature superconducting (HTS) cable and 275 kV/3 kA high voltage HTS cable using REBCO tapes. These HTS cables are expected as a compact size with large capacity and low loss power transmission. We have examined AC loss, thermal characteristics of the cables under over-current, the optimum cable design and so on. After the design studies and elemental tests are completed, long cable systems will be built for verification purposes. This paper described the overview and current status of these HTS cables development in the M-PACC project. (PACS: 84.71.Fk) (C) 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors.

Sumitomo Electric (SEI) has been involved in the development of 66 kV-class HTS cables using REBCO wires. One of the technical targets in this project is to reduce the AC loss to less than 2 W/m/phase at 5 kA. SEI has developed a clad-type of textured metal substrate with lower magnetization loss compared with a conventional NiW substrate. In addition, 30 mm-wide REBCO tapes were slit into 4 mm-wide strips, and these strips were wound spirally on a former with small gaps. The AC loss of a manufactured 4-layer cable conductor was 1.5 W/m at 5 kA at 64K. Given that the AC loss in a shield layer is supposed to be one-fourth of a whole cable core loss, our cables are expected to achieve the AC loss target of less than 2 W/m/phase at 5 kA. Another important target is to manage a fault current. A cable core was designed and fabricated based on the simulation findings, and over-current tests (max. 31.5 kA, 2 s) were conducted to check its performance. The critical current value of the cable cores were measured before and after the over-current tests and verified its soundness. A 5 kA-class current lead for the cable terminations was also developed. The current loading tests were conducted for the developed current leads. The temperature distribution of the current leads reached to the steady-state within less than 12 h, and it was confirmed that the developed current lead has enough capacity of 5 kA loading. (C) 2011 Elsevier B.V. All rights reserved.

A 275 kV-3 kA high temperature superconducting (HIS) cable has been developed in the Materials & Power Applications of Coated Conductors (M-PACC) project.
AC loss reduction of a two-layer HTS conductor was undertaken by removing the edges of YBCO tapes with low critical current density. The HTS conductor using these tapes was fabricated, and low loss of 0.235 W/m at 3 kA(rms) was achieved.
The 275 kV-3 kA cable was designed, and the 2 m model cables were fabricated. This cable had 325 mm(2) copper stranded former inside the HTS conductor and a 310 mm(2) copper shield layer on the HTS shield layer for over-current protection. These cables withstood an over-current of 63.0 kA for 0.6 s, which is the worst situation for 275 kV systems.
The partial discharge (PD) and V-t characteristics of a liquid nitrogen (LN(2))/polypropylene (PP) laminated paper composite insulation system have been integrated into the design of the insulation for the 275 kV cable. The results revealed that the PD inception stress (PDIE) did not depend on the insulation thickness, and that lifetime indices of V-t characteristics at PD inception were as high as about 80-100. (C) 2011 Elsevier B.V. All rights reserved.

The thermal behavior of a high-temperature superconducting (HTS) coil is significantly different from that of low-temperature superconducting (LTS) coil because it has a greater volumetric heat capacity at the temperature required for practical use. Therefore, the possibility of quench in HTS coils is much lower than that in LTS coils. In the application of the YBCO coil to Superconducting Magnetic Energy Storage (SMES) system, electrical charging and discharging are repeated; therefore, the superconducting characteristics of the YBCO coated conductor may deteriorate as a result of cyclic subjection to tensile strain. To enhance the reliability and safety of HTS coil, protection scheme assuming a quench is also required for HTS coil. In this study, we focus on the coil wound with YBCO laminated bundle (parallel) conductor supposing SMES application and investigate the characteristics of normal-zone propagation and the thermal behavior within the coil during a quench by using a newly developed computer code based on the finite element method and an equivalent circuit. And we also propose a quench detection method using observation of nonuniform current in YBCO laminated bundle conductor and discuss the validity of the detection method comparing with conventional quench-voltage detection.

In Japan, the development of the 66-kV-class superconducting power cable was begun in 2008 as a national project. A high-temperature superconducting (HTS) power cable typically consists of a copper former, HTS conductor layers, electrical insulation layers, HTS shield layers, and copper shield layers. 66-kVclass superconducting power cables may be subjected to a fault current of 31.5 kA(rms) for 2 s. Therefore, in order to ensure the stability and feasibility of superconducting power cables, we need to investigate these cables with respect to their thermal characteristics and current distribution under fault conditions. In this study, we carried out over-current experiments on a 2-m-long HTS model cable. We also performed numerical simulations on the model cable by using a computer program developed by us on the basis of a 3D finite element method (FEM) and an electrical circuit model.

In Japan, a project for the development of a 275-kV class YBCO power cable started in 2008. High-temperature superconducting (HTS) power cables typically consist of a copper former, an HTS conductor layer, an electrical insulation layer, an HTS shield layer, and a copper shield layer. In practical applications, the 275-kV class transmission line may be subjected to short-circuit fault currents such as 63 kA(rms) for a duration of 0.6 s. Therefore, in order to ensure the stability and demonstrate the feasibility of the cable, it is important to estimate the current distribution and temperature increase under the fault condition. We designed the copper former, copper shield layer, and copper plating of the YBCO coated conductor carefully so as to fulfill the requirements of practical applications. In this study, we carried out over-current experiments on a 2-m long YBCO model cable and performed numerical simulations by a computer program developed using the finite element method (FEM) and an electric circuit model. We investigated the electromagnetic and thermal behaviors of the cable under fault conditions from the experimental and simulation results.

The application of yttrium barium copper oxide (YBCO) coated conductors in power cables is anticipated. In practical applications, high-temperature superconducting (HTS) cables can be subjected to short-circuit fault current and are expected to operate for over 30 years. In order to safeguard the current margin of YBCO coated conductors against fault current, we carried out preliminary experiments on overcurrent characteristics using an HTS model cable. Meanwhile, we performed numerical simulations to clarify the electromagnetic and thermal behaviors of the HTS model cable. In this study, we numerically simulated the fault current waveform in each YBCO coated conductor that constitutes an entire cable. We then carried out experiments focusing on the current-voltage (I - V) characteristics and the current margin without causing critical current (I(c)) degradation against the fault current by varying the amplitude of the waveform obtained numerically. Furthermore, we repeatedly applied the fault current to a YBCO coated conductor to experimentally investigate the I(c) degradation caused by iterative fault current.

YBCO coated conductors are candidate materials for future electric power devices such as transmission cables, transformers, and fault current limiters. In practical applications, YBCO coated conductors are subject to short-circuit fault currents greater than the operating current. These fault currents cause heat generation, resulting in degradation of the characteristics of YBCO coated conductors. It is therefore necessary to identify the maximum temperatures under which YBCO coated conductors can operate without suffering degradation. To date, we have carried out preliminary experiments on the degradation of YBCO coated conductors due to an over-current pulse. We also focused on the relationship between increases in temperature and Ic degradation. In this study, we prepared YBCO coated conductors fabricated by TFA-MOD method. We also carried out over-current tests to investigate the Ic degradation and performed numerical simulations on thermal stress to estimate the stress and strain acting on the YBCO layer.

The critical current of a cable-in-conduit conductor (CICC) for an ITER TF coil was measured using the SULTAN test facility. It was found that a non-uniform current distribution was established due to a non-uniform joint resistance. To evaluate the critical current performance accurately, we used a solder-filled joint. To study the effect of this solder-filled joint on the current distribution, we used a lumped circuit model and a distributed circuit model for the conductor and the joint, respectively, and combined the two to develop a new analysis model. This enabled us to solve the distributed circuit equation using only a few iterations, resulting in considerable reduction of the calculation time. Simulation results showed that, although the current distribution was non-uniform due to the ramping current, it improved at the current sharing temperature. This indicated that the solder-filled joint effectively improved the current distribution.

In recent years, rapid improvements have been made to the quality of high-temperature superconductors (HTSs). We carried out a feasibility study on the application of HTS coils in medical accelerators, which are used for particle cancer therapy. In this application, HTSs are exposed to radiation. As a result, the HTS conductors become radioactive, thereby making the operation and maintenance of the reactors or accelerators difficult. Further, radiation affects the superconducting characteristics and causes an increase in the amount of nuclear waste generated. Therefore, it is necessary to investigate the effect of radiation on HTSs. In this study, we carried out irradiation experiments on Bi-2223 and YBCO tapes at room temperature by using 14-MeV neutrons (dose: 77 kGy). We also performed radioactive analyses on HTS tapes and carried out current-voltage (I - V) measurements on the tapes immersed in a liquid-nitrogen (LN(2)) bath, before and after irradiation. The irradiation to YBCO had been conducted in the past[1]. We irradiated neutrons to YBCO and Bi-2223 tapes on the assumption that they will be applied by the medical accelerator.

Fuel supply is a fundamental issue in application of inertial nuclear fusion. To address this issue, we propose the use of an HTS superconductor capsule containing nuclear fuel, which can levitate in a magnetic field. During magnetic levitation, positional stability is important because it affects levitation accuracy to the focal point of giant lasers. We have developed a numerical simulation program based on a 3-D hybrid finite element and boundary element method to evaluate positional stability during magnetic levitation. Moreover, we have optimally designed and constructed a multi-coil system wherein spherical HTS bulk of 5 mm diameter can levitate stably. Finally, we have conducted the experiments for the evaluation of magnetic levitation, and compared the results with our simulation.

High-temperature superconducting (HTS) cables are considered the next generation transmission line because they are compact, lightweight, and demonstrate large capacity and low loss compared to conventional cables. In particular, since a coated conductor (YBCO wire) provides high critical current, high magnetic-field property, low AC loss, and low cost, it is expected to make the HTS cable more attractive than other superconducting wire. In Japan, 66/77 kV HTS cables have developed for about 20 years. We started developing 275 kV class HTS cables three years ago based on 66/77 kV HTS cables. The goal is a 275 kV 3 kA cable with a capacity of 1.5 GVA, the same capacity as a typical overhead transmission line, which serves as the backbone of Japanese power networks.
The following technical developments will be carried out: high current and low AC loss cable conductors and high voltage insulation and low dielectric loss cables. Regarding high current and low AC loss cable conductors, 3-kA cables have been fabricated, and AC losses have been measured. We found that using thin YBCO wire reduced AC losses in experiments.

In high temperature superconductor applications used in electric power devices, YBCO coated conductors may be subjected to short-circuit fault-currents that are 10-30 times the normal operating current. These over-currents cause heat generation, resulting in 1, degradation of the YBCO coated conductor. Therefore, it is necessary to fully investigate the degradation characteristics of YBCO coated conductors. We previously conducted preliminary experiments on the degradation of YBCO sample tapes resulting from the over-current pulse drive.
In this study, we analyzed the waveform of fault-current that is supposed to flow into the YBCO coated conductor when a short-circuit fault occurs in a power cable application, by using a newly developed numerical simulation program. We also carried out experiments in which an over-current with the analyzed fault-current waveform was applied to two YBCO sample tapes. We then investigated the degradation characteristics, focusing on the amount of current permitted. Experimental results showed that both the sample tapes (initial I(c) = 118.6 A and 123.2 A) were degraded by applying an over-current with a peak value of 320 A. (C) 2010 Published by Elsevier B.V.

In japan, the development of a 275 kV class YBCO power cable was started in 2008 with the support of the New Energy and Industrial Technology Development Organization (NEDO). In the designing of 275 kV YBCO power cable, the thickness of insulator is required to be 26 mm. Thus, in order to design the 275 kV YBCO power cable, it is important to estimate the thermal diffusion through the electrical insulation layer for designing the high-voltage YBCO power cable. Furthermore, YBCO power cables might be subjected to short-circuit fault currents that are 10-30 times a normal operating current. Therefore, in order to ensure stability and feasibility of superconducting power cables, we need to investigate the thermal characteristics and current distribution in the cable in the normal and fault conditions. The objective of this study is to investigate the thermal characteristics and dielectric performance in 275 kV class power cables. We carried out experiments on dummy cables in a steady state to clarify the distribution of temperature. In order to estimate the thermal conductivity and the heat capacity of the insulation layer, we performed numerical simulations using a developed computer program on the basis of the 3D finite element method. We also measured a tans and permittivity of an insulation layer in order to investigate the dielectrical performance. (C) 2010 Published by Elsevier B.V.

In the new national project, as wire and cable development, Sumitomo Electric Industries, Ltd. (SEI) is trying to develop a 66 kV/5 kArms high temperature superconducting (HIS) cable with its own REBa(2)Ca(3)O(x) (RE-123, RE = rare earth element) coated conductors. In an actual power grid, an inflow of current higher than the normal rated power (fault-current) can occur. The maximum fault-current condition is set at 31.5 kArms for 2s for 66 kV-class transmission system in this project. Therefore, one of the technical targets in this project is to design the wires and cables to survive at this fault-current condition. As the first step, the prototype HIS cable was designed against the fault-current condition of 31.5 kArms for 2 s. The wires have copper plating as a stabilizer layer not to be damaged by over-currents. The designed HIS cable has a copper former and copper shield layers in parallel with the HIS conducting layers and HTS shield layers, respectively, to control the temperature rises in the cable core at fault-current conditions. As the second step, a sample HIS cable core was fabricated and fault-current tests were conducted to check its soundness. The temperature rises in the cable core were measured under various fault-current conditions. The critical current of the cable core was measured before and after the fault-current tests, and no deterioration in the critical current characteristics was observed. However, there was slight degradation of the n value. The investigation will be conducted, and the results will be fed back to designs of the RE-123 wires and cables. (C) 2010 Elsevier B.V. All rights reserved.

The new Japanese national project of development of 66 kV/5 kA high-temperature superconducting (HIS) power cables has been started since 2008. In an AC power system, a fault current may flow in the event of a short circuit accident. The maximum fault current condition is defined to be 31.5 kA(rms) for duration of 2 s for 66 kV class transmission line in this project. Therefore, one of the technical targets in this project is to design the HIS power cable to survive at this fault current condition. The coated conductors have copper layer as a stabilizer not to be damaged by over-currents. The HIS cable has a copper former and copper shield layers in parallel with the HIS conductor layers and HIS shield layers, respectively, to control the temperature rises in the HIS cable at a fault accident. In this study, we simulated the maximum temperature rises in the HIS power cable at the fault current condition of 31.5 kA(rms) for 25 in order to design the copper stabilizer and copper shield layers by using a computer program we developed on the basis of 3D finite element method (FEM) and electrical circuit model. (C) 2010 Published by Elsevier B.V.

The possibility of the occurrence of quench caused by mechanical disturbance in high-temperature superconductors (HTS&apos;s) is much lower than that in low-temperature superconductors. However, to enhance the reliability and safety of HTS coils, it is necessary to establish a stability criterion and a protection scheme on the basis of the assumption that the quench is caused by the failure in power supply, cooling systems, etc. In this study, we assume that the stored magnetic energy in the HTS coil is dissipated by an external resistance that acts as a quench protection. Under this assumption, we derive a function that examines the heat balance of quenching YBCO coil under an adiabatic condition and propose a criterion using the derived function to determine an appropriate stabilizer thickness of YBCO conductors. (C) 2010 Published by Elsevier B.V.

Recently, affinity magnetic beads have been widely used in immunomagnetic cell sorting (IMCS) technology. Today, we can easily sort and analyse DNA and antibodies (immunoglobulin) using various types of affinity magnetic beads available in the market. The diameters of the affinity magnetic beads used in immunomagnetic cell sorting are above approximately 1 mu m because of the low magnetic fields induced by permanent magnets. At present, nanosized affinity magnetic beads are strongly desired to achieve high resolutions. We have been studying and attempting to develop a high-gradient magnetic separation (HGMS) system that employs a superconducting magnet to induce a considerably higher magnetic field than that induced by a permanent magnet. The objective is to trap smaller nanosized affinity magnetic beads using a filter made of fine stainless steel wool. In a previous study, we constructed a prototype of a desktop-type HGMS system using a cryocooler-cooled LTS magnet; we conducted preliminary experiments on trapping nanosized magnetic particles and investigated the magnetic field distribution and magnetic force around a magnetic wire in the filter by means of a numerical simulation.
In this study, we succeeded in producing prototype nanobeads covered with the biosurfactant of a high-affinity ligand system for immunoglobulin G and M. Furthermore, we attempted to improve the recovery of nanobeads by adding a resonance circuit to the HGMS system. In practice, the trapped nanobeads attract one another and agglomerate due to their remaining magnetization when the magnetic field is decreased to 0 T. Therefore, the nanobeads and wire are demagnetized in the AC magnetic field by the resonance circuit, making good use of the superconducting magnet of the HGMS system.

In Japan, the development of a 275 kV/3 kA class YBa(2)Cu(3)O(7) (YBCO) power cable was started in 2008. A high-temperature superconducting (HTS) power cable typically consists of a copper former, HTS conductor layers, an electrical insulation layer, HTS shield layers, and copper shield layers. 275 kV/3 kA class YBCO power cables may be subjected to a fault current of 63 kA(rms) for 0.6 s. Therefore, in order to ensure stability and feasibility of superconducting power cables, we need to investigate the thermal characteristics and current distribution in the cable under fault conditions. In this study, we performed numerical simulations on a YBCO model cable under fault conditions by using a computer program we developed on the basis of 3D finite element method (FEM) and electrical circuit model.

The thermal behavior of a high-temperature superconducting (HTS) coil is significantly different from that of a low-temperature superconducting (LTS) coil. A HTS conductor has a greater volumetric heat capacity at the operating temperature envisaged for practical applications. Therefore, a HTS coil is much less likely to be quenched than a LTS coil by mechanical disturbances such as the heat generated by the cracking of the impregnation material or by the friction resulting from wire movements. However, the HTS conductor is cyclically subjected to tensile strain because electrical charging and discharging are repeated in real applications involving the Superconducting Magnetic Energy Storage (SMES) system. The superconducting characteristics may locally deteriorate due to this cyclic strain. Therefore, to enhance the reliability and safety of the HTS coil, a quench protection scheme is needed. Because the normal-zone propagation velocity is quite low, detecting a non-recovering normal zone is difficult in HTS coils, and quenching produces excessive overheating that may cause the conductor to melt. In this study, we focus on a coil wound with a YBCO bundle conductor used in SMES applications and investigate the redistribution characteristics of the transport current in and the thermal behavior of the coil during a quench; we use a newly developed computer code based on the finite element method (FEM) and an equivalent circuit. We also discuss a protection scheme to dump the magnetic energy stored in the coils on an external resistance connected in parallel.

In the ITER Engineering Design Activity (EDA), four NB3Sn model coils were developed and successfully tested. However, it was revealed that the critical current of the conductor degraded with the increase of electromagnetic force. One of the explanations of this phenomenon is a strand bending caused by enormous electromagnetic force. The authors therefore developed a simulation code using the distributed circuit model to investigate dependency of the critical current performance on the periodic bending deformation. The simulation results were in good agreement with the experiments. The dependence of the critical current on the periodic transverse load, temperature, periodic load pitch, thickness of Ta barrier which prevents Cu stabilizer from being contaminated by Sn, twist pitch of the strand, and RRR of the bronze matrix was investigated using the developed code. The results showed that the critical current degraded less with decreasing the pitch of the transverse load and increasing the Ta barrier thickness. It suggests that the shorter cabling pitch and the larger bending stiffness prevent the critical current degradation. (C) 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 171(3): 7-15, 2010; Published online in Wiley Inter Science (www.interscience.wiley.com). DOI 10.1002/eej.20923

High-temperature Superconducting (HTS) power cables transmit bulk power with lower loss than conventional cables. Moreover, HTS cables are expected to be constructed as a new underground cable in urban areas at lower cost compared to a high voltage XLPE cable. To put promising HTS cables to practical use, we need (RE)BCO tapes with long length, high critical current, and low cost. Recently many organizations have improved the performance of the (RE)BCO tapes, such as YBCO tapes, or other coated conductor tapes that are made with a variety of different processes. We have fabricated the conductors for the HTS power cable that was constructed of different kinds of(RE)BCO tapes and measured the l(c) and AC losses. We achieved significantly low AC loss of 0.1 W/m at 1 kA in the HTS conductor using narrow slit tapes that were cut by laser. Moreover, a 20 m long HTS power cable model and a cable intermediate joint were developed. Short circuit current tests were conducted on the cable system that consisted of two 10 m cables, a cable joint, and two terminations. The cables and the joint withstood the short circuit current of 31.5 kA for 2 s without damage. (C) 2009 Elsevier B.V. All rights reserved.

To achieve large current capacity and high mechanical flexibility, YBa(2)Cu(3)O(7) (YBCO) superconducting cables consist of a number of YBCO coated conductors that are assembled and wound spirally on a Cu former. In practical applications, superconducting cables are vulnerable to short-circuit fault currents that are 10-30 times greater than the operating current. Therefore, in order to ensure the stability of YBCO superconducting cables in such a situation and to protect them from the fault currents, it is important to investigate the redistribution of the transport current and electromagnetic coupling between the conductor layer, shield layer, and Cu former. In this study, we carried out experiments on a 10-m-long YBCO model cable, which was manufactured by Furukawa Electric. An over-current with a peak of 31.8 kA(rms) and a duration of 2.02 s was applied to the model cable. We performed numerical simulations using a novel computer program developed using the 3D finite element method to elucidate the electromagnetic and thermal behavior of the YBCO model cable in the presence of an over-current. (C) 2009 Published by Elsevier B.V

To achieve large current capacity and mechanical flexibility, high-temperature superconductor (HTS) power transmission cables consist of a number of YBCO coated conductors, which are assembled and wound spirally on a Cu former. In practical applications, superconducting cables might be subjected to short-circuit fault currents that are 10 to 30 times the operating current. Therefore, in order to ensure the stability and feasibility of HTS power cables and protect them from fault currents, it is important to estimate the redistribution of the transport current and electromagnetic coupling among the conductor layer, shield layer, and Cu former. In this study, we carried out experiments on a 20-m-long YBCO model cable, which was composed of two jointed 10-m-long YBCO model cables. Over-current with a peak of 31.8 kA(rms) and a duration of 2.02 s was applied to the model cable. We performed numerical simulations using a newly developed computer program based on the 3D finite element method (FEM) in order to clarify the electromagnetic and thermal behaviors of the YBCO model cable in the presence of an over-current.

We have been developing an active magnetic levitation system, which is composed of a field-cooled disk-shaped or sphere-shaped high-temperature superconducting bulks (HTS bulks) and vertically piled ring-shaped electromagnets without an iron core. We suppose that this active magnetic levitation system can be applied to inertial nuclear fusion. In inertial nuclear fusion, one of the most important issues is to achieve high-accuracy position control of the fusion fuel, which consists of deuterium and tritium in order to evenly illuminate the entire surface of the target. Therefore, active magnetic levitation is applied to the levitation and position control of a sphere-shaped superconducting capsule containing nuclear fusion fuel. In this study, we designed and constructed a three-coil position control system in order to achieve the stable levitation of the sphere-shaped HTS bulk with a diameter of 5 mm by using the numerical simulation based on the hybrid finite element and boundary element analysis. Then, we carried out the experiments on the position control and restoring force characteristics of the sphere-shaped HTS bulk in the constructed levitation system.

Recently, affinity magnetic beads have been widely used in immunomagnetic cell sorting (IMCS) technology. Today, we can easily sort and analyze DNA and antibodies (immunoglobulin) using various types of affinity magnetic beads available in the market. The diameters of these affinity magnetic beads used in immunomagnetic cell sorting are limited to above approximately 1 because of the low magnetic fields induced by permanent magnets. Now, nano-sized affinity magnetic beads are strongly desired to achieve high resolutions. We have been studying and attempting to develop a high-gradient magnetic separation (HGMS) system that employs a superconducting magnet to induce a considerably higher magnetic field than that induced by a permanent magnet in order to trap smaller nano-sized affinity magnetic beads by a filter made of fine stainless steel wool. In this study, we constructed a prototype of a desktop-type HGMS system using a cryocooler-cooled LTS magnet and conducted preliminary experiments on trapping the nano-sized magnetic particles. Furthermore, we investigated the magnetic field distribution and magnetic force around a magnetic wire in the filter by means of a numerical simulation.

The quality of high-temperature superconductors ( HTSs) is rapidly increasing, and they can be used in power systems such as SMES systems and in industries as Si single-crystal growth pullers. In the future, HTS conductors are expected to be used for the magnets in nuclear fusion reactors or accelerators. In such applications, HTS conductors will be exposed to radiation. As a result, HTS conductors become radioactive making the operation and maintenance of the reactors or accelerators difficult; this also leads to an increase in the amount of nuclear waste generated. Therefore, we carry out experiments on the radiation damage in HTS conductors to study the characteristics of HTS conductors in radiation environments and to create an industrial database of the radiation effects on HTSs. We also carry out radioactive analyses on HTS conductors in order to clarify the relationship between the irradiation dose and the activity of HTS conductors. In this study, we prepared YBCO and Bi-2223 tapes and carried out the following preliminary experiments: ( a) irradiation of the sample HTS tapes with 2-MeV neutrons at room temperature without transport current and (b) study of neutron-induced radioactivity by gamma-ray spectrometry using a Ge detector after the irradiation.

YBCO tapes are expected to be used in future high temperature superconducting (HTS) applications because of their good characteristics at high temperatures and in high applied magnetic fields. In applications to electric power devices such as transmission cables, transformers, and fault current limiters, the HTS conductors will be subjected to short-circuit fault currents that are 10 to 30 times the normal operating current. These overcurrents are greater than the critical current, and degrade or burn-out the HTS conductors. Therefore, it is important to clarify the mechanism of the degradation caused by such overcurrent pulses. We carried out preliminary experiments on damage caused by overcurrent pulse drive, focusing on the temperature limitation without suffering degradation for overcurrent pulse operation. A 10-mm-wide YBCO tape was cut into 2-mm-wide sample tapes by a laser beam, and the sample tapes were soldered on silver-deposited 100-thick copper plates. Overcurrent tests were carried out on these sample tapes and I(c) degradation was investigated. In addition the contact interface between YBCO and the Ag layer or buffer layer before and after the overcurrent drives has been investigated in order to clarify the correlation between the degradation and delamination of sample tapes.

For the application to power transmission cables, a number of YBCO tapes would be assembled and wound in spiral on a Cu-former. YBCO tapes and the Cu-former are connected in parallel and they might be subjected to short-circuit fault currents 10-30 times the operating current. In this study, we constructed a 1 m long YBCO model cable. Over-current with a peak of 31.5 kArms and a duration of 2.0 s, which was established by JEC (Japanese Electrotechnical Committee), was applied to this cable in a liquid nitrogen bath. The redistribution of the transport current between YBCO tapes and the Cu-former were examined by using Hall sensors. The numerical simulations were carried out using a newly developed computer program based on the 3D finite element method (FEM) in order to clarify the over-current characteristics in the cable. From the comparison of the experimental and simulation results, the validity of the developed computer program was confirmed. Therefore, we performed the simulations of a 10 m long model cable when carrying a fault current of 31.5 kArms applied for a durations of 2.0 s, and also estimated the influence of the cross-section area of the Cu-former and the Cu shield layer on the thermal behavior of the model cable by using the developed computer program. (C) 2008 Elsevier B.V. All rights reserved.

The critical current of the ITER model coil degraded with an increase in the electromagnetic force. Experiments in which a periodic transverse load was artificially applied to a single coil strand showed this degradation. However, since these experiments were carried out under a certain condition, a general relation between the transverse load and the critical current was not obtained. Therefore, we have developed a numerical simulation code to investigate this general relation. The results of the numerical simulations are in good agreement with the experiments. This indicates that our numerical code is valid for the evaluation of the critical current performance of strands in CIC conductor. Next, the general dependence of the critical current degradation on Ta barrier thickness and resistivity of bronze was evaluated by using the developed simulation code. The results of the calculation showed that the Ta barrier thickness affects the critical current characteristics because of the large flexural rigidity of Ta barrier. On the other hand, the resistivity of bronze and the twist pitch of the filament rarely affect the critical current degradation since current transfers between filaments do not occur within the practical resistivity range of bronze.

YBCO tapes are expected to be used in future high temperature superconductor (HTS) applications as they have better J(c) characteristics at high temperatures and in high magnetic fields. For power applications such as transmission cables, YBCO tapes and a copper former are connected in parallel and they might be subjected to a short-circuit fault current that is 10 to 30 times the normal operating current. Therefore the over-current behavior, including the hot-spot and the distribution of transport current between the copper-laminated YBCO tapes and the copper former, is very important to examine the stability and feasibility of the cable. This paper describes the experimental results for over-current pulses of a 1-m single-layer cable fabricated with five copper-laminated YBCO tapes and a copper former. Further, we performed numerical simulations by using a newly developed computer program based on the 3D finite element method.

It is widely accepted that magnetoencephalography (MEG) is a promising tool for investigating human brain activity in good temporal and spatial resolution. However, the use of MEG is currently quite limited, mostly due to excessive diversity in localization techniques to estimate regional brain activity using MEG data. Because source localization in MEG is an ill-posed problem, an adequate localization technique varies depending on the task design and the timing of interest in MEG signals, which sometimes confuses investigators when choosing an analysis technique or comparing the results with those obtained with other modalities, such as functional magnetic resonance imaging. This chapter reviews the introductory theories and applications of currently available MEG source localization techniques as well as principles of MEG signals and its measurement for beginners and possible future MEG users. The physiological and mathematical backgrounds of cerebral MEG source are briefly introduced followed by the technical requirements for MEG data acquisition. Modern localization techniques for inverse problem solving with MEG, from a simple dipole model to an underdetermined type of norm estimation or spatial filter technique, are thoroughly described.

To establish a stability criterion for Yttrium barium copper oxide (YBCO)-coated conductors, it is necessary to clarify their transient thermal characteristics. Thermocouple thermometers, resistance thermometers, or the relation between the temperature and resistance of the stabilizer used for the conductors are generally employed for measuring their temperatures. However, these methods can only be employed to observe fixed-point data. Hence, it is difficult to observe the longitudinal and transversal distributions of temperatures in coated conductors. In this study, we adopt a method for the visualization of the temperature distribution of coated conductors by using fluorescent paints whose color changes with temperature. By adopting this method, we can measure the two-dimensional distribution of temperature in magnetic fields and at cryogenic temperatures. Therefore, local heat generation in the YBCO tapes can be observed with a higher spatial resolution and the uniformity in the critical current density of YBCO tapes can be identified. In this paper, we describe the developed thermal visualization system and the preliminary experimental results of the visualization of normal-zone initiation and propagation in YBCO sample strips. Further, we also report a part of the normal-zone initiation and propagation mechanism by electromagnetic field and heat transfer analyses by employing a three-dimensional finite element method (FEM).

We have developed an active magnetic levitation system that comprises a field-cooled disk-shaped or sphere-shaped HTS bulk and multiple ring-shaped electromagnets. In this system, the levitation height of HTS bulk can be controlled by adjusting the operating current of each electromagnet individually. Further, the application of the vertical noncontact levitation system is expected due to its levitation stability without mechanical supports. We assume that this system is applied to inertial nuclear fusion. However, one of the important issues is to achieve position control with high accuracy of the fusion fuel in order to illuminate the target evenly over the entire surface. Therefore, this system is applied to the levitation and position control of a sphere-shaped superconducting capsule containing nuclear fusion fuel. In this study, we designed and constructed a position control system for the sphere-shaped HTS bulk with a diameter of 5 mm by using numerical simulation based on hybrid finite element and boundary element analysis. We then carried out the experiment of levitation height and position control characteristics of the HTS bulk in this system. With regard to position control, accuracies within 59 mu m are obtained.

In this paper, we report the fatigue characteristics of IBAD/PLD YBCO coated conductors. A YBCO coated conductor used in the superconducting coil of a SMES system is repeatedly subjected to mechanical tensile or compressive strain due to the Lorentz force during electrical charging or discharging. The superconducting characteristic of this conductor may deteriorate because of this cyclic strain. Therefore, it is necessary to investigate the effect of cyclic strain on the superconducting characteristics of YBCO coated conductors that have a laminated structure. We developed an experimental apparatus with a U-shaped sample holder in order to apply cyclic strain to the sample tape. This apparatus was used to perform the fatigue tests on YBCO coated conductors in liquid nitrogen in the absence of an external magnetic field. The strain cycles with the maximum strain epsilon(max) (zero external strain -&gt; epsilon(max) -&gt; zero external strain) were applied and repeated up to 5000 times, and the I-c measurements were performed at epsilon(max). Therefore, the application of cyclic strain with emax ranging from 0.3% to 0.5% did not result in any significant deterioration of the superconducting characteristics of the conductor.

In applications to power transmission cables, a number of YBCO tapes would be assembled and wound spirally on a Cu former. The YBCO tapes and the Cu former would be connected in parallel and could be subjected to short-circuit fault currents 10 to 30 times the operating current. The fault currents would drive the YBCO tapes into the normal state and thus generate Joule heating for the duration of the fault. Therefore, in order to examine the stability and feasibility of YBCO power transmission cables, it is important to demonstrate the redistribution of the transport current and the electromagnetic coupling between the YBCO tapes and the Cu former during a short-circuit fault current. We constructed a 1 m long superconducting model cable and subjected it to an overcurrent with a peak of 31.5 kA(rms) for a duration of 2.0 s, as established by JEC (Japanese Electrotechnical Committee), in a liquid nitrogen bath. We examined the redistribution of the transport current between the YBCO sample tapes and the Cu former by using Hall probes. In addition, we developed a novel computer code based on the finite element method and an equivalent circuit in order to clarify the characteristics of the redistribution of the transport current and the thermal behaviour within the cable. Finally, we designed a 10 m long cable that would reach the degradation temperature of the YBCO tape due to the fault currents.

Overcurrent characteristics and reduction of AC loss are essential for high temperature superconducting (HTS) cable in a real grid. AC loss in an HTS conductor using YBCO could be potentially small but protection for overcurrent was needed.
A 0.1 mm thick copper tape soldered to the YBCO tape was effective as protection from overcurrent and did not affect the increase in AC loss. The 2 m HTS conductor with Cu strands of 250 mm(2) and YBCO tapes with copper was fabricated. This conductor could withstand overcurrent of 31.5 kA for 2 s.
To reduce AC loss, 10 mm wide YBCO tapes were divided into five strips using YAG laser. Using narrower strips and decreasing the space between the strips were effective in reducing AC loss. In consideration of this configuration, a three-layer conductor was fabricated, and AC loss of 0.054 W/m at 1 kA rms was achieved even though it had a small outer diameter of 19.6 turn. (C) 2007 Elsevier B.V. All rights reserved.

The Adaptive Beamformer draws attention as an MEG inverse solution for multiple sources. The method has a drawback that we cannot apply it to the temporally correlated sources. We had developed a method to overcome this drawback. Signal-subspace eigenvectors of an original data-covariance matrix was decomposed by the GA-SA method (the hybrid method of Genetic Algorithm and Simulated Annealing) and a new decorrelated data-covariance matrix was reconstructed. The GA-SA method, however, requires the number of components of the eigenvector as a foreknowledge. In this study, we introduced the minimum-norm method, that requires no foreknowledge, instead of the GA-SA method. We carried out a three-dimensional simulation study using the proposed method, and applied to the auditory evoked response field measured by using the Vectorview system. As a result, multiple sources with time-correlations were successfully estimated. © 2007 Elsevier B.V. All rights reserved.

Critical current of ITER model coil degraded with the increase of electromagnetic force. Analysis based on results of the experiments in which periodic transverse load is artificially applied to single strand showed that periodic bending of the strand by transverse load caused this degradation. However, since the experiments were carried out under a certain condition, general relation between transverse load and critical current was not obtained. We have therefore developed a numerical simulation code to investigate the general relation. However, there were some disagreements between the simulation and experimental results in our previous study. Our simulation model was consequently modified, resulting in good agreement with the test results. The general dependence of the critical current on periodic transverse load, temperature, and pitch of periodic load is investigated using the modified simulation code. The simulation results reveal that the reduced critical current depends on the temperature, and is a function of the bending strain independent of the pitch of the periodic load.

YBCO coated conductors are expected to be used in future high-temperature superconductor applications. For the application to power transmission cables, a number of YBCO coated conductors would be connected in parallel and they might be subjected to short-circuit fault currents. Therefore, in order to examine the stability and feasibility of YBCO power transmission cables, it is important to investigate the normal-zone initiation and propagation characteristics of the YBCO tapes and demonstrate the redistribution of transport current and the electromagnetic coupling between YBCO tapes during a short-circuit fault current. In this study, over-current pulses were applied to the two parallel-connected YBCO sample tapes in a liquid nitrogen bath, and the characteristics of the normal-zone initiation and propagation were examined experimentally. Additionally, we also show the experimental results of the redistribution of the transport current between parallel-connected YBCO sample tapes.

We have been developing a pipeline transporter system using high-temperature superconducting bulks (HTS bulks). In this paper, we propose a pipeline transporter system using pinning effect of HTS bulks. This system has the advantage of the application in harsh or dangerous environment such as in hazardous gas. This system requires large lateral force at the certain gap between the permanent magnets and the HTS bulks. For the realization of this pipeline transporter system, we investigated the influences of the arrangement of HTS bulks and permanent magnets on the lateral force characteristics through experiments and numerical simulations by using a computer program based on the three-dimensional hybrid finite element and boundary element (3D hybrid FE-BE) method. And we also designed a model system with a lateral force of 1500 N.

The purpose of this study is to propose a criterion for determining stabilizer thickness of YBCO coated conductors. The permissible disturbance energy density in high-temperature superconductors (HTSs) is two or three orders of magnitude greater than that in low-temperature superconductors (LTSs). On the other hand, to enhance the reliability and safety of HTS coils, an establishment of a protection scheme assuming a quench caused by a failure in power supply, cooling system or similar problem is also required for HTS coil systems. Quenching produces excessive temperature or voltage in the coil winding; overheating may cause the conductor to meltdown, or high voltage may punch a hole in the winding. To avoid these types of damages in YBCO coils, a suitable stabilizer thickness must be determined for the sophisticated structure of YBCO coated conductors, which are composed of highly textured thin-film on a flat flexible substrate. Therefore, we investigate the appropriate structure for YBCO coated conductors from the perspective of energy dumping (i.e., quench protection). Based on the protection scheme that the stored magnetic energy in the coils is dumped by an external dump resistance prepared as a quench protection, we derive a function that examines the heat balance of a quenching YBCO coil in an adiabatic condition and propose a criterion using the function to determine an appropriate stabilizer thickness for YBCO coated conductors.

Aluminum stabilized superconductors are used in accelerators, SMES, and fusion devices, such as the LHD helical coils. These superconductors have large cross-sectional area of high purity aluminum to improve their stability. However, one of the important properties of these superconductors is the transient stability, which is caused by a long duration of transport current transfer from the superconducting strands into the aluminum in a normal state region. Once a normal zone is initiated in such superconductors, excess joule heat is generated in a small region of the aluminum stabilizer near the superconducting strands during the transport current diffusion time. It hence deteriorates the transient stability. Therefore, it is important to investigate the characteristics of the transient stability by numerical analysis. The latest experiments of the LHD helical coil conductor showed an asymmetrical propagation of normal zone along the longitudinal direction of the conductor. The Hall effect is clearly one of the causes of this phenomenon. The Hall effect prevents the transport current from transferring between the superconducting strands and the aluminum stabilizer. It causes the asymmetrical transport current distribution, and affects the stability of the superconductor. In order to simulate the normal zone propagation in the superconductor more precisely and to clarify the cause of the asymmetrical propagation, we have developed a 2D finite element analysis code taking account of the Hall effect and investigated the characteristics of the normal zone propagation of large aluminum stabilized superconductors.

For HTS (High-Temperature Superconductor) applications to electric power devices, superconductors are subjected to short-circuit fault currents 10 to 30 times the normal operating current. These over-currents will drive the HTS conductors normal and degrade or burn-out the HTS conductors. Therefore it is important to establish a novel thermal stability criterion for over-current pulse drive.
In this study, we carried out preliminary experiments on the damage caused by over-current pulse drive focusing on the limitation of the temperature rise without degradation. We prepared five YBCO sample tapes fabricated with IBAD/PLD and IBAD/MOCVD methods. Measurements were performed at 60 K and 80 K (Conduction cooling with Gifford-McMahon cryocooler was adopted) in self-field.
The experimental results indicated that the degradation of the sample tapes for the over-current pulse with a duration of 1 second depended on not the initial critical current I-c, but the peak temperature T-peak. The permissible peak temperature without degradation existed at about 400-600 K. The sample tapes were also observed before and after the experiments by magneto-optic (MO) imaging to find the distribution of degradation area.

A YBCO coated conductor is expected to be a next generation high performance superconductor. To design superconducting machines using YBCO conductors, evaluation of not only superconducting properties but also mechanical properties is very important. Hence, we applied bending and torsion to the YBCO conductor and measured degradation of the conductor against the strain. To compare with the YBCO's data, a Bi2223 conductor was also tested under the same experimental condition. From the bending test, degradation of the Bi conductor was faster than that of the YBCO conductor and the critical current gradually decreased with increasing bending strain. The YBCO conductor kept better performance than the Bi conductor, and degradation suddenly occurred. And when the superconducting layer in the YBCO conductor was subjected to compressive strain in the bending test, the critical current hardly decreased at approximately one percent of the bending strain.

We have applied compressive stress perpendicular to YBCO coated tapes, and measured the degradation of critical currents. When the tape was pushed from the silver surface of the tape, the performance of the tape hardly decreases. On the other hand, compressing from the hastelloy face, degradation of the tape occurred. We observed the damage of the tapes by a magneto-optical imaging technique. The observed results were consistent with the degradation data. From those results, we think that it is better not to compress strongly from the hastelloy side.

To establish a stability criterion for YBCO coated conductors, it is required to clarify their transient thermal characteristics. In this study, we adopt a method for visualization of. temperature distribution using fluorescent paints whose color changes depending on the temperature. In the visualization method, firstly, commercial fluorescent paints are coated on the surface of sample strips. And then, the sample is irradiated by a black light or UV-LEDs, and the image of the fluorescence is observed by a CCD camera. These captured images are manipulated image processing by a computer, and show the temperature distribution. This system can be applied to cryogenic and high magnetic field circumstances. By adopting this visualization method to the investigation of transient thermal characteristics of YBCO tapes, local heat generation in the YBCO tapes can be observed by higher resolution, and it is expected to identify the uniformity of critical current density of YBCO tapes. In this paper, we describe the developed thermal visualization system and the preliminary experimental results on visualization of normal-zone initiation and propagation in YBCO sample strip fabricated with IBAD/PLD method.

We have constructed and demonstrated levitating transporter models composed of the superconductor bulks (HTS bulks) and permanent magnets. The dynamic characteristics of lift force, lateral force and magnetic stiffness are very important for the design of superconducting devices such as a levitation system. The electromagnetic phenomenon should be clarified through the accurate analyses of the magnetic field and shielding current distributions within HTS bulks. In this study, we developed a new simulation program based on the three-dimensional hybrid finite element and boundary element (3D hybrid FE-BE) method, which combines with the fast multipole method (FMM). By using the FMM, it is possible to solve the large-scale problem with a reduction of the required memory size and improvement of the accuracy. This simulation program can consider the anisotropic and nonlinear E - J characteristics of HTS bulks. The results of experiment and numerical simulation are in good agreement.

Aluminum stabilized superconductors are used in accelerators, SMES, and fusion devices, such as the LHD helical coils. These superconductors have large-cross sectional area of high purity aluminum to improve their stability. However, one of the important properties of these superconductors is the transient stability, which is caused by a long duration of transport current transfer from the superconducting strands into the aluminum in a normal-state region. Once a normal zone is initiated in such superconductors, excess joule heat is generated in a small region of the aluminum stabilizer near the superconducting strands during the transport current diffusion time. It hence deteriorates the transient stability. Therefore, it is important to investigate the characteristics of the transient stability by numerical analysis. The latest experiments of the LHD helical coil conductor show an asymmetrical propagation of normal zone along the longitudinal direction of the conductor.
The Hall current generation is clearly one of the causes of this phenomenon. The Hall current generation prevents the transport current from transferring between the superconducting strands and the aluminum stabilizer. It causes the asymmetrical transport current distribution, and affects the stability of the superconductor. In order to simulate the normal-zone propagation in the superconductor more preciously and to clarify the cause of the asymmetrical propagation, we have developed a 2D finite element analysis code taking account of the Hall effect and investigated the transient stability of large aluminum stabilized superconductors.

To demonstrate the applicability of Nb3Sn Cable in Conduit Conductors (CICC's) to International Thermonuclear Experimental Reactor (ITER) coils, four Nb3Sn model coils have been constructed and tested. The experimental results showed that the measured critical current of the conductors degraded compared with what is expected from the ability of the strands. In addition, the larger is the applied electromagnetic force, the larger the magnitude of the degradation is. A degradation in n-value was also observed. One of the explanations of this degradation is supposed to be a local strand bending. This consideration has been supported by test results, in which a similar degradation in the critical current and n-value was observed when applying periodic bending strains to a single strand. However, general dependence of critical current on periodic bending strain has not been clarified in this test since the experiments were carried out at a certain magnetic field, temperature and strain. Therefore, a numerical simulation code was developed to study the general dependence of the critical current and n-value of Nb3Sn strand on periodic bending strain. A distributed constant circuit model is applied to simulate current transfer among the filaments in the strand. The simulation results show relatively good agreement with the experimental results but some modification in modeling is required for more accurate simulation.

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It is difficult to accurately simulate supercurrent in high-temperature superconducting bulk even by adopting the finite-element method. The equivalent electrical conductivity of high-temperature superconducting bulk, which has a strong nonlinearity according to the E-J power law, is introduced for the supercurrent analysis. However, the strong nonlinearity results in bad convergency of the nonlinear equation solvers, i.e., the Newton-Raphson (NR) method. Consequently, the unsuitable and/or undulating supercurrent is observed. Therefore, in this paper, the accuracy of the supercurrent analysis in the high-temperature superconducting bulk is discussed and a new method is proposed for the accurate high-temperature superconducting bulk simulation. Then, the method combined a line search with the NR method is adopted as a nonlinear equation solver, and the improvement of convergency and computation time are investigated

It is expected that electric devices would be high efficiency, high output and downsized by using high-temperature superconducting (HTS) bulk materials. In the previous study, we have fabricated and tested a linear synchronous actuator with double-sided primary and YBCO bulk secondary; the linear actuator consists of a field-cooled YBCO bulk for secondary (mover) and copper windings with an iron core as a primary. The primary, which was excited by a three-phase VVVF (variable voltage variable frequency) power source to generate sinusoidal traveling magnetic fields, was divided into two sections: 1) the starting section in which the secondary is accelerated as an induction machine; and 2) the synchronous section in which the secondary moves with a specified synchronous speed. And we have evaluated both of the static and dynamic characteristics on this linear synchronous actuator. However, for practical use of linear actuator with field-cooled HTS bulk secondary, it is required to examine the distribution of magnetic field within the HTS bulk which is exposed to the traveling magnetic field in a realistic operational environment of linear actuator. Therefore, in this study, we repeated the mode of acceleration, constant-speed, deceleration and stop with the control circuit in the synchronous section. The direction of the mover can be turned automatically by the switching device and it made round-trip running possible. Then, we carried out many times round-trip operations and evaluated the change of the magnetic field on the surface of the YBCO bulk in various conditions with trapped field of the secondary and traveling magnetic field generated by the primary windings.

We have been developing an active magnetic levitation system composed of field-cooled disk-shaped YBCO bulk and multiple ring-shaped electromagnets which are vertically piled up. We have demonstrated and reported that the levitation height, as well as stability, can be remarkably improved by adjusting the operating current of each electromagnet individually. In this study, we constructed a noncontact position control system in the vertical direction based on the feedback control theory and the electromagnetic field analyses considering superconducting characteristics of the YBCO bulk. Depending on levitation height, the operating current in the electromagnets is controlled automatically. The system consists of two ring-shaped copper-winding electromagnets without iron core and a ring-shaped YBCO bulk magnetized by the field-cooling process. We carried out experiments to verify the feasibility of noncontact position control system. In the experiment for position control, the levitation height maintained a target position accurately, and responded smoothly to changing target position. In addition, we aimed at the construction of the position control system with higher accuracy using the numerical simulation based on the finite element method analyses. And, the position control system that piled up three electromagnets was designed and constructed. Concretely, accuracies of the position control within 28 mu m were obtained. Final goal of our study is to apply the levitation system to the inertial nuclear fusion in which the accuracy as high as several mu m would be required.

Recently, the properties of high temperature superconducting tapes have been in advance and high temperature superconducting magnets have been constructed and demonstrated. However, the high temperature superconducting tapes have different thermal characteristics compared with low temperature superconducting wires. Therefore, it is necessary to consider these characteristics of high temperature superconducting tapes at the magnet design stage. We proposed an optimal design method for superconducting coils wound with Bi2223/Ag tapes. In this paper, the configuration of 72 MJ SMES coils wound with Bi2223/Ag tapes are optimized.

Trapped field characteristics are one of the key factors in the application of high-temperature superconducting bulk, such as magnetic levitation systems, motors, bulk magnets and so on. The immutable trapped field is required in superconducting bulk applications as a quasi-permanent magnet, however, the trapped field is influenced and changed by a time-varying external magnetic field in a realistic operational environment of electrical devices. This means that shielding current distribution within the bulk is changed by the time-varying magnetic field and the transient magnetic flux movement results in temperature rise and finally reduction of the trapped field. In this study, we observed the abrupt reduction of the trapped field for the external AC magnetic field in the range of frequency up to 50 Hz. And we also performed the numerical simulation using a newly developed computer program based on the finite element method (FEM). In the simulation program, the voltage-current (E-J) characteristics, which depend on the irreversibility temperature are taken into account. From these results, it can be considered that the abrupt reduction of the trapped field is caused by the temperature rise due to AC loss.

We have been developing a mice biomagnetic measurement system using a dc superconducting quantum interference device magnetometer for a comparative magnetoencephalogram study of transgenic and wild mice. In this study, we adopted a magnetometer with a pickup coil of 1.0 mm diameter and 0.7 mm lift-off distance to improve spatial resolution and magnetic field sensitivity. The magnetocardiogram of a wild mouse was measured as the initial application of the system. The bipolar peak positions in a magnetic field map separated by 5.6 mm were clearly detected with the maximum amplitude of 88 pT. (C) 2004 American Institute of Physics.

Very-large-current composite superconductors are used in SMES coils and fusion applications. These superconductors have large cross-sectional areas of high purity aluminum to improve their stability. Once a normal zone is initiated in such superconductors, the current transfers from the superconducting strands to the aluminum stabilizer according to the Maxwell's equations and the temperature distribution. However, the time constant of current diffusion in the aluminum stabilizer is very long as electrical resistivity of aluminum is very low. Therefore, excess Joule heating is generated in a small region of aluminum stabilizer near superconducting strands, and the temperature increases locally. Some 2D numerical analyses have been carried out in order to investigate the transient stability of the superconductor applied to the Helical Coil of LHD in National Institute for Fusion Science. But, as the performance of computers have improved, huge numerical simulations are new feasible. So we wrote a 3D finite element analysis code ourselves to carry out some now analyses that we compared with 2D analysis.

Since 1999, a Japanese national project to realize commercially feasible SMES for designated applications has been promoted. Model coils for power system stabilization, load fluctuation compensation and frequency control have been fabricated and tested. In this paper, we introduce the present status of the project, including feasibility studies of an HTS SMES.

We have been developing a magnetic levitating device with two-dimensional movement, namely a 'levitating X-Y transporter'. For the real design of a levitating X-Y transporter, it is necessary to clarify the levitation characteristics, such as the lift, the levitation height and the stability against mechanical disturbances. Furthermore various kinds of force may be applied to the levitating part and cause mechanical oscillation. Therefore the characteristics of oscillation are also important factors in the dynamic stability of such a levitation system. In this paper, we examine experimentally the lift and the restoring force and develop a new simulation code based on the three-dimensional hybrid finite and boundary element method to analyse the dynamic electromagnetic behaviour of the HTS bulk. We have investigated a suitable permanent-magnet arrangement to enhance the levitation characteristics through experiment and numerical simulation. We can then determine the suitable conditions for stable levitation from those results.

We propose a novel classification method based on the Bayes rule to utilize the magnetocardiogram (MCG) in noninvasive mass screening. The cardiac excitation is directly tracked by maps of the MCG field generated by myocardial excitation current through the excited wave front. To adopt the characteristics of the excited wave fronts as a parameter for the Bayes theorem, we developed a parameterization procedure that consists of a two-dimensional wavelet approximation and a cluster analysis of magnetic field maps. With the parameter determined by this procedure, the probability of a subject to belong to a disease group or to the normal group is estimated by the Bayes theorem. The subject is classified into the group of the highest probability. We applied the proposed method to ST-T period of MCG data of 6 old myocardial infarction (OMI) patients and 15 normal controls. The method showed sensitivity of 83%
specificity, 100%
positive predictive value, 100%
and negative predictive value, 94% in the classification of OMI patients and normal controls. The processing time is less than 5 seconds per one subject. It suggests a possible application of the proposed method in mass screening of abnormal MCG patterns.

A small, low-magnetic-noise nondestructive inspection (NDI) system that uses a high-T_c superconducting quantum interference device (SQUID) cooled by a coaxial pulse tube cryocooler (PTC) has been developed. The key components for realizing the compact, low-noise system developed in this study, include the following: a high-T_c SQUID gradiometer, the coaxial PTC, and a SQUID-mount stage separated from the cold head of the cryocooler. The high-T_c SQUID gradiometer contributes not only to stable system operation when subjected to the environmental noise without magnetic shielding, but also a decrease in magnetic noise caused by the mechanical vibration of the SQUID, which is transmitted from the cryocooler. By using the coaxial PTC, which was constructed of non-magnetic materials, both magnetic noise and the mechanical vibration are well suppressed compared to conventional two-axial PTCs. In addition, the coaxial PTC is designed to be compact (50 mm diameter, 400 mm height, and 4 kg weight) for practical use. The SQUID-mount stage separation contributes to stable operation by reducing the transmitted vibration to the SQUID, and thermal stability at the SQUID-mount stage. The magnetic flux noise spectrum of the cryo-cooled system was measured and compared with that of a liquid-nitrogen-cooled system using the same SQUID. From the comparison, no increase in magnetic noise due to the cryocooler was observed, although the magnetic flux noise level of the cryo-cooled system was dominated by SQUID operation noise, which was rather high, (i.e., around 100 μΦ₀/Hz^1/2 at 100 Hz). The relation between the mechanical vibration at the SQUID and magnetic noise is discussed.

Superconducting fault current limiter (SCFCL) is expected to be a first application of high-T-C superconductors to electric power systems. We have been developing a magnetic shielding type of SCFCL. In this paper, we propose a design method for the magnetic shield SCFCL. The method is based on a newly developed computer program which can evaluate the dynamic thermal and electromagnetic behaviors of high-T-C superconducting cylinder in fault current limiter operation. As an example, the trial design of a model system, which is 6.6 kV (rms) in rated voltage, 400 A(rms) in rated current, and 12.5 kA(rms) in maximum prospective short-circuit current, is shown. (C) 2003 Wiley Periodicals, Inc.

Recently, the properties of high temperature superconducting (HTS) tapes have been in advance and HTS magnets have been constructed and demonstrated. However, the HTS tapes have thermal different characteristics compared with the low temperature superconducting (LTS) wires. Moreover, Bi2223/Ag tapes and YBCO coated conductors have the different properties about the dependence on the degree of external magnetic field. Therefore it is necessary to consider these characteristics of HTS tapes at magnet design stage. We proposed an optimal design method for superconducting coils using Bi2223/Ag tapes. And the proposed configuration is optimized by using the simulated annealing (SA), which is one of the optimization algorithms, under a lot of constraints such as B-I characteristic, storage energy, and so on. The details of the optimization method and an example of its application to SMES coils (72 NU) using YBCO coated conductors are shown.

We have been developing a new type of active-maglev system that is a transporter in the vertical direction, consisting of high-temperature bulk superconductors and multiple electromagnets piled up on the vertical axis. In a previous paper, using an active-maglev system composed of a disk-shaped YBCO bulk and five electromagnets, we have demonstrated continuous levitation and verified that its levitation height, as well as stability, call be remarkably improved by adjusting the operating current of the electromagnets individually. Electromagnetic behavior within the bulk has been also investigated numerically by a newly developed computer program based on the finite-element method adopting the Bean model. Agreement of levitation force and height between experiments and numerical analyses was good. The levitation force properties strongly depend on the field-cooling condition and the distribution and magnitude of the external magnetic field generated by the electromagnets.,We clarify electromagnetic phenomena within the bulk superconductor by the computer program developed to improve the levitation properties for applications of continuous levitation to real maglev systems.

High-T-c. superconducting bulk has been used for magnetic levitation systems such as flywheel energy storage system and noncontact transport system. We have been investigating the electromagnetic behavior of high temperature superconductor (HTS) bulks to realize a X-Y (two-dimensional) magnetic levitating transporter without any fixed guides. We have investigated qualitatively the characteristics. of the lift and the restoring force of an YBCO bulk under various field-cooling conditions and permanent-magnet arrangements. In this paper, we evaluate the most suitable 4-permanent-magnet arrangement through lift and restoring force measurements and numerical analysis by the finite element method (FEM) using the current vector potential.

It is expected that HTS bulk materials can be applied to various electric devices. We have fabricated and tested a linear actuator with a single-sided primary and an HTS bulk secondary. The linear actuator was comprised of a zero-field-cooled YBCO bulk plate tor the secondary (mover) and copper windings with iron core for the primary. In this study, to improve the characteristics of the linear. actuator, we designed and fabricated a double-sided and short-secondary type of linear synchronous actuator which has a field-cooled YBCO bulk plate as a secondary (mover). The static synchronous thrust forces were measured. To evaluate the static characteristics of the linear actuator, we also numerically investigated the electromagnetic behavior within the bulk in an operational environment of the linear actuator by using a computer program based on the finite element method (FEM). The influence of the supercurrent distribution in the bulk on the thrust-angle characteristics of the model linear actuator are discussed.

Although immutable trapped field is required in superconducting bulk applications as a quasipermanent magnet, the trapped field is influenced and changed by time-varying external magnetic field in a realistic operational environment of electrical devices. This means that shielding current distribution within bulk is changed by the time-varying magnetic field and the transient magnetic flux movement results in temperature rise and finally reduction of the trapped field. In this paper, we experimentally investigated the transition of trapped field while applying external AC magnetic field with various amplitude and frequency to a disk-shaped. YBCO bulk. And we also numerically investigate the relationship between characteristic of trapped-field attenuation and shielding current distribution within the bulk using a newly developed simulation Program. This program is based on the finite element method (FEM) considering the voltage-current (E-J) characteristics. It is found that the shielding current distribution is dependent on frequency of AC external field and trapped field attenuation is closely related-to the shielding current.

This paper describes an optimal design method for SMES coils wound with high temperature superconducting (HTS) tapes (e.g., Bi2223/Ag tapes). Recently, the properties of HTS tapes have advanced and HTS magnets have been constructed and demonstrated [1]-[3]. However, the HTS tapes have thermal different characteristics compared with the low temperature superconducting (LTS) wires. Therefore, it is necessary to consider these characteristics of HTS tapes at magnet design stage. We propose an optimal design method for SMES coils using Bi2223/Ag tapes. And the proposed configuration is optimized by using the simulated annealing, (SA), which is one of the optimization algorithms, under a lot of constraints such as B-I characteristic, storage energy, and so on. The details of the optimization method and examples of its application to small SMES coils (0.5 MJ) using Bi2223/Ag tape are shown.

We present a, new type of active-maglev system consisting of a disk-shaped superconducting bulk (YBCO) and multiple electromagnets. Using the active-maglev system composed of five electromagnets, we demonstrated. continuous levitation and verified that the levitation height, as well as stability, could be remarkably improved by adjusting operating current of electromagnet individually. Electromagnetic behavior within the bulk was investigated numerically by the finite element method (FEM) adopting the Bean model. Agreements of levitation force and height between experiment and analysis were good. Suitable electromagnet operation for continuous levitation in terms of consumed operating energy was also investigated. It was found in analysis that continuous levitation could be realized efficiently by adopting a three-electromagnet operation and the operating procedure is applicable to multiple-electromagnet system. Therefore, based on the assumption of constant total operating current of three electromagnets, we numerically investigated the relationship between the operating procedure and levitation force in a five-electromagnet system as a function of levitation height. Maximum allowable weight of float (superconducting bulk and load) was evaluated through the estimation of minimum levitation force during continuous levitation as a function of air gap between electromagnets.

It is very important to clarify characteristics of trapped field in HTS bulk exposed to an external AC magnetic field in application of high-temperature superconducting bulk to motors, magnetic levitation systems, bulk magnets and so on. We have experimentally investigated the transition of trapped field while applying external AC magnetic field to a disk-shaped YBCO bulk. The observed trapped-field attenuation is different from that by flux creep, i.e., zero external field. It may be considered that the trapped-field attenuation is affected by external AC magnetic field, particularly temperature rise due to AC loss. In this paper, electromagnetic behaviors within the bulk exposed to external AC magnetic field, especially supercurrent distribution, are investigated numerically using a simulation code based on the finite element method (FEM) considering voltage-current (E-J) characteristic. We focus on the relationship between the transition of supercurrent distribution within the bulk while applying external AC magnetic field and both n-value of E-J characteristic and temperature rise by AC loss.

High T-c superconducting bulk has been used for magnetic levitation systems such as flywheel energy storage system, noncontact transport system and so on. We have been investigating electromagnetic behaviors of HTS bulk to realize a X-Y (two-dimensional) magnetic levitating transporter without any fixed guides. We have investigated qualitatively the characteristics of lift and restoring force of an YBCO bulk for various field-cooling conditions and permanent-magnet arrangements. In this paper, we evaluated the most suitable conditions of air gap in field-cooling process and permanent-magnet arrangement through lift and restoring force measurements and numerical analysis by the finite element method (FEM) based on the current vector potential method. the characteristics of oscillation in the vertical direction of levitating X-Y transporter were investigate by the measurements of free and forced vibrations. The theoretical characteristics of the free and forced vibrations were compare with the experiments.

It is expected that HTS bulk materials are applied to various electric devices. In a previous work we have fabricated and tested a linear actuator with single-sided primary and HTS bulk secondary. The linear actuator was comprised of zero-field-cooled YBCO bulk for secondary (mover) and copper windings with iron core (primary). The primary was excited by a three-phase VVVF power source to provide sinusoidal traveling field. To improve the characteristics of the linear actuator, we newly designed and fabricated a linear actuator with double-sided primary and zero-field-cooled HTS bulk secondary. We measured starting thrust force and magnetic field distribution in the air gap between the bulk and the primary iron core at the stationary state. To evaluate dynamic characteristics of the linear actuator, we numerically investigated electromagnetic behavior within the bulk exposed to time-varying magnetic field using a simulation program based on the finite element method (FEM) taking the voltage-current (E-J) characteristic into consideration. Computed starting thrust force, displacement characteristic and field distribution between the bulk and the primary iron core were compared with the experimental results. Agreement between experiments and analyses was good. We also investigated the electromagnetic characteristics as a linear synchronous actuator with double-sided primary and field-cooled HTS bulk secondary.

The use of carbon-fiber-reinforced plastic (CFRP) in aircraft and space structures is recently increasing. We investigated the possibility of nondestructively detecting a delamination in CFRP using a low-Tc first-order gradiometric superconducting quantum interference device (SQUID) loop. A CFRP plate including a thin insulator sheet between the central layers was prepared. The injected current method was adopted. The magnetic field due to the sheet was measured by a SQUID nondestructive evaluation system using the lock-in detection technique. The position and size of the sheet were successfully detected by mapping the measured field. The result agreed with that by ultrasonic testing. Future application of this method for detecting deep-lying flaws was discussed.

The widespread use of the SQUID-NDE requires the visualization of defects, We have developed a method to obtain depth information by monitoring the SQUID output while changing the frequency of the current flowing in a sample, The effectiveness of this method was verified by experiment and simulation.

One of the advantages of magnetic levitation using high-temperature superconducting (BTS) bulk is that stable levitation can be achieved without any control systems. We have been investigating the electromagnetic behaviors of HTS bulk to realize a two-dimensional magnetic levitating transporter without any aired guides, The characteristics of lift and stability are key parameters to design and optimize such a device. We measured the lift and the restoring force of a YBCO bulk, displaced by a distance in lateral direction, for various field-cooling conditions and permanent-magnet arrangements, Both lift and restoring force are closely related to the air gap in the field-cooling process, distance between the permanent magnets, number of permanent magnets, and permanent magnet arrangement, that is, external magnetic field distribution. The most suitable arrangement of permanent magnets depends on the required levitation height and the weight of the levitating part. It can be considered that the size of the levitating part and geometry are also very important to determine the optimal arrangement in the maglev device.

The trapped field characteristics of disk-shaped YBCO bulk exposed to external AC magnetic field are investigated experimentally, The magnetic flux density an the top surface of the disk defined as "trapped flux" in this paper, was measured in both short and long tel ms as functions of amplitude and frequency of the AC external magnetic field. The observed trapped flux attenuation was obviously different from that of flux creep, i.e,, with no external magnetic field; this Implies that the trapped flux density within the YBCO dish is reduced by a temperature rise due to AC loss, The abrupt attenuation of trapped flux density in the first several cycles was observed at frequencies of 0.1 and 1 Hz, while not observed at 10 Hz The attenuation rate after seven minutes of applying AC magnetic field, however, became almost the same regardless of frequency. The trapped Bur attenuation in one cycle of, AC magnetic field decreases with the frequency and increases with the amplitude of AC magnetic field. These results imply that the characteristic of trapped pus is closely related to AC loss, especially the hysteresis-loss, and the AC loss depends on the frequency and the amplitude of the AC external magnetic field. at can he considered that transient electromagnetic behavior within HTS bulk, especially supercurrent distribution, is a key factor. of the relationship between the trapped flux attenuation and the AC loss.

This paper describes an optimal design method for superconducting magnets wound with high temperature superconducting (HTS) tapes (e.g. Bi2223/Ag tapes). Recently, the properties of HTS tapes have been in advance and HTS magnets have been constructed and demonstrated [1]-[4]. However, the HTS tapes have thermal different characteristics compared with the low temperature superconducting (LTS) wires. Therefore it is necessary to consider these characteristics of HTS tapes at magnet design stage. We propose a new configuration of the superconducting magnets using Bi2223/Ag tapes. And the proposed configuration is optimized by using the simulated annealing (SA), which is one of the optimization algorithms, under a lot of constraints such as B-I characteristic, central magnetic filed, held homogeneity and so on. The details of the optimization method and an example of its application to 12-tesla superconducting magnet using Bi2223/Ag tape are shown.

We have constructed a model Superconducting Cylindrical (Tubular) Linear Induction Motor (SCLIM) to clarify the behaviors (AC loss, stability and so on) of AC superconducting windings in a realistic operational environment of electrical rotating machines. The studies of an AC superconductor's development in regard to the reduction of AC loss and the improvement of current capacity have been advanced From now on, the valuation of characteristic of AC superconducting windings in a realistic operational environment should be investigated. In the previous paper we presented the design and construction of SCLIM model system composed of AC superconducting primary windings, FRP cryostat and VVVF power supply with quench detection and protection circuit, and the results of fundamental driving experiments. In this paper, we optimize the current distribution in primary windings to enhance the thrust characteristics and the quench detection and protection system for AC superconducting machines by a numerical approach based on the developed FEM computer program. The measurements of the thrust force, the distributions of the magnetic flux density and primary current in the SCLIM model are also shown.

Superconducting fault current limiter (SCFCL) is expected to be the first application of a high-temperature superconductor (HTS) to electric power systems. We have been developing a magnetic shielding type of SCFCL that uses a cylindrical Bi-2223 HTS bulk Short-circuit fault tests in a small SCFCL model were performed experimentally. A computer program based on the finite element method (FEM) taking the voltage-current (E-J) characteristics of the bulk material into account was developed to analyze the performance in the short-circuit fault tests and to investigate the dynamic electromagnetic behavior within a bulk superconductor. Because the E-J characteristic of HTS bulk depends on temperature and magnetic field, we investigated experimentally the E-J characteristics of a bulk superconductor in various operating temperatures and magnetic fields. The computer program considering the measured E-J characteristics simulated the electromagnetic behaviors in an SCFCL test model successfully.

The Magnet technology of a high-temperature superconductor (HTS) with a conduction cooling system by a GM cryocooler has been advanced. It is very important to understand the thermal and electromagnetic phenomena in HTS tapes for HTS coil design. We have measured the quench characteristics of HTS tapes with a conduction cooling system of a GM cryocooler and developed a computer program based on the finite element method (FEM) to investigate the thermal and electromagnetic behaviors within HTS tapes, From these experimental and analytical results, the validity of the computer program has been confirmed. In this paper, we discuss a stability criterion for cryocooler-cooled HTS coils by using the computer program. The current at which "thermal runaway" occurs, which depends on the relationship between the cooling power of the GM cryocooler and the heat generation within HTS coils, is chosen as a stability criterion. We also investigate the transient thermal and electromagnetic behavior in HTS coils.

To investigate characteristics of a linear actuator with high-temperature superconducting (HTS) bulk secondary, a short secondary type linear actuator has been designed and constructed. The actuator is comprised of an YBCO bulk secondary (mover) and copper windings with iron core (primary). A zero-fieid-cooled balk located at the center of the actuator plays a role of generating thrust, while four field-cooled (trapped field) bulb al e used for levitation and guidance of the secondary. We measured the starting thrust force and magnetic Weld distribution in the air gap. We developed a simulation program based on the finite element method (FEM) taking the voltage-current (E-J) characteristics of the bulk into consideration to investigate electromagnetic behaviors within the bulk exposed to a time-varying magnetic field,agreement between experiment and simulation Is good, and it validates the simulation program and the presented assumptions in the numerical approach. Using the simulation program, we investigated the dependency of n-value and critical current density of the bulk material on the magnetic flux density in the air gap and the starting thrust force. Supercurrent density within the bulk is a key factor for the magnetic flux density and the thrust force.

The superconducting fault current limiter is expected to be the first application of high-Tc superconductors (HTSs) in power systems. To develop a magnetic shielding-type superconductor fault current limiter, we have carried out some fundamental experiments on the magnetic shielding characteristics of an HTS bulk cylinder. In this paper, the experimental results of magnetic flux penetration into Bi-2223 cylinders are shown. An ac magnetic field is applied to the sample by a primary winding (copper coil) excited by ac triangular- and sinusoidal-waveform currents in the frequency range from 0.1 Hz to 100 Hz. We also developed a finite element method computer program for evaluation of the dynamic electromagnetic behavior of the HTS cylinder in a time-varying external magnetic field. The results of computer simulations considering the voltage-current (E-J) characteristic are compared with experiments. Next we carried out a current-limiting test with a small limiter model, and the developed finite element method computer program successfully simulated the electromagnetic behavior in current-limiting operation. (C) 2000 Scripta Technica.

We present results of comprehensive study of electromagnetic behavior of high-temperature superconducting bulk YBCO exposed to time-varying magnetic field. The characteristic of trapped magnetic flux is one of the key factors in application of high-temperature superconducting bulk, such as magnetic levitation system, rotor of motor and bulk magnet. Especially, time inconstancy of trapped flux, such as flux creep, is undesirable for its application system. The relationship between time-varying external magnetic field and trapped flux is investigated experimentally using field-cooled (FC) YBCO disk. Magnetic flux density just above the disk is measured by Hall probes lined up along the disk radius. The magnetic field decay which may be independent of flux creep is observed in the experiment and is influenced significantly by frequency and amplitude of AC external magnetic field. The effect of changing rate of applied magnetic field in field-cool process on trapped flux is also investigated experimentally.

We studied influences of growth temperature on superconducting characteristics in YBa2Cu3O7-x (YBCO) thin films prepared by pulsed laser deposition (PLD). The films deposited at low growth temperature (below 730 degrees C) have a high c-axis orientation, good in-plane alignment and large amount of a-axis grains. With increasing growth temperature, the number of a-axis grains in the films were decreased, whereas superconducting properties were degraded due to the formation of misoriented grain boundaries. The crystal defects acting as barriers to supercurrent flow were observed by magneto-optical imaging (MOI) technique. Using this technique, we confirmed that the critical current densities of the films were limited by these defects. Dynamic magnetic flux behaviors in flux flow state were also observed by MOI system.

This paper presents the characteristic of linear actuator with high-temperature superconducting (HTS) bulk for secondary. We designed and constructed a single-sided and short-secondary type linear actuator. The actuator is comprised of YBCO bulk secondary (rotor) and copper winding (primary) with iron core. Using this model, we measured the distributions of magnetic flux density in the air gap. We developed simulation code based on a finite element method (FEM) taking the voltage-current (E-J) characteristic into consideration to investigate electromagnetic behavior within the bulk exposed to time-varying magnetic field. The computed magnetic flux density distribution is compared with the experiment. Numerical investigation of the effect of pole pitch in the primary on thrust is also shown.

We studied influences of growth temperature on superconducting characteristics in YBa2Cu3O7-x (YBCO) thin films prepared by pulsed laser deposition (PLD). The films deposited at low growth temperature (below 730 degrees C) have a high c-axis orientation, good in-plane alignment and large amount of a-axis grains. With increasing growth temperature, the number of a-axis grains in the films were decreased, whereas superconducting properties were degraded due to the formation of misoriented grain boundaries. The crystal defects acting as barriers to supercurrent flow were observed by magneto-optical imaging (MOI) technique. Using this technique, we confirmed that the critical current densities of the films were limited by these defects. Dynamic magnetic flux behaviors in flux flow state were also observed by MOI system.

This paper presents the characteristic of linear actuator with high-temperature superconducting (HTS) bulk for secondary. We designed and constructed a single-sided and short-secondary type linear actuator. The actuator is comprised of YBCO bulk secondary (rotor) and copper winding (primary) with iron core. Using this model, we measured the distributions of magnetic flux density in the air gap. We developed simulation code based on a finite element method (FEM) taking the voltage-current (E-J) characteristic into consideration to investigate electromagnetic behavior within the bulk exposed to time-varying magnetic field. The computed magnetic flux density distribution is compared with the experiment. Numerical investigation of the effect of pole pitch in the primary on thrust is also shown.

The committee on applied superconducting apparatuses and characteristics of superconductors, the Institute of Electrical Engineers of Japan (IEEJ) has made a data base on low temperature superconducting magnets which have been built over the last ten years in Japan. Based on the data base trends of magnet technology are investigated. In this paper (Part I), out-line of the data base is explained and based on the data base the recent trends of the magnet technology are investigated focusing on sizes and configurations of magnets, current density of magnet windings and conductor configurations. The stability and quench protection of magnets are also investigated in other paper (Part II).

In this paper, we present the results of experiments and computer simulations to make clear the quench propagation properties in Bi-2223/Ag superconducting multifilament tape. We developed a computer code based on the two-dimensional finite element method (2-D FEM). Computed voltage and temperature traces during a quench agree well with the experimentally recorded voltage and temperature traces. Good agreement validates the computer code, making a useful tool in developing protection strategies for high-temperature superconducting (HTS) coils. Therefore, we simulate the quench process in a single pancake coil wound with Bi-2223/Ag superconducting multifilament tape with background magnetic filed up to 10 T at 20 K using the developed computer code. The quench propagation properties and stability in HTS coils are discussed and compare with those in low-temperature superconducting (LTS) coils.

Stability tests have been carried out on short samples of the aluminum/copper stabilized composite-type superconductors developed and used for the pool-cooled helical coils of the Large Helical Device. The waveform of the longitudinal voltage initiated by resistive heaters shows a short-time rise before reaching a final value, which seems to correspond to the diffusion process of transport current into the pure aluminum stabilizer. The propagation velocity has a finite value even for the transport current being lower than the recovery current, and it differs depending on the direction with respect to the transport current.

The superconducting fault current limiter (SCFCL) is expected to be the first application of high-T-c superconductors (HTSs) in power systems. To develop a magnetic shielding type superconducting fault current limiter, we have carried out some fundamental experiments concerning the magnetic shielding characteristics of an HTS bulk cylinder. In this paper, the experimental results of magnetic flux penetration into a Bi(2223) cylinder are shown. AC magnetic field is applied to the sample by a primary winding (copper coil) excited by AC triangular- and sinusoidal-waveform currents in the frequency range of 0.1 Hz to 100 Hz, We also developed a Finite Element Method (FEM) computer program for evaluation of the dynamic electromagnetic behavior of the HTS cylinder in a time-varying external magnetic field. The results of computer simulations considering the voltage-current (E-J) characteristic are compared with experiments.

Levitated metal casting and superconducting elevators are but two of the potential applications of Superconducting Cylindrical Linear Induction Motors (SCLIMs). We are constructing a model device to clarify the behavior of AC superconducting windings in a realistic operational environment of electrical rotating machines. This paper presents design, construction, and experimental results of the model SCLIM.

The committee on applied superconducting apparatuses and characteristics of superconductors, the Institute of Electrical Engineers of japan (IEEJ) has made a data base on superconducting magnets which were built over last ten years in Japan. The outline of the data base is explained in the paper (Part I). In this paper (part II), stability and hot-spot temperature of a magnet are investigated. Minimum quench energies, minimum propagating zones, and Stekly's stability factors are calculated using the data base. Dependence of those stability parameters on the magnet scale is studied and the design trends of the stability are discussed. Hot-spot temperatures of the pool-cooled magnets at quench and energy dump process are also calculated based on the data base, and the recent trends of design rules for the quench protection are studied.

Very large current composite superconductors have been considered and adopted to use in SMES coils and fusion applications, such as the Large Helical Device (LHD), These superconductors have large cross-sectional area of high purity aluminum stabilizer to improve their stability and to enhance the overall current density. Once a normal-zone is initiated in such a composite superconductor, the current transfers to the aluminum stabilizer according to the temperature distribution. The time constant of current diffusion in the stabilizer, however, is very long due to the low electrical resistivity of aluminum and the large conductor size. Therefore, an excess joule heat is generated in a small area near superconducting filaments and the temperature increases locally. In this paper, to evaluate this peculiar property we carry out some simulations with regard to quench process in the superconductor applied to the helical coil of LHD in National Institute for Fusion Science, The simulations by using a newly developed computer code are compared with the experimental results of the stability tests on the short samples of LHD conductor. Furthermore, we focus on the influence of the CuNi alloy clad adopted to the LHD conductor on the normal transition and normal-zone propagation properties.

A current lead with kA capacity and small AC loss is being developed for use in superconducting current limiting devices cooled with liquid helium. A gas-cooled type current lead using Bi-based oxide superconducting bulk materials was examined to clarify the effects of the current lead geometry and the use of multiple current leads. This paper presents the results of experiments and numerical simulations based on the finite element method taking consideration of non-uniform transport current distribution among multiple current leads connected in parallel. The simulations are performed to investigate the relationship between the geometry of current leads and the amount of heat flow into a cryostat. In the simulations, the AC loss caused by both mutual and self-induced magnetic fields of multiple current leads is taken into account. The influence of AC loss, the joule heat generated by the contact resistance at joints of NbTi/CuNi multi-strand cables and the HTS bars, and the return passage for gas on the operating characteristics of the HTS current leads are evaluated. © 1999 IEEE.

AC application, it is necessary to estimate the stability of multi-strand superconducting cable. Therefore, we have been studying the transient stability of non-insulated multi-strand cable when one strand in a cable turns into the normal state locally. In the quench process, local temperature rise produced by current redistribution among strands is not desirable for stability. In a previous work, we discussed the effect of Cu matrix allocated to each strand on the transient stability and showed that the Cu matrix allocation can improve the stability of non-insulated multi-strand cable through mainly numerical simulations. In this paper, we carried out experiments on three kinds of non-insulated three-strand cables
one consists of NbTi/CuNi strands and the others consist of NbTi/Cu/CuNi strands having different cross-sectional arrangement. These sample strands have almost the same diameter, the same matrix to superconductor ratio and the same BJ characteristics to evaluate the effect of Cu allocation quantitatively. We choose to define the transient stability in terms of the minimum quench energy (MQE) at each DC transport current. We also investigated the transient stability of sample cables when quench is initiated in two or three (all) strands simultaneously. © 1998 Elsevier Science B.V. All rights reserved.

We developed a method to estimate multi-dipole sources in human brain by using Genetic Algorithm for source localization of the olfactory magnetoencephalography (MEG). Two current dipoles in each hemisphere of the brain are supposed as a source model. To investigate the ability of this method, source localization in simulation was performed by the presented method. As a result, if the noise level is restricted under 2 percent (S/N = 20), the source localization of the simulation was obtained with the error less than 1cm. © 1998 IEEE.

It is expected that the normal-zone propagation of a high-temperature superconductor (HTS) is slower than that of a low-temperature superconductor (LTS) because of its large heat capacity. If the normal-zone propagates too slowly, hot spots will develop, possibly resulting in permanent conductor damage. Accordingly, there is a need to measure propagation quantitatively for designing HTS magnets. This paper describes the electromagnetic and thermal behavior of Bi-2223/Ag superconducting multifilament tapes during normal transition by conduction cooling. Some experiments were carried out under the conditions of operating temperature at 10-40 K and transport current as parameters in a zero magnetic field. Sample tapes were transited normally with a heater to initiate a quench. The voltage and temperature properties corresponding to normal transition were measured by voltage taps and thermocouples attached to the sample tapes. We also calculated longitudinal and transverse-direction normal-zone propagation velocities from the voltage and temperature traces. In the case of conduction cooling, we investigated the cooling effect due to the length of the sample tape because its cooling condition is different from the case of using a coolant - for example, liquid helium. © 1998 Published by Elsevier Science Ltd. All rights reserved.

This paper describes an optimal design method for highly homogeneous superconducting magnet systems with ferromagnetic shield. The presented design technique is a combination of the equivalent magnetization current method for the computation of the magnetic field problem, which includes nonlinear and saturated iron, and the simulated annealing for solving the corresponding optimization problem. The equivalent magnetization current method is superior in estimating the field homogeneity of the center of the magnet systems. By using the simulated annealing, the positions of each solenoid coil are optimized as continuous design variables, while the number of turns and layers of the coil windings are treated as discrete design variables. The details of the algorithm and some examples of its application to 1.5-tesla magnet systems with three types of magnetic shielding are shown. © 1997 IEEE.

This paper presents numerical results of the finite element method(FEM) with Bi-2223/Ag superconducting multifilamentary tapes in order to estimate the transient stability, and to find out the most suitable analytical model and the behaviors at normal transition of high-Tc superconductors. We investigated the analytical model and the normal transition in the longitudinal direction using the three dimensional FEM and in the transverse direction using the two dimensional FEM and compared these with the case of low-Tc superconductors.

For A.C. application, superconducting strands are bundled and twisted to make a cable with large current capacity, The quench characteristic of the cable depends on the contact thermal and electrical conductivities between strands, significantly. For improvement of the stability of non-insulated multi strand cables, it is necessary to decrease the local rise of temperature by the Joule heating in the current redistribution among strands, For this purpose, we have analytically investigated non-insulated strand cables in which the surface of each strand is covered with Cu-sheath, The transient stability of this cable is discussed analytically and experimentally.

This paper describes an optimal design method for high-field and highly homogeneous superconducting magnet systems with hybrid iron and active shielding. The presented design technique is a combination of the equivalent magnetization current method for the computation of the magnetic field problem, which includes nonlinear and saturated iron, and the simulated annealing for solving the corresponding optimization problem. The equivalent magnetization current method is superior in estimating the field homogeneity at the center of the magnet systems. By using the simulated annealing, the positions of each coil are optimized as continuous design variables, while the number of turns and layers of the coil windings are treated as discrete design variables. In this paper, the detail of the algorithm and the example of its application to 9-tesla magnet system with hybrid iron and active shield are shown.

The design and the characteristics analysis of two kinds of superconducting tubular linear induction motor are described. One has Oxide superconducting primary windings, the other has NbTi superconducting primary windings, The basic structure and design values are shown in this paper.

Highly homogeneous and high-field superconducting magnets are difficult to be designed for its optimal configuration, which has the minimum winding volume. In this paper, we proposed an optimal design method, utilizing a modified Genetic algorithm, which is newly developed, to realize the minimum winding volume design for multi-section superconducting magnets. The detail of the algorithm and two examples of its application to three-section superconducting magnets are shown.

For AC application, superconducting strands are bundled and twisted to make a cable with targe current capacity, The quench characteristic of the cable depends on the contact thermal and the contact electrical conductivities between strands, significantly, Also, the inter-strand contact resistance in the case of non-insulated cable and the material of the matrix of each strand have influence on the quench characteristics. In this paper, the thermal and the electromagnetic behaviors of non-insulated three-strand cable against local disturbances in one strand of multi-strand cables, taking the influence of inter-strand contact resistance and its joule heating, and the current redistribution among strands into account, are investigated, Particularly, the influence of the matrix material of cables on quench process is analytically investigated, and the results which show the influence of the joule heating by the contact conductance after local normal transition in one strand on quench process of whole cable is determined experimentally.

We measured operating temperature dependence of normal zone propagation velocity in longitudinal direction and thermal property with regard to Ag sheathed Bi-2223 superconducting single core and multifilamentary tapes, Each experiment, with the test sample cooled by a cryocooler, covers the operating temperature range 10-40K and It/Ic range 0.4 similar to 0.9 (where It is the transport current and Ic is the critical current in each operating temperature, Top). And, we also carried out numerical analysis by a three-dimensional finite element method in order to make dear the mechanism of normal zone propagation in high-temperature superconducting (HTS) conductors.

It is important for superconducting utilization, such as MRI, that superconducting magnets generate a highly homogeneous magnetic field. In order to enhance the field homogeneity, some notches are placed outside of a solenoid. The optimal design of notched solenoid arrangement becomes a problem which has both continuous and discrete design variables. In this paper, we propose an optimal design method utilizing a modified simulated annealing algorithm to realize the minimum winding volume design for notched multi-section superconducting magnets.

In AC multi-strand superconducting cables, AC quench current is much less than that of DC. This AC quench current degradation is caused by AC loss, mechanical damage by cabling, non-uniform current distribution among strands due to self and mutual inductances between strands, and different electrical resistivity at the joint between superconducting strands and current lead. In this paper, the current distribution and the thermal behavior of non-insulated 7- and (6+1)- strand cables applying AC transport current are analyzed and the influence of contact electrical resistivity on the current redistribution and the thermal diffusion in quench process are investigated.

For AC application, a number of superconducting strands are twisted to make a cable with a large current capacity. However, it has been observed in such cables that the AC quench current is much less than that of DC. This AC quench current degradation is caused by mechanical damage, localized current distribution due to non-uniform self and mutual inductance between strands and due to irregular contact electrical resistivity at the joint between superconducting strands and current leads. In non-insulated 7-strand cable, we investigated the current distribution near the joint between each strand and current leads affected by contact electrical resistivity in stationary state applying AC transport current. Then, we calculated current distribution in non-insulated 7-and (6+1)-strand cables applying AC transport current to investigate the influence of contact electrical resistivity on current redistribution in the quench process. (Initial quench occurs in one strand and the current in the strand are redistributed to the other strands.) In these analysis we adopt distributed constant circuit to solve the current distrbution. From these analytical results, the characteristics of current distribution in non-insulated 7-and (6+1)-strand cables are discussed.

The finite element method (FEM) is one of the most versatile methods of numerically analyzing physical phenomena described by partial differential equation. It is suitable to the analysis of such a complicated region which includes non-linear materials. It has some disadvantages, however, in dealing with the field extending infinitely. The phenomena in the electromagnetic field intrinsically spread over the infinite space. We sometimes have to consider the infinite region rigorously, e.g. in the magnetic stray field of superconducting magnets. The boundary element method (BEM) is useful to analyze very large linear fields. It reduces the domain problem to the boundary problem and is particularly suitable for analyzing the infinite space. Furthermore, the required potentials and those derivatives inside the domain are calculated by using theoretical expressions. Owing to these features, the boundary element method is especially useful for the analysis of electromagnetic phenomena. Taking account of advantages in both the finite element and the boundary element methods, the hybrid utilization of both methods (the hybrid FE-BE method) has been proposed. In this review, the recent development and several analytical results of the three-dimensional boundary element method and the hybrid FE-BE method are described.

This paper describes an optimal design of a highly homogeneous superconducting coil system for magnetic Resonance Imaging (MRI). The presented optimal design technique is based on a combination of a numerical Method for analysis of the magnetic field distribution and a mathematical programning method for solving the corresponding optimization problem. In this paper, the coil current density, the croos-sectional shape of coil, and the configuration of coil are optimizer so that a highly homogeneous magnetic field is realized. © 1988 IEEE

A new optimal design technique of magnetic field systems, which utilizes a coupling procedure of the hybrid finite element-boundary element method (HFB method) and the mathematical programming, is proposed. This technique is applied to the design of the high homogeneous field superconducting magnet system with the environmental shield for whole-body NMR imaging. Results of the optimal design for three 1.0-teslamagnet systems with different types of shields (an iron shield, a shield magnet, and a combination of the iron shield and the shield magnet) are shown. © 1987 IEEE.