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DOI： 10.1109/TASC.2023.3344427

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/tasc.2023.3338594

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/tasc.2023.3334254

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/tasc.2023.3338151

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DOI： 10.1109/TASC.2024.3358773

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DOI： 10.1109/TASC.2024.3378212

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DOI： 10.1109/TASC.2024.3360935

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DOI： 10.1109/TASC.2024.3354669

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DOI： 10.1109/TASC.2024.3361874

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

The development of high-performance HTS magnets requires a method to correctly measure the contact resistance between turns of no-insulation (NI) coils. We have proposed a method to measure the turn-to-turn contact resistance of NI coils using low frequency alternating current (LFAC). In the LFAC method, AC current is used, and it is assumed that all the AC current flows in the radial direction of the NI HTS coil. To confirm the reliability of the proposed measurement method, AC current distribution characteristics were evaluated using four (quarter circle, 1/4 size) pickup coils in a previous study. The pickup coils were mounted under the NI test coils and the current bypass characteristics from circumferential to radial direction were evaluated as a function of frequency. However, the size of the fabricated four pickup coils was relatively large compared to the test coil, so its sensitivity and spatial resolution were not sufficient. Therefore, in this article, we prepared eight pickup coils smaller than the 1/4 size pickup coil used in the previous study. Two test coils with winding tensions of 0.5 and 1 kg were prepared, and the non-uniformity of the contact resistance of both NI coils was detected using eight pickup coils.**We also identified locations in the test coil where the contact resistance changed due to multiple cooling cycles by liquid nitrogen.

DOI： 10.1109/tasc.2023.3248519

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We proposed a large-capacity wireless power transmission (WPT) system for the electric vehicle in which a high-temperature superconducting (HTS) coil is installed only on the ground side. On the other hand, the design guideline of the 100 kW class WPT system using the HTS and the copper coils for EVs has not been sufficiently discussed. Therefore, we analyzed the electric power transmission characteristics of the 100 kW class proposed WPT system using the HTS coil and the copper coil by an electromagnetic field analysis using the finite element method (FEM) and an equivalent circuit analysis. Also, we investigated the effect of a cooling temperature of the HTS coil on the receiving power and the transmission efficiency of the proposed WPT system for EVs. As a result, we found that the proposed WPT systems using the high-inductance primary and secondary coils are suitable for a low-frequency operation around 1 kHz. On the other hand, the proposed WPT systems with high-frequency operation above 10 kHz are suitable for reducing the weight of the secondary coil. In addition, although the system efficiency considering the cryocooler power loss is reduced by decreasing an operating temperature of the primary HTS coil, it is possible to realize a large-capacity WPT system with the receiving power of more than 100 kW.

DOI： 10.1109/tasc.2023.3247685

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

REBCO wires have high critical current density and thermal stability, and are being applied in various fields such as NMR, MRI, and accelerators. We are currently designing a 1.3-GHz LTS/HTS NMR magnet that operates in a persistent current (PC) mode. However, there are issues with using REBCO coils that need to be resolved. One problem unique to REBCO coils is non-uniform electromagnetic forces caused by screening currents. During the charging and discharging of the coil, screening currents are induced, resulting in a significantly non-uniform current distribution within the coil. As a result, non-uniform electromagnetic forces are generated within the coil due to these screening currents. In this study, we performed an electromagnetic stress analysis using a two-dimensional axisymmetric structural analysis that considered tension during winding, thermal stress during cooling, and electromagnetic force during charging. First, we compared the results of the numerical simulation of the layer-wound REBCO coil with the experimental results. Then, we conducted numerical simulations of an insert REBCO coil in a design example of a 1.3-GHz NMR to evaluate the stress and deformation of the REBCO coil.

DOI： 10.1109/tasc.2023.3259923

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

A wireless power transmission (WPT) system using high-temperature superconducting (HTS) coils has been investigated. However, it is difficult to precisely match the positions of the primary and secondary coils. Thus, the HTS coil structure with high misalignment tolerance is required. In this paper, we investigated the non-circular HTS coil structure with a high quality factor for the WPT system, such as a rectangular coil and a racetrack coil. Also, we evaluated the power transmission characteristics of the WPT system using the non-circular HTS coils. As a result, the quality factor of the non-circular HTS coils was maximum when the radial gap between turns was almost the same as the HTS wire width, and increased with increasing coil length in a rail longitudinal direction. Also, the receiving power and the system efficiency using the non-circular HTS coils were more robust against misalignment between coils than those using the circular HTS coils. On the other hand, the receiving power decreased with increasing the coil length of the non-circular HTS coils in the rail longitudinal direction due to thermal radiation from normal temperature. From the above, the receiving power of the WPT system with small lateral misalignments can be improved by using the rectangular coils with a short coil length in the rail longitudinal direction in parallel. Moreover, the system efficiency of the WPT system with large lateral misalignments can be improved by using the rectangular coils or the racetrack coils with a long coil length in the rail longitudinal direction.

DOI： 10.1109/tasc.2023.3252490

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

In recent years, Jelly-rolled Nb/Al composite monofilament Nb3Al superconducting wires with an outer diameter of 30 μm, and multi-stranded Nb3Al wires were successfully fabricated by NIMS in Japan. So, it is necessary to measure the temperature dependence of the critical current of Nb3Al wires in cryocooler system for development of superconducting applications cooled by conduction cooling method. The single and multi-stranded ultrafine Nb3Al superconducting sample wires with different outer diameter were prepared to evaluate the critical current properties. The critical currents of sample wires were evaluated by the current sweep method and constant current method. In this study, the sample wires were soldered to the copper wire to prevent burnout of the sample wire during critical current measurements. Therefore, the amount of current shunted to the copper wire was estimated to accurately evaluate the critical current of the superconducting wire. Also, the critical current of Nb3Al wires due to bending strain (bending radius of 5 to 15 mm) was experimentally investigated. From the critical current measurement results of Nb3Al wires with different cross-sectional structures, it was confirmed that the superconducting properties of the wires were improved by reducing the wire diameter. It was shown that the critical current density of the developed Nb3Al wire is lower than that of REBCO wire, but higher than that of commercial NbTi and Nb3Sn wires.

DOI： 10.1109/tasc.2023.3258911

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In this paper, we discuss two points: (a) the accuracy of contact resistance measurement using the low-frequency AC (LFAC) method and (b) the relation between the contact resistance and the contact resistivity. The LFAC method is used to measure the turn-to-turn contact resistance of a no-insulation (NI) rare-earth barium copper oxide (REBCO) pancake coil. We have previously shown the validity of the LFAC method by experiment. In this paper, the current distribution of aNI REBCO pancake coil on the applyication of the LFAC method is investigated using numerical simulation. The simulation results show the possibility of improving the measurement accuracy. Hence, a correction method is also proposed using the phase difference of the coil voltage and current. The LFAC simulation results also indicate the inconsistency of the well-known relation between contact resistance and contact resistivity. Therefore, we investigated the relation through the LFAC and sudden-discharging test simulations.

DOI： 10.1088/1361-6668/aca5b9

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2022.3149232

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2022.3171195

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2022.3175454

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

In recent years, an ultrafine Nb3Al wires with a diameter of less than 50 mu m has been developed by a research group at the National Institute for Materials Science (NIMS) in Japan. Therefore, we are planning to use the developed high performance Nb3Al wires for superconducting applications operated by cryocooler. In order to design the superconducting applications cooled by cryocooler, it is necessary to measure the temperature and magnetic field dependence of the critical current of Nb3Al wires under conduction cooling conditions. Single and seven-stranded Nb3Al wires with different diameters (phi 67 mu m and phi 81 mu m) fabricated by the Jelly-rolled method were prepared. The barrier material of the two samples is the same Nb, but the core materials are Nb (phi 67 mu m) and Ta (phi 81 mu m), respectively. The Cu/nonCu ratio of the two samples is 1. In this study, the critical currents of sample wires were measured by a four-probe method with current sweep and constant current, and they were compared to define a reasonable critical current value under conduction cooling. Under conduction cooling, it was confirmed that the critical current measurement by the constant current method was more accurate than the current sweep method.

DOI： 10.1109/tasc.2022.3156927

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We devised an HTS cable termination applying a wireless power transmission (WPT) system. The WPT system for the HTS cable termination can mechanically separate the HTS cable (low temperature part) from the copper cable (normal temperature part). In order to clarify the feasibility and design guidelines of the WPT system for the HTS cable termination, it is necessary to investigate the power transmission characteristics of the WPT system using the HTS and copper coils. In this study, we analyzed the power transmission characteristics of the simple WPT model system for the HTS cable termination by an equivalent circuit model and a finite element method analysis, and investigated the basic coil structure for the HTS cable termination applying the WPT system. As a result, the high-efficiency WPT system for the HTS cable termination can be realized by increasing the transport current of the HTS coil more than that of the copper coil. Also, we found that the HTS-Cu system using the primary-side HTS coil and the secondary-side copper coil is suitable for the HTS cable termination applying the WPT system.

DOI： 10.1109/tasc.2022.3167933

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We have been investigating a wireless power transmission (WPT) system using a high-temperature superconducting (HTS) coil for a railway vehicle. In the previous research, we investigated the conceptual design of the WPT system using the HTS coils for the railway vehicle. However, in order to suppress the AC loss per HTS coil below the cooling power of a cryocooler, it was necessary to install multiple HTS coils in parallel on the vehicle and ground sides. Therefore, we investigated the low-loss and high-energy density coil structure in the WPT system for the railway vehicle. In this study, we analyzed the AC loss and the coupling coefficient of various HTS coil structures using the narrow REBaCuO (RE: Rare Earth elements, REBCO) wires in parallel by finite element method (FEM) calculation, and evaluated the power transmission characteristics of the WPT system using the HTS coil structures using the narrow REBCO wires in parallel for the railway vehicle. As a result, we found that the AC loss of the HTS coil structure using the narrow REBCO wires in parallel was lower than that using the wide REBCO wire. The coupling coefficient between coils using a single pancake coil structure was higher than that using a double pancake coil structure. Also, the receiving power density of the WPT system was improved by the HTS coil structures using the narrow REBCO wires. Particularly, the receiving power density of the single pancake coil using the twisted narrow wire in parallel in the coil radial direction increased to about 1.5 times that of the single pancake coil using the wide wire.

DOI： 10.1109/tasc.2022.3169997

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

In recent years, a development of a high magnetic field magnet using a high temperature superconducting (HTS) wire has been advanced. On the other hand, we are planning to use the HTS wires for superconducting applications cooled by cryocooler. So, it is necessary to measure the temperature dependence of the magnetic properties of the metal substrates (Hastelloy, stainless steel and NiW) for HTS wire under conduction cooling conditions. We have been developing a single sheet tester (SST) that can evaluate the magnetic properties of electromagnetic steel sheets and other materials. However, since the size of an excitation coil in the conventional SST is large, it is difficult to install them in the conduction cooling system. In this study, we fabricated the compact SST and its measurement accuracy was compared with the conventional SST. The magnetic properties of metal substrates were measured at room temperature and liquid nitrogen temperature (77 K). As a result, although the measurement accuracy of the compact SST was slightly reduced by decreasing the coil size, we found that the error of the magnetic field homogeneity of the compact SST was within 2% at the center region of the 5 mm length. The magnetic properties of Hastelloy and stainless steel were almost unchanged at both room temperature and liquid nitrogen temperature. However, the magnetic properties of the NiW substrate changed significantly at liquid nitrogen temperature. The B-H characteristics of the NiW substrate in the rolling and transverse directions were clearly different, and the iron loss increased significantly at liquid nitrogen temperature.

DOI： 10.1109/tasc.2022.3159499

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We are developing a high-temperature superconducting (HTS) air-core compact cyclotron, named Skeleton Cyclotron (Ueda et al. 2013), (Ueda et al. 2019), that can accelerate various particles and variable energy for radioisotope (RI) production. The coil system of Skeleton Cyclotron consists of circular main coils and noncircular sector coils which are wound by REBa2Cu3O6+delta (REBCO) tape applied no-insulation (NI) winding technique for both high current densities and thermal stabilities. The electromagnetic and thermal behaviors in NI coil differs from those in conventional insulated coils during charging, discharging, and quenching. Therefore, it is necessary to investigate the mechanical behavior and reinforcement structure of multiple NI coils in Skeleton Cyclotron. In this study, we report on the numerical evaluation of mechanical properties of NI REBCO coils in Skeleton Cyclotron at charging and quenching considering the current distributions and electromagnetic stresses unique to NI coils. In the quenching process, the radial current in a quenching pancake of the sector coil became dominant, resulting in generating the rotating force and non-symmetric deformation. YOROI-coil (Y-based oxide superconductor and reinforcing outer integrated coil) structure is effective for the additional magnetic stress.

DOI： 10.1109/tasc.2022.3163693

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We have proposed a cyclotron for cancer therapy, called Skeleton Cyclotron, which is based on non-insulation (NI) high temperature superconducting (HTS) technology. The Skeleton Cyclotron consists of circular main coils for the isochronous field and non-circular sector coils for the azimuthally varying field (AVF). In order to establish the development technology of HTS multi-coil system for the skeleton cyclotron, we have now designed a small half-size multi-coil system for demonstration, called Ultra-Baby Cyclotron, at the operating current of 540 A, the operating temperature of 20 K and the magnetic field of 1.6 T at the extraction radius of 0.2 m. The NI HTS coil can make both high current density and high magnetic field, so it can achieve miniaturization of its size and high intensity beams. Furthermore, in NI coils, the current flows not only in the coil winding direction but also in the radial direction. Therefore, even if the local normal transition occurs in the coil windings, the current can bypass into the neighboring wires and thermal stability can be improved. On the other hand, when the normal transition and quench occur, the electromagnetic and mechanical behaviors in NI coils are different from those of conventional insulated coil. Therefore, we developed a current distribution analysis code for non-insulation HTS coils and simulated the current distributions during the normal transition and quench in NI REBCO coil system for Skeleton Cyclotron.

DOI： 10.1109/tasc.2022.3174811

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

In recent year, there has been remarkable progress in the use of high-field rare-earth barium copper oxide (REBCO) coils in NMR, MRI, and accelerator applications. No-insulation (NI) winding techniques are adopted to achieve high thermal stability and high current density. Thus, each winding in a non-impregnated NI REBCO coil can deform separately and move freely because of the thermal strain and electromagnetic force. The additional force and stress due to a screening current has also become an issue in REBCO coils. The screening current leads to a non-uniform force distribution in REBCO tapes and forces exceeding an acceptable value. It has been reported that REBCO coils have experienced mechanical deterioration or damage when operating under high magnetic fields. However, the detailed mechanism for this damage or deterioration has not been clarified. In order to clarify the detailed mechanism of this damage to a REBCO coil, the stress and strain during the winding, cooling down, charging, and discharging processes need to be evaluated. Furthermore, in NI REBCO coils, the turn-to-turn contact configurations are very important for the current bypassing, the thermal and mechanical stability. In previous studies, we have developed the numerical simulation code on the electromagnetic behavior of screening current and mechanical behavior of the turn-to-turn contact state. This study numerically evaluated the deformation and stress in REBCO tape and the turn-to-turn contact in NI REBCO coils.

DOI： 10.1109/tasc.2022.3171160

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

In recent years, high field REBCO coils have been applied with great interest to NMR and accelerators due to their high thermal stability and high current density. However, deteriorations and damages of REBCO coils are occurring. During the charge and discharge of REBCO coils, screening currents are induced to circulate in the windings, resulting in a non-uniform current distribution in the wires. It has been reported that the electromagnetic force distribution in the wire becomes significantly non-uniform, resulting in degradation of REBCO coils. In this study, the effects of stress and deformation of REBCO coils were investigated through electromagnetic stress analysis using two-dimensional axisymmetric structural analyses, taking into account thermal stress during cooling, tension during winding and electromagnetic force during not only charge and but also discharge, and the suppression of stress and deformation by the currently devised YOROI-coil (Y-based oxide superconductor and reinforcing outer integrated coil) structure and over-banding was evaluated.

DOI： 10.1109/tasc.2022.3177389

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

No-insulation (NI) winding technique provides high thermal stability, and the turn-to-turn contact resistance is an important factor to characterize the thermal stability and charging delay of NI HTS magnets. Therefore, a method that can accurately measure the contact resistance between turns of NI HTS magnets was required. We have previously proposed a turn-to-turn contact resistance measurement method using a low frequency AC current. The proposed low-frequency-AC-current (LFAC) can accurately evaluate the contact resistance between turns only when the whole AC current flows in the radial direction of the NI HTS pancake coil (there is almost no coil inductance). In addition, the AC current flowing in the radial direction needs to have a uniform current distribution in the plane of winding wire (REBCO wire). Therefore, in this study, an experimental study was conducted to prove the reliability of the proposed LFAC method. In the experiment, three NI test coils with different winding tension (0.5, 1 and 2 kg) were prepared, and four pickup coils were placed under each test coil to evaluate the radial current bypass characteristics. The contact resistance of each test coil was decreased from a few percent to several tens of percent by multiple cooling cycles from room temperature to liquid nitrogen temperature. Although it is a qualitative observation, the validity of the LFAC method was proved by the prepared pickup coils, and it was shown that even a small change of contact resistance in NI coils can be measured.

DOI： 10.1109/tasc.2022.3175791

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We proposed the air-core cyclotron using high-temperature superconducting (HTS) technology, named Skeleton Cyclotron, as high intensity compact, variable-energy and multi-particle cyclotron for RI production. Skeleton Cyclotron consists of HTS split circular main coils generating the isochronous field and HTS non-circular sector coils generating the azimuthally varying field (AVF). Now, we are currently developing the small-scale cryocooler-cooled HTS coil system of Skeleton Cyclotron, which accelerates the proton up to energy of 5 MeV at an extraction radius of 20 cm. Skeleton cyclotron is multi-coil system consists of HTS split circular main NI (no-insulation) coils and HTS non-circular sector NI coils. Therefore, the electromagnetic, thermal, and mechanical behaviors are expected to be very complicated. In this study, the structure analysis based on finite element method using COMSOL Multiphysics is performed for a non-circular sector coil. The coils in Ultra-Baby Cyclotron are reinforced with YOROI-coil structure. Therefore, the effect of YOROI-coil structure was numerically investigated during cool-down process and excitation process. YOROI-coil structure is effective for non-circular sector coil against the electromagnetic force.

DOI： 10.1109/tasc.2021.3072871

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We have been developing a new-generation accelerator named 'Skeleton Cyclotron', which is an ultra-compact and high-intensity accelerator for the stable production of 211At used for targeted alpha-particle therapy. Coil system for Skeleton Cyclotron consists of circular and noncircular high-temperature superconducting magnets and it can cause the output energy to vary without being bounded by the non-linearity of the iron core. This will enable not only the production of 211At, but also the production of radioisotopes for positron emission tomography and neutron irradiation for boron neutron capture therapy with a single accelerator. A high-precision magnetic field distribution and a high temporal stability are required for stable beam acceleration, but the screening current, which is induced in the REBa2Cu3Ox (REBCO, RE: Rare Earth) coated conductor, generates an irregular magnetic field and deteriorates the quality of the magnetic field spatially and temporally. In this study, we numerically evaluated the temporal stability and reproducibility of the screening current-induced field considering the variable energy operation for the Ultra-Baby Skeleton Cyclotron, which is a small demonstration model of Skeleton Cyclotron.

DOI： 10.1109/tasc.2021.3059725

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

A wireless power transmission (WPT) system using the high temperature superconducting (HTS) coil for a railway vehicle is required to transmit the electric power of several hundred kW in a short time (60 s). In order to design the WPT system using HTS coil with the thermal stability, and it is necessary to clarify the thermal characteristics of the HTS coil at the operating current in kHz band and the operating conditions considering thermal stability in the WPT system. We analyzed the frequency characteristics of the critical current in the HTS coil above 1 kHz and investigated the operating conditions considering thermal stability in the WPT system for the railway vehicle using HTS coil in liquid nitrogen. As a result, the critical current of the HTS coil in the kHz band decreased with the frequency. This is because the AC loss in the HTS coil increased with the frequency, and the critical current density of the HTS coil decreased with increasing the surface temperature. In order to design the HTS coil with thermal stability in the kHz band, and it is necessary to operate the HTS coil on alternating current less than about 50% of the critical current measured by direct current. We found that the thermally stable WPT system capable of fast charging for 60 seconds can be realized by operating current at the frequency of about 2 kHz.

DOI： 10.1109/tasc.2021.3059234

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

To achieve high-field rare-earth barium copper oxide (REBCO) coils, the mechanism of the deterioration of or damage to such coils needs to be clarified. A REBCO coil experiences various stresses and strains during the winding, cooling down, charging, and discharging processes. Furthermore, the additional force and stress due to a screening current has become an issue in REBCO coils. To protect REBCO coils from large electromagnetic stress in a high field, a reinforcement structure called the Y-based oxide superconductor and reinforcing outer integrated (YOROI) coil was proposed. This study numerically evaluated the effect of the YOROI coil structures against thermal and electromagnetic stress using a two-dimensional axisymmetric structural analysis. This simulation considered the following: 1) the mechanical stress induced by 10 N of winding tension, 2) thermal stress during cooling from 300 K to 10 K, and 3) electromagnetic stress, which included the additional stress due to a screening current during the charging process. The mechanical behavior due to each process was investigated, and the effect of the YOROI structures was also examined.

DOI： 10.1109/tasc.2021.3060702

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We have been developing a no-insulation (NI) REBCO coil which is expected to be applied to medical accelerators and magnetic resonance imaging as a technology that can achieve both high thermal stability and high current density. Due to the flat shape and much greater, than the thickness, width of a REBCO-coated conductor, the screening currents are remarkably induced by changing the magnetic flux that penetrates the tape vertically. For insulated (INS) coils, this screening current causes an irregular magnetic field (screening current-induced magnetic field (SCIF)). Also, in the NI coil, an excitation delay occurs because of the current flowing in the radial direction with coil excitation. Therefore, as the cause of irregular magnetic field, it is necessary to simultaneously consider the influence of both the excitation delay and SCIF. In this study, we numerically investigate the magnetic field in the NI coil. The SCIF and an excitation delay reduce magnetic accuracy in the NI coil. Thus, we propose an energization waveform that can improve the magnetic field accuracy even if there is an excitation delay. Besides, we verified its effectiveness.

DOI： 10.1109/tasc.2021.3064540

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We have been studying no-insulation (NI) high temperature superconducting (HTS) coils that do not use turn-to-turn electrical insulation. The no-insulation winding technique can provide high stability of the HTS coils against the quenching, but it cannot prevent mechanical deterioration of the HTS coil due to electromagnetic force and multiple cooling. To solve these problems, we propose the installation of metal protection ring on the outermost turn of NI HTS coil to improve thermal, mechanical, and electrical stability. In this study, the current bypass characteristics in the transverse direction within an NI test coil wound with 37 turns of REBCO wire with/without copper protection ring were experimentally investigated for various amounts of heating by a heater and various heater positions. Eight repeated cooling experiments with liquid nitrogen proved that the proposed copper protection ring was effective against shape deformation of the NI test coil and deterioration of contact resistance between turns due to thermal stress.

DOI： 10.1109/tasc.2021.3063654

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The intra-layer no-insulation (LNI) method is effective for protecting a layer-wound REBCO coil from a quench. However, a recently reported hoop stress modification (both increase and decrease) due to screening currents is a potential problem causing conductor degradation during charging. Epoxy impregnation can suppress the hoop stress modification and in this work, we develop prototypes of epoxy-impregnated LNI-REBCO coils to be charged at 77 K. Degradation of the coil performance due to cooling down can be prevented by mechanically isolating each layer. The coil shows a reasonably short field delay and current bypassing behavior, which are major technical benefits of the LNI-REBCO coil. A structural analysis with screening current-induced electromagnetic forces qualitatively suggests that the epoxy-impregnated LNI-REBCO coil can support a compressive hoop stress which causes conductor degradation due to buckling. The tensile hoop stress, however, is still very high at the upper and lower ends of each layer, causing fracture of a superconducting layer; i.e., additional reinforcement is required.

DOI： 10.1109/TASC.2021.3065875

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2021.3073634

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The characteristics of no-insulation high-temperature superconducting (HTS) manet is dominated by the magnitude of the turn-to-turn contact resistance. A few techniques have been proposed to increase the turn-to-turn contact resistance. The relation between the turn-to-turn contact resistance and the magnet stability has also been investigated in simulations. Although the turn-to-turn contact resistance measurement was only the sudden-discharging method before, we proposed a low-frequency-AC current (LFAC) method. Using our proposed method, it is possible to measure the turn-to-turn contact resistance under various conditions. In a previous paper, we measured it when charging DC current to a single pancake coil. In this paper, the turn-to-turn contact resistance was measured by the LFAC method when an external field of 1-3 T was applied to a single pancake coil. The coil strain was also measured, and the relation of the turn-to-turn contact resistance and the coil strain was also discussed.

DOI： 10.1109/TASC.2021.3070232

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Thecharacteristics of no-insulation high-temperature superconducting (HTS) manet is dominated by the magnitude of the turn-to-turn contact resistance. A few techniques have been proposed to increase the turn-to-turn contact resistance. The relation between the turn-to-turn contact resistance and the magnet stability has also been investigated in simulations. Although the turn-to-turn contact resistance measurement was only the sudden discharging method before, we proposed a low-frequency-AC current (LFAC) method. Using our proposed method, it is possible to measure the turn-to-turn contact resistance under various conditions. In a previous paper, we measured it when charging DC current to a single pancake coil. In this paper, the turn-to-turn contact resistance was measured by the LFAC method when an external field of 1-3 T was applied to a single pancake coil. The coil strain was also measured, and the relation of the turn-to-turn contact resistance and the coil strain was also discussed.

DOI： 10.1109/TASC.2021.3070232

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The no-insulation (NI) coil is a technology expected to realize both high current density and high thermal stability which are essentially trade-off relationship in REBCO coil application. This technique has been studied mainly for application to small-diameter inner coils of NMR magnets exceeding 30 T. In this case, the coil is cooled using 4.2-K liquid He. We are developing an NI-REBCO coil system for high-magnetic-field whole-body MRI, and for a medical cyclotron to be used for cancer therapy. The NI-REBCO coil which we aim for development has a diameter of similar to 1 m, a generated magnetic field is similar to 10 T, and a conduction cooling around 30 K is assumed. Because the size, operating temperature, and magnetic field differ from those of NMR coil, it is possible that the electromagnetic, thermal, and mechanical behaviors will be quite different. In this paper, we report on these behaviors when a local deterioration occurs in NI-REBCO double-pancake coils using numerical analysis considering coil size, operating temperature, and magnetic field as parameters. For the numerical simulation, we conducted a coupled analysis of current distribution analysis based on PEEC (Partial Element Equivalent Circuit) model and thermal analysis by the 2-dimensional finite element method.

DOI： 10.1109/TASC.2020.2974847

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

In this study, we propose the installation of multiple metal protection rings on the outermost turn of no-insulation (NI) coils to improve thermal, mechanical, and electrical stability. The current bypass characteristics in the transverse direction within an NI test coil wound with 10 turns of GdBCO wire and wrapped with 8 turns of Cu tape around the outermost layer were experimentally investigated for various amounts of heating by a heater and various heater positions. To confirm the electrical effectiveness of the metal protection rings, 8-turn Cu tape was used as the rings. A bypass current from the outermost part of the test coils to the Cu tape was observed. The local temperature increase due to thermal disturbance was suppressed.

DOI： 10.1109/tasc.2020.2976066

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

No-insulation (NI) (RE)Ba tex-math notation="LaTeX">$_3$ (REBCO) magnets are very promising for practical applications to generate high magnetic field. The NI winding technique provides high thermal stability so that quench protection is unnecessary. However, the turn-to-turn contact resistance (sometimes called characteristic resistance) is an important factor to characterize the thermal stability and charging delay of NI REBCO pancake magnets. Although the conventional sudden-discharging method is widely used to measure the turn-to-turn contact resistance, it is not applicable to various conditions; e.g., the change of DC operating current and temperature. Therefore, we have previously proposed a turn-to-turn contact resistance measurement method using a low-frequency AC (LFAC) current. In this paper, we measured the contact resistance for different DC operating currents or change of DC current to confirm the validity of LFAC method. In addition, by heating a test coil, the contact resistances were also measured during the change of DC operating current. Since the electromagnetic stress is small enough not to change the contact condition, the change in contact resistance is negligible. The measured contact resistance was unchanged, so we could conclude that the LFAC method is effective in measurement of the contact resistance when DC operating current was applied.

DOI： 10.1109/tasc.2020.2970381

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Authorship：Last author   Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

Biomedicine is indispensable for the treatment of various diseases such as cancer and diabetes mellitus. In particular, antibodies utilized by the immune system are believed to represent the future of medical supplies as they offer an effective treatment for diseases with minor side effects. The development of antibody drugs requires a large-scale, sequential, fast separation, and refinement process for the medical proteins. Thus, in the previous study, we proposed a medical protein screening system that is based on high-gradient magnetic separation using a superconducting magnet. In the proposed system, the protein is specifically attached to the magnetic beads and captured by the magnetic force around a magnetic filter generated by a high magnetic field. In this paper, the behaviour of the magnetic beads was investigated using particle trace simulation coupled with electromagnetic and fluid analyses for three types of commercially available magnetic filters. Simulations were performed using the filter stack intervals and number of filters as design parameters. Finally, we designed the suitable filter for capturing the magnetic beads.

DOI： 10.1088/1742-6596/1293/1/012081

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

© 2002-2011 IEEE. The turn-to-turn contact resistance (characteristic resistance) is a very important factor to a no-insulation (NI) (RE)Ba-2 Cu-3O-y (REBCO) pancake coil. It characterizes the stability and charging delay of NI REBCO pancake coils. However, the ordinary sudden-discharging method cannot measure the turn-to-turn contact resistance under different conditions, such as an operating current, a temperature, and an external magnetic field. Although the turn-to-turn contact condition is strongly affected by the pressure inside of the NI REBCO coils, the feature of the contact resistance must be clarified to estimate the stability of NI REBCO coils. In this paper, we have proposed a new method to measure the turn-to-turn contact resistance applying ac current. The theory and the measurement results are also shown. The measured turn-to-turn contact resistance is reasonable, compared with that measured by the ordinary sudden-discharging method. In near future, we will measure the turn-to-turn contact resistance under different conditions by means of the proposed method.

DOI： 10.1109/TASC.2019.2903643

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

© 2019 IOP Publishing Ltd. The development of NMR/MRI REBa 2 Cu 3 O y (REBCO, RE = rare earth) magnets is happening all over the world. It has been found that screening current-induced magnetic fields (SCMFs) are a serious problem for NMR/MRI magnets wound with REBCO tapes. The reduction of SCMFs is strongly desired, and the estimation of SCMFs is also desired at the design stage of REBCO magnets. In order to evaluate a SCMF, a finite element method (FEM) or a boundary integration method is needed thus far, and a high-level simulation technique is required. In this paper, we develop an easy method to estimate a SCMF without a high-level simulation technique. In this method, the inductance of a winding turn is calculated and then a screening current is estimated according to the magnetic field penetrating into the winding turn. The SCMF is obtained from the estimated screening current. The SCMFs computed by the proposed method were compared with measurements and simulation results of the FEM. The results agreed well, but we can see a large difference. However, the SCMFs found by the proposed method are accurate enough to know they are at the design stage without the use of a high-level simulation technique.

DOI： 10.1088/1361-6668/aafe26

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.2221/jcsj.54.119

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DOI： 10.2221/jcsj.54.90

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Authorship：Last author   Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP Publishing  

DOI： 10.1088/1742-6596/1054/1/012039

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Other Link： http://stacks.iop.org/1742-6596/1054/i=1/a=012039?key=crossref.94732ed5f8ebb67f3bccc61f3d01a080

Authorship：Last author   Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP Publishing  

DOI： 10.1088/1742-6596/1054/1/012085

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Other Link： http://stacks.iop.org/1742-6596/1054/i=1/a=012085?key=crossref.d830a6f3b705d0aca2baed2da5510a49

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

The compact NMR magnets consisted of the stacked higherature superconducting (HTS) GdBCO bulk annuli trapped by field cooling (FC) method have been developed. The target values of strength and homogeneity of magnetic field for proposed NMR relaxometry are 1.5 T and 150 ppm/cm3, respectively, at liquid nitrogen temperature. Since last year, we have been developing a hardware and software to detect the NMR signals by ourselves. Therefore, we needed to determine the final structure of HTS bulk magnet to be used in the NMR relaxometry. We can obtain the trapped magnetic field over 1.5 T at 77.4 K experimentally. However, it is still difficult to obtain the magnetic field homogeneity under 150 ppm/cm3 experimentally, due to their different superconducting properties. Therefore, in this study, four types of HTS bulk magnets to determine the final structure of compact NMR relaxometry were designed and their field properties including the degraded HTS bulk were investigated using three-dimensional finite-element method analysis. The field properties of 10-stacked HTS bulk magnets with the ring-shaped irons for field compensation and the degraded HTS bulk were investigated experimentally.

DOI： 10.1109/TASC.2017.2785260

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

The no-insulation (NI) winding technique to improve the transient stability of high-temperature superconducting (HTS) magnets has been studied and a cowinding method using a metallic tape was tested. Normally, the operating current in NI HTS coils is bypassed into the transverse direction due to the normal region generated within the coils. Therefore, a method that controls such current bypassing into the turn-to-turn direction of NI HTS coils was investigated as a means to regulate the effective number of turns. In this study, we experimentally investigated the characteristics of current bypassing in the transverse direction within a test coil wound with a GdBCO tape. First, to find an optimized heater location, a 20-turn HTS pancake with two partial insulation shapes was prepared and tested experimentally. The current bypassing characteristics in the normal region created by heaters (strain gauges) were studied while varying the number of heaters, heater positions, and the amount of heating at liquid nitrogen temperature. Using both short and long heating periods, we found that we could effectively control the number of turns (coil inductance) via long-duration heating.

DOI： 10.1109/TASC.2017.2787108

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

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.

DOI： 10.1109/TMAG.2017.2752083

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Physics Publishing  

High temperature superconducting (HTS) magnets wound with REBCO wires are used in many applications, and the superconducting magnet which is operated in persistent current mode has many advantages. Therefore, we have been developing compact NMR relaxometry devices using HTS coils operated at liquid nitrogen temperature. The required strength and homogeneity of magnetic field of proposed NMR relaxometry devices are 1.5 T and 150 ppm/cm3 respectively. The proposed HTS magnet for NMR relaxometry device consists with stacked HTS double pancake coils wound with REBCO wires and operated by persistent current mode (PCM) using superconducting joint between REBCO wires. In PCM operation, the ability of persistent current switch (PCS) is very important, so, in this study, the thermal properties of YBCO wire against the various thermal inputs by heater were investigated experimentally to design the PCS for PCM HTS magnets. The thermal behaviors of the YBCO wires were measured as functions of amount of heat input using two types of epoxy resin. The current bypassing properties on the YBCO loop coil with developed PCS were studied experimentally.

DOI： 10.1088/1742-6596/871/1/012087

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2017.2652918

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2017.2656858

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

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.

DOI： 10.1109/TASC.2017.2656381

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2017.2657636

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2016.2645839

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

High-temperature superconducting (HTS) magnets have several advantages for ac and pulse operation due to having a high critical temperature. However, fast magnetic field changes generate severe ac losses and eddy current losses. Therefore, loss estimation and reduction is a significant issue for HTS magnet cooling design. For future cooling design, we have measured losses of a Bi-2223 HTS magnet for beamline switching in the cyclotron facility at the Research Center of Nuclear Physics, Osaka University. The time-varying magnetic field losses for triangular wave excitation with a perpendicular magnetic field amplitude of 0.8 T in wire and a minimum period of 42 s were obtained from measured cryocooler temperatures and preliminary measured cooling capacity curve. The eddy current losses of the metal components were calculated using transient electromagnetic finite element analysis. The ac losses of the Bi-2223 wire were derived from the measured time-varying magnetic field losses and the calculated eddy current losses. The frequency dependence of their losses indicates the importance of accurate evaluation and reduction of the eddy current losses for the future magnet with higher charging speed.

DOI： 10.1109/TASC.2017.2653222

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

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.

DOI： 10.1109/CEFC.2016.7815982

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

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).

DOI： 10.1109/CEFC.2016.7816041

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

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.

DOI： 10.1088/0953-2048/29/10/104001

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Language：English   Publishing type：Research paper (international conference proceedings)  

DOI： 10.18429/JACoW-IPAC2016-TUPMB035

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We have developed a rapid beam emittance measurement system for the injection beam of the K140 azimuthally varying field (AVF) cyclotron at Research Center for Nuclear Physics (RCNP). So far, a conventional emittance monitor has been used in a section of a medium energy beam transport (MEBT) system to evaluate the quality of the injected beam to the K400 ring cyclotron. Two kinds of emittance monitors were supplemented in the low energy beam line for evaluation of ion beams from ion sources. One of them is a conventional type consisting of two sets of position-variable slits and a three-wire profile monitor (TPM), similar to the one installed in the MEBT system of the AVF cyclotron. It takes about 30 min to get emittances in both the horizontal and vertical planes. For quick emittance measurements, we have developed a new system equipped with a set of fast moving slits with a fixed gap and a real-time beam profile monitor (BPM83) with a rotating helical wire. With this system the measurement time was considerably reduced to 70 s for both the horizontal and vertical emittances. Moreover the data analysis and graphical processing were completely automated. The overall measurement and analysis time was successfully minimized within 75 s. This rapid emittance measurement system has contributed to improve the beam quality by optimizing parameters of ion sources and the beam transport system.

DOI： 10.1109/TNS.2016.2542865

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2016.2536736

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2016.2535863

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2016.2535966

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2016.2542980

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2016.2530301

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We have developed a Bi-2223 high-temperature superconducting dipole magnet for beam line switching at the cyclotron facility of the Research Center for Nuclear Physics, Osaka University. Exit beam lines are periodically switched by increasing and decreasing the magnetic field between 0 and 1.6 T with a switching time of 10 s. The magnet is equipped with two sets of Bi-2223 coil assemblies, which are conduction cooled by two 10-K Gifford-McMahon cryocoolers. We evaluated the superconducting property of the Bi-2223 coil assemblies in liquid nitrogen before installation in the cryostat. There were no degradation in wire performance during the assembly process. Magnetic field, strains, and temperatures of the coil assembly were investigated for performance verification after the magnet fabrication. The magnetic field and the temperatures meet specifications, and the deformation of the coil assembly is considered to be successfully suppressed by reinforcing structure. Furthermore, from a viewpoint of temperature, it is indicated that the magnet can make switching time and cycle time faster than that of the specified operation.

DOI： 10.1109/TASC.2016.2528543

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2016.2529841

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELSEVIER SCIENCE BV  

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.

DOI： 10.1016/j.phpro.2016.04.034

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.71.1.0_2664

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELSEVIER SCIENCE BV  

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/).

DOI： 10.1016/j.phpro.2015.05.122

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

A series of feasibility studies are carried out on the applications of a high-temperature superconducting (HTS) magnet system to a heavy-ion accelerator for particle cancer therapy. A novel HTS cyclotron is expected to be more compact and have higher efficiency than a conventional heavy ion synchrotron accelerator. A high current density is required for the HTS magnet to realize a compact HTS cyclotron. Further, a high-strength reinforcing structure is absolutely essential for the HTS magnet owing to the large Lorentz force caused by the high current density to prevent the magnet deformation and maintain the required high accuracy field. In a previous study, a novel coil structure called the Y-based oxide superconductor and reinforcing outer integrated "YOROI coil" was proposed and tested at 4.2 K in 8-T backup fields. The YOROI model coil exhibited no degradation after the excitation test with a maximum hoop stress of 1.7 GPa determined by the product of the magnetic field, current density, and coil radius. The mechanism of stress sharing between the coil winding and the reinforcing structures of the YOROI coil was numerically determined. In addition, the electromagnetic and mechanical properties of a reinforcing structure based on the YOROI coil for circular REBCO coils assuming part of the HTS cyclotron magnet system was also numerically simulated in the previous paper. In this study, a new reinforcing structure is proposed on the basis of the YOROI coil for a non-circular REBCO coil assuming part of the HTS cyclotron magnet system. The electromagnetic andmechanical properties for the new reinforcing structure are numerically determined. As a result, the proposed reinforcing structure 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.

DOI： 10.1109/TASC.2014.2365626

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

In REBCO coils, the screening current flows in the tape in response to the normal component of the linkage flux to the tape's wide face. Generally, the radial component of the magnetic field is large at both ends of the coil winding. Therefore, changes in the winding arrangement may be expected to reduce screening current induction. Furthermore, multifilamentary or narrow REBCO tapes with widths of a few millimeters can be also expected to reduce the magnitude of the screening current. Based on the above considerations, in this paper, we numerically investigated the reduction effect on a magnetic field generated by a screening current in a REBCO coil, due to thinning tape width and varying coil winding arrangements. It was found that the screening field is inversely proportional to the number of tape filaments, whereas a smaller coil winding aspect ratio leads to a decrease in the screening field.

DOI： 10.1109/TASC.2014.2375953

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

In a superconducting coil using a REBCO tape, a large screening current is induced by the magnetic field perpendicular to the tape. The irregular magnetic field generated by this screening current affects the magnetic-field distribution in high-field magnet applications, which require a magnetic field with high accuracy and temporal stability. Therefore, in these applications of the REBCO coil, we have to investigate and suppress the influence of the screening current in the REBCO tapes. In this study, we carried out experiments and numerical simulations focusing on the irregular magnetic field generated by the screening currents in multiple-REBCO-coil models assuming real operations in magnetic applications such as NMR/MRI system. The results of the numerical simulation are in good agreement with the behaviors of the screening field observed in the experimental results. In addition, we discussed the differences in the screening field for various excitation conditions using the numerical results of the current distribution in the coil.

DOI： 10.1109/TASC.2014.2371133

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

A high gradient magnetic separation system for medical protein using affinity magnetic nano-beads has been developed. Medical protein such as monoclonal antibody or immunoglobulin is an important substance as a medicine for cancer etc. However; the separation system of these medical protein has very low separation rate and the cost of product is extremely high. The developed system shows very high separation efficiency and can achieve low cost by large production rate compared to the system now using in this field. The system consists of a 3T superconducting magnet cooled by a cryo-cooler, a filter made of fine magnetic metal wires of about 30 mu m diameter and a demagnetization circuit and a liquid circulation pump for solvent containing medical protein. Affinity magnetic nano-beads is covered with the medical protein after agitation of solvent containing the protein and nano-beads, then the solvent flows through the system and the beads are trapped in the filters by high gradient magnetic field. The beads are released and flow out of the system by the AC demagnetization of the filters using LC resonance circuits after discharge of the magnet. The test results shows 97.8% of the magnetic nano-beads in pure water were captured and 94.1% of total beads were collected.

DOI： 10.1063/1.4860832

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2013.2297317

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2014.2299537

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We have successfully developed a high-gradient magnetic separation system for medical proteins using affinity magnetic nanobeads. Our system shows very high separation efficiency and can also be expected to realize lower cost due to larger production rate compared to the conventional system. The developed system consists of a 3-T superconducting magnet and a filter made of fine magnetic metal fibers. The superconducting magnet is wound with a NbTi twisted multifilamentary wire, and cooled by a 4-K Gifford-McMahon cryocooler. In order to achieve high recovery ratio of the magnetic nanobeads trapped on the filter located in a room-temperature clear bore of the cryostat, the ac degaussing system for the filter is fabricated using an inductance capacitance resonance circuit composed of a series connection with the superconducting magnet and an additional capacitor. To perform the inductance capacitance resonance more than a few cycles between superconducting magnet and capacitor, the superconducting magnet has a slit in the bobbin to prevent an eddy current coupled with an alternating magnetic field. It also has a control system for a high-speed switching circuit. This magnet can successfully generate a magnetic field of 3.0 T in the clear bore of the cryostat with a diameter of 30 mm in a relatively fast sweep time of 150 s due to the slit in the magnet bobbin. Using our degaussing system, a high recovery ratio of the nanobeads in pure water has been performed about 94.1%.

DOI： 10.1109/TASC.2013.2281364

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Medical proteins such as monoclonal antibodies and immunoglobulins are important substances for the manufacture of medicines for cancer, etc. However, the conventional separation system for these medical proteins has very low separation rate and the cost is extremely high. To address these issues, we have developed a high gradient magnetic separation system for medical proteins using affinity magnetic nanobeads. Our system shows very high separation efficiency and can achieve low cost owing to its large production rate compared with conventional systems. The system consists of a 3T superconducting magnet cooled by a cryocooler, a filter made of fine magnetic metal fibers of about 30 mu m in diameter with demagnetization circuit and liquid circulation pump for the solvent containing the medical proteins. Reducing the size of the system entails reduction of the cryocooler size, thereby resulting in reduced cooling capacity. Therefore, the heat load on the cryocooler has been considered carefully in the design of the cryogenic system. The calculated heat load of the 1st and 2nd stages was made to satisfy the cooling capacity of the cryocooler. As a result, a magnet temperature of 4.2 K and a thermal shield temperature of 60 K have been achieved, enabling smooth operation and good performance of the HGMS separation system.

DOI： 10.1109/TASC.2013.2281020

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

An 18 GHz superconducting electron cyclotron resonance ion source is installed to increase beam currents and to extend the variety of ions especially for highly charged heavy ions which can be accelerated by cyclotrons of Research Center for Nuclear Physics (RCNP), Osaka University. The beam production developments of several ions from B to Xe have been already done [T. Yorita, K. Hatanaka, M. Fukuda, M. Kibayashi, S. Morinobu, H. Okamura, and A. Tamii, Rev. Sci. Instrum. 79, 02A311 (2008) and T. Yorita, K. Hatanaka, M. Fukuda, M. Kibayashi, S. Morinobu, H. Okamura, and A. Tamii, Rev. Sci. Instrum. 81, 02A332 (2010)] and the further studies for those beam extraction and its transport have been done in order to increase the beam current more. The plasma electrode, extraction electrode, and einzel lens are modified. Especially extraction electrode can be applied minus voltage for the beam extraction and it works well to improve the extracted beam current. The extraction voltage dependences of transmission and emittance also have been studied for beam current improvement which is injected into azimuthally varying field cyclotron at RCNP. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4849675

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Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.2221/jcsj.49.25

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2012.2234813

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2012.2232955

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In recent years, properties of high temperature superconducting (HTS) tapes, especially in-field performance and mechanical strength, have been continuously improved. The HTS tapes have been widely used for high field (&gt
20 T) magnet researches and there are several technical challenges including field attenuation of an HTS magnet by screening currents induced within the HTS tapes. Several publications reported that the screening currents, induced by penetration of self magnetic fields into HTS tapes within an HTS magnet, weakened a field constant of the HTS magnet. The result may demonstrate that the screening current changes an overall current density distribution in HTS tapes and, as a consequence, the generated magnetic field. Therefore, it is necessary to investigate the screening current distribution in an HTS tape. This paper reports numerical simulation of the screening current distribution in an HTS tape of high field magnets using 2D finite element method with the E-J characteristic of the HTS tape taken into account. Self magnetic field distribution and its orientation to the HTS tape are also considered to compute critical currents and locally generated electric fields, two key components to figure out the distribution of screening currents.

DOI： 10.1016/j.physc.2012.03.040

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2011.2174578

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

As the upgrade program of the azimuthally varying field (AVF) cyclotron is at the cyclotron facility of the RCNP, Osaka University for the improvement of the quality, stability, and intensity of accelerated beams, an 18 GHz superconducting (SC) ECR ion source has been installed to increase beam currents and to extend the variety of ions, especially for highly charged heavy ions which can be accelerated by RCNP AVF cyclotron. The production development of several ions such as B, O, N, Ne, Ar, Ni, Kr, and Xe has been performed by Yorita et al. [Rev. Sci. instrum. 79, 02A311(2008); 81, 02A332 (2010)]. Further studies for the beam transport have been done in order to improve the beam current more for injection of cyclotron. The effect of field leakage of AVF main coil is not negligible and additional steering magnet has been installed and then beam transmission has been improved. The emittance monitor has also been developed for the purpose of investigating correlation between emittance of beam from ECR ion sources and injection efficiency. The monitor consists with :BPM82 with rotating wire for fast measurement for efficient study. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3671748]

DOI： 10.1063/1.3671748

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2010.2098383

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2010.2086413

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2010.2092396

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2010.2093491

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2010.2085413

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Publishing type：Research paper (international conference proceedings)  

An upgrade program for increase of beam intensity and improvement of beam quality is in progress at the cyclotron facility of Research Center for Nuclear Physics(RCNP). A 2.45 GHz ECR proton source was modified to increase 392 MeV proton beam intensity. Rated output power of a microwave power supply was increased from 200 W to 2 kW. A water-cooled waveguide system for high power microwave transfer is being installed for the proton source. A 15 keV proton beam with intensity of 0.6 mA was stably produced for output microwave power of 200 W. Emittance of 50 to 100 mm-mrad was obtained for 0.05 to 0.1 mA proton beams in the LEBT system. Beam transmission, defined by the ratio of the beam intensity between a Faraday cup placed in the axial injection beam line and an inflector electrode of the AVF cyclotron, was improved from 25 % for a 0.07 mA proton beam to more than 90 % for 0.03 mA, which indicated that the beam transmission was limited by the acceptance of the axial injection beam line. A new active gradient corrector is being designed for horizontal beam focusing in the extraction region of the AVF cyclotron and for reducing a beam loss at the vacuum chamber in the extraction region. An asymmetric dee voltage distribution at an acceleration gap is being analyzed for improvement of a flat-top acceleration system of the AVF cyclotron. Copyright © 2011 by IPAC'11/EPS-AG.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

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.

DOI： 10.1016/j.physc.2010.05.106

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

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.

DOI： 10.1016/j.physc.2010.05.165

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

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.

DOI： 10.1016/j.physc.2010.05.164

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

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.

DOI： 10.1016/j.physc.2010.05.166

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The possibility of the occurrence of quench caused by mechanical disturbance in high-temperature superconductors (HTS'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.

DOI： 10.1016/j.physc.2010.05.224

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2010.2040158

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2010.2041642

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

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

DOI： 10.1002/eej.20923

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

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

DOI： 10.1016/j.physc.2009.05.034

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2009.2018441

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2009.2017898

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2009.2018316

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2009.2018734

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2009.2017713

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1541/ieejpes.128.853

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2008.920610

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

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.

DOI： 10.1109/tasc.2007.899124

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/tasc.2007.899708

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/tasc.2007.898005

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J-GLOBAL

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

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.

DOI： 10.1109/tasc.2007.899966

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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 colls, 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. © 2007 IEEE.

DOI： 10.1109/tasc.2007.897998

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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. © 2007 IEEE.

DOI： 10.1109/tasc.2007.898113

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SCRIPTA TECHNICA-JOHN WILEY & SONS  

A magnetic levitation device with two-dimensional movement, the so-called "levitating X-Y transporter," has been developed. In order to develop a working levitating X-Y transporter, it is necessary to clarify the levitation characteristics, such as the lift force, levitation height, and stability against mechanical disturbances. In this paper, we examine the lift and the restoring force experimentally and propose a new simulation program based on the three-dimensional hybrid finite and boundary element method to analyze the dynamic behavior of electromagnetic characteristics of YBCO bulk. Using the numerical simulation and experiments, we investigated a suitable arrangement of permanent magnets to enhance the levitation characteristics. We also designed a levitating transporter which can carry a load of 200 kg with a gap of 16 mm. (C) 2007 Wiley Periodicals, Inc.

DOI： 10.1002/eej.20220

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DOI： 10.1109/TASC.2006.871310

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

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. © 2006 IEEE.

DOI： 10.1109/tasc.2006.871280

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We have been developing an X-Y (two-dimensional) magnetic levitating transporter, which is composed of the HTS bulks and permanent magnets. To accomplish the design of a real levitation system, we have to investigate the influence of permanent magnet arrangement on levitating characteristics. In this study, we prepared several types of permanent magnets and evaluated how magnetic flux distribution influences the levitating characteristics through lift and restoring force measurements and numerical simulation. For the simulation, we developed a computer program based on the 3D hybrid finite and boundary element (FE-BE) methods, taking into account the anisotropic and nonlinear I - V characteristics of HTS bulks. The simulation results agreed well with experiments. Therefore, we designed and constructed a levitating transporter model, which can levitate a load of 200 kg with a gap of 16 mm, using the developed computer program. © 2005 IEEE.

DOI： 10.1109/tasc.2005.850567

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It is expected that HTS bulk materials will be applied to various electric devices. We have fabricated and tested a linear synchronous actuator with double-sided primary and HTS bulk secondary. The linear actuator consists of a field-cooled YBCO bulk for secondary (mover) and copper windings with iron core as a primary. The primary, which was excited by a three-phase VVVF power source to generate sinusoidal traveling magnetic field, 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. As the secondary, two field-cooled bulks were arranged in certain interval. In this paper, for various intervals of bulks, we experimentally investigated the static characteristic in points of synchronous thrust force and the pull-in characteristics. And we developed a simulation program based on the finite element method (FEM) to investigate electromagnetic behavior within the bulk exposed to the magnetic field generated by the primary winding. We investigated the influence of the size, the number, and the interval of bulks on the static characteristic in points of synchronous thrust force by using the developed computer program. © 2005 IEEE.

DOI： 10.1109/tasc.2005.849619

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

Thermal behavior of high-temperature super-conducting (HTS) coil is exceedingly different from that of low-temperature superconducting (LTS) coil. Therefore for the sake of the application of HTS coil to practical use, establishment of a new stability criterion for coil design becomes indispensable. In this study, a small-sized cryocooler-cooled HTS coil wound with Bi-2223/Ag tape was constructed and tested to evaluate the electromagnetic and thermal characteristics in the cryocooler-cooled HTS coil. We carried out experiments subjected to pulse current operation supposing SMES for power system stabilization. The simulation by a newly developed computer program based on the finite element method (FEM) agreed well with experimental results. In the computer program, short-sample data concerning the E-J characteristics, which depend on the orientation of applied magnetic field to the wide face of Bi-2223Ag tape, was adopted. By using the computer program, we also evaluate the thermal behavior of HTS coil graded according to the magnetic field distribution. © 2005 IEEE.

DOI： 10.1109/tasc.2005.849321

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Authorship：Lead author,　Corresponding author   Language：Japanese  

DOI： 10.1541/ieejpes.124.1357

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Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical Engineers of Japan (IEE Japan)  

We have been developing a magnetic levitating device with two-dimensional movement, namely a ’levitating X-Y transporter’. For a real design of the levitating X-Y transporter, it is necessary to clarify the levitation characteristics, such as lift, levitation height and stability against mechanical disturbances. In this paper, we examine the lift and the restoring force experimentally and newly developed a simulation code based on the three-dimensional hybrid finite and boundary element method to analyze the dynamic electromagnetic behaviors of YBCO bulk. We investigated the suitable permanent-magnet arrangement to enhance the levitation characteristics through experiments and numerical simulation. And we also performed the design of a levitating transporter which can levitate a load of 200kg with a gap of 16mm.

DOI： 10.1541/ieejpes.124.1200

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

We have been investigating the electromagnetic behaviour of HTS bulks for realizing an X-Y (two-dimensional) magnetic levitating transporter without any fixed guides. The transporter is mainly composed of disc-shaped YBCO bulks and Nd-Fe-B permanent magnets or field-cooled YBCO bulks. The lower part and the upper part (levitating part) are magnetically connected. In such a 'levitating X-Y transporter', it is necessary to comprehend the characteristics, such as the lift, the levitation height and the stability against mechanical disturbances. In this paper, we evaluated the lift and the restoring force characteristics experimentally in the case where a field-cooled YBCO bulk is adopted instead of permanent magnets and also developed a numerical simulation program based on the finite element method to clarify the electromagnetic phenomenon in the HTS bulks.

DOI： 10.1088/0953-2048/17/8/017

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

The preliminary experiments of a tubular linear actuator in the vertical direction using YBCO bulk was studied. In active-maglev systems the leviation height was adjusted by varying the operating current through an electromagnet. Synchronous operations by a travelling magnetic field generated by using a three-phase aiternating current was achieved. The primary consists of six ring-shaped copper-winding coils without iron core and was supplied with three-phase sinusoidal excitation. The electromagnetic phenomenon within the bulk superconducted was also studied using a computer program based on the finite element method.

DOI： 10.1088/0953-2048/17/5/017

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

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.

DOI： 10.1088/0953-2048/17/5/016

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.2221/jcsj.39.560

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

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, can 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.

DOI： 10.1109/tasc.2003.813023

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SCRIPTA TECHNICA-JOHN WILEY & SONS  

We have designed and constructed a new type of active-maglev system using YBCO bulk and two electromagnets and have demonstrated that levitation height was remarkably improved by continuous levitation. In the active-maglev system, however, the upper and lower coils differed in the coil size and an iron plate beneath the lower coil was used to enhance the magnetic field around the bulk. Considering the real application to maglev transporter in the axial direction, we must realize continuous leviation using a larger number of electromagnets without such iron plate and enable the levitation height to be controlled freely. In this paper, we demonstrate continuous levitation while controlling the leviation height using three- and five-electromagnet systems and investigate electromagnetic behavior within the bulk using an FEM computer program. (C) 2002 Wiley Periodicals, Inc.

DOI： 10.1002/eej.10055

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2002.1018547

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1541/ieejpes1990.122.8_944

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

One useful feature of the active-maglev system is that levitation height is adjustable by varying operating current in electromagnets. To improve the levitation height, we have designed and constructed a new type of active-maglev system with two electromagnets instead of using a larger single electromagnet. The levitation height, as well as stability, was remarkably improved by adjusting the operating current of each electromagnet individually. Although the bulk is levitated up to high position, the lower electromagnet is continued to operate in continuous levitation using multiple electromagnets. It is waste that the lower electromagnet is continued to operate. The required energy for the electromagnet operation should be minimized. In this paper, we tried to realize continuous levitation with keeping the total operating current constant in five-electromagnet active-maglev system and investigate electromagnetic behavior within the bulk using a newly developed FEM computer program. Continuous levitation was successfully achieved under the desired operating condition. Agreements between experiment and analysis were excellent. (C) 2002 Published by Elsevier Science B.V.

DOI： 10.1016/S0921-4534(02)01050-X

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2002.1018528

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/TASC.2002.1018527

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/TASC.2002.1018548

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Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1541/ieejpes1990.121.10_1331

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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.

DOI： 10.1109/77.920135

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.2221/jcsj.36.68

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DOI： 10.2221/jcsj.35.246

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Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

DOI： 10.2221/jcsj.47.623

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