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

DOI： 10.1109/tasc.2023.3252490

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

DOI： 10.1109/tasc.2023.3247685

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

DOI： 10.1109/tasc.2023.3248519

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Publishing type：Research paper (scientific journal)  

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

DOI： 10.1109/tasc.2023.3258911

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

DOI： 10.1109/tasc.2023.3259923

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

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)  

DOI： 10.1109/tasc.2022.3163693

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

DOI： 10.1109/tasc.2022.3171160

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

DOI： 10.1109/tasc.2022.3174811

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

DOI： 10.1109/tasc.2022.3175791

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

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)  

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)  

DOI： 10.1109/tasc.2022.3167933

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

DOI： 10.1109/tasc.2022.3159499

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

DOI： 10.1109/tasc.2021.3059234

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

DOI： 10.1109/tasc.2021.3072871

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

DOI： 10.1109/tasc.2021.3063654

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

DOI： 10.1109/tasc.2021.3060702

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

DOI： 10.1088/1361-6668/abcd24

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Other Link： https://iopscience.iop.org/article/10.1088/1361-6668/abcd24/pdf

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

DOI： 10.1109/tasc.2021.3070232

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

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：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

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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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical Engineers of Japan (IEE Japan)  

Uniform magnetic field using superconducting bulk magnets has been developed and new research and practical targets associated with commercial market have appeared. The analysis and measurement of a uniform field with the bulk HTS are introduced and reviewed.

DOI： 10.1541/ieejpes.140.162

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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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DOI： 10.1109/TASC.2019.2903643

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

DOI： 10.1109/tasc.2019.2900557

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

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

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

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

The compact NMR magnets consisted of the stacked high-temperature 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/cm(3), 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/cm(3) 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 (IEEE)  

DOI： 10.1109/tasc.2017.2779491

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

DOI： 10.1109/tmag.2017.2752083

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

DOI： 10.1088/1742-6596/871/1/012104

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Other Link： http://stacks.iop.org/1742-6596/871/i=1/a=012104?key=crossref.c2102776e707d04829f0ccb8bde7ef80

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

DOI： 10.1088/1742-6596/871/1/012088

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Other Link： http://stacks.iop.org/1742-6596/871/i=1/a=012088?key=crossref.8bd24b0680d956e44eb2a009bfd93a97

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

DOI： 10.1088/1742-6596/871/1/012096

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Other Link： http://stacks.iop.org/1742-6596/871/i=1/a=012096?key=crossref.44063e95cb9417a2a5b8c06123cdf4e7

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

DOI： 10.1109/tasc.2016.2637937

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

DOI： 10.1109/tasc.2017.2655262

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Authorship：Lead author   Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Cryogenic Association of Japan  

DOI： 10.2221/jcsj.52.11

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

We have been studying the compact magnet for NMR device that consists of a stacked high-temperature superconducting (HTS) GdBCO bulk annuli trapped by a field cooling method. It was difficult to trap the uniform magnetic field above 4.7 T (200-MHz-class NMR magnet) and field homogeneity under 0.01 ppm/cm(3) at liquid nitrogen temperature (77.4 K) because of the low Jc-B characteristics of present HTS bulks. On the other hand, the strength and homogeneity of the magnetic field required for NMR relaxometry device are 1.5 T and 150 ppm/cm(3), respectively. Therefore, we have been investigating the development of the compact magnet for NMR relaxometry device. In our previous work, we obtained the trapped magnetic field over 1.5 T at 77.4 K using the stacked HTS bulk magnet with 80-mm height, and 150-ppm/cm(3) field homogeneity was obtained using the fabricated field compensation methods. However, the asymmetric problem of the magnetic field uniformity occurs by the different superconducting characteristics of HTS bulks when we produce the compact NMR magnet by stacking a large number of HTS bulks. Therefore, in this study, the field homogeneity of HTS bulk magnet by the arrangement of the degraded HTS bulk was investigated using 3-D FEM analysis. We examined the effects of the stacking position of degraded HTS bulk using various Jc-B characteristics.

DOI： 10.1109/TASC.2016.2521381

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

DOI： 10.1109/tasc.2016.2535665

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

DOI： 10.1109/tasc.2016.2518487

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

DOI： 10.1109/tasc.2016.2536440

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

DOI： 10.1109/tasc.2016.2518021

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

DOI： 10.1109/tasc.2016.2540162

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DOI： 10.1109/tasc.2014.2374337

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

DOI： 10.1109/tasc.2014.2383411

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

DOI： 10.1109/tasc.2014.2376992

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

DOI： 10.1109/tasc.2014.2379946

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

DOI： 10.1109/tasc.2014.2365549

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

DOI： 10.1109/tasc.2015.2393055

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

DOI： 10.1109/tasc.2014.2372896

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.phpro.2015.05.135

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MYU K.K.  

DOI： 10.18494/sam.2015.1145

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DOI： 10.1016/j.phpro.2015.05.088

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.phpro.2015.05.129

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.phpro.2015.05.138

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DOI： 10.1016/j.phpro.2015.05.141

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DOI： 10.1016/j.phpro.2015.05.132

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DOI： 10.1016/j.physc.2014.08.001

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

DOI： 10.1109/tasc.2013.2290591

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

DOI： 10.1109/tasc.2013.2283103

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

DOI： 10.1109/tasc.2013.2284021

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

DOI： 10.1109/tmag.2013.2276736

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.phpro.2014.09.073

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.phpro.2014.09.074

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.phpro.2014.09.072

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

Magnetic chromatography is an effective system for fine magnetic particle separation because of its strong magnetic field gradients. We have been developing the magnetic chromatography system to separate two or more kinds of fine magnetic particle with different sizes. To evaluate the performances of our developed system, we have also developed a simulation code with taking into account the fluid dynamics and magnetics. It is, however, difficult to consider two or more kinds of fine magnetic particles with different sizes in the simulation, because magnetic interferences between two or more kinds of magnetic particles need to be considered. Therefore, we have newly developed a simulation code to deal with magnetic interferences between two or more kinds of fine magnetic particles.

DOI： 10.1109/TMAG.2011.2174207

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/s0921-4534(02)00922-x

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

YBCO films on LaAlO3 and IBAD (YSZ/Hastelloy) substrate were made by TFA-MOD (metalorganic deposition using trifluoroacetate) process. Improved coating solution resulted in high Jc at 77.3K and 0 T of 7 MA/cm2 and 1.7 MA/cm2 for LaAlO3 and IBAD substrates, respectively. The magnetic field dependence of Jc at 77.3K also showed superior property: 105 A/cm2 at 4T (B//c-axis) for the LaAlO3 substrate and at 3T (B//c-axis) for IBAD substrate. Furthermore, the difference in Jc between single crystal LaAlO3 and IBAD was small up to 3T. The present results imply that TFA-MOD process is promising for the practical YBCO coated conductor process.

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Language：English   Publisher：Institute of Electrical and Electronics Engineers Inc.  

This paper proposes an advanced method to satisfy both the stability and charging/discharging time constant requirements of second generation high-temperature superconductor (2G HTS) magnets. This novel method entails installing metal–insulator transition (MIT) material between the turns of the 2G HTS magnets. The MIT material acts as an insulator when the temperature is lower than a certain value. Above this temperature, the MIT material becomes a conductor. This transition can be used as a switch between the turns of the 2G HTS magnets. We refer to this temperature-dependent smart switch as “smart insulation.” Considering the operating temperature, we selected V2 O3 as the material for smart insulation. We have experimentally verified that the advantages of both the insulation magnet (in the superconducting state) and no-insulation magnet (during quenching) can be simultaneously realized.

DOI： 10.1109/TASC.2017.2779491

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Language：English   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   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   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

This paper proposes an advanced method to satisfy both the stability and charging/discharging time constant requirements of second generation high-temperature superconductor (2G HTS) magnets. This novel method entails installing metal–insulator transition (MIT) material between the turns of the 2G HTS magnets. The MIT material acts as an insulator when the temperature is lower than a certain value. Above this temperature, the MIT material becomes a conductor. This transition can be used as a switch between the turns of the 2G HTS magnets. We refer to this temperature-dependent smart switch as “smart insulation.” Considering the operating temperature, we selected V2 O3 as the material for smart insulation. We have experimentally verified that the advantages of both the insulation magnet (in the superconducting state) and no-insulation magnet (during quenching) can be simultaneously realized.

DOI： 10.1109/tasc.2017.2779491

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Language：English   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

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   Publisher：Institute of Physics Publishing  

In previous study, non-contact rotating system consisting of the ring-shaped high temperature superconducting (HTS) bulks, ring-shaped permanent magnets (PMs) and stator coil was proposed. In this system, HTS bulks were magnetized by PMs and PMs were levitated with strong restoring force. In our previous study, we have constructed the rotating system with the ring-shaped HTS bulks with ID 20 mm, OD 60 mm, and 15-mm thickness. However, since these bulks costs too much, we switched to use HTS bulks with ID 20 mm, OD 60 mm, and 5-mm-thickness to miniaturize the system. However, this system have potential to fail the radial stability of rotating shaft. Therefore, we focused on the rotating and the radial restoring force in terms of the stability of the rotating shaft in the rotating system with 5-mm thickness HTS bulks.

DOI： 10.1088/1742-6596/871/1/012096

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Language：English   Publisher：Institute of Physics Publishing  

The magnetic drug delivery system (MDDS) is a key technology to reduce the side effects in the medical applications, and the magnetic force control is very important issue in MDDS. In this application, the strength of magnetic field and gradient required to MDDS devices are 54 mT and 5.5 T/m, respectively. We proposed the new magnetic force control system that consists of the multiple racetrack HTS magnets. We can control the magnetic field gradient along the longitudinal direction by the arrangement of the multiple racetrack HTS magnets and operating current of each magnet. When the racetrack HTS magnets were used, the critical current was reduced by the self-magnetic field. Therefore, the shape design of HTS magnet to reduce the magnet field into the surface of HTS tapes was required. Therefore, the electromagnetic analysis based on finite element method (FEM) was carried out to design and optimize the shape of multiple racetrack HTS magnet. We were able to suppress the reduction of critical current by placing the magnetic substance at upper and lower side of the HTS magnets. It was confirmed that obtained maximum values of magnetic field strength and field gradient were 33 mT and 0.18 T/m, respectively.

DOI： 10.1088/1742-6596/871/1/012104

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Language：English   Publisher：Institute of Physics Publishing  

The strength and homogeneity of the magnetic field required for NMR relaxometry device are 1.5 T and 150 ppm/cm3 respectively. It is relatively easy to generate the trapped magnetic field over 1.5 T at 77.4 K using the stacked HTS bulks, and 150 ppm/cm3 field homogeneity was obtained using the fabricated field compensation methods on HTS bulks magnet with 20 mm inner diameter. However, it is still hard to obtain 150 ppm/cm3 field homogeneity without field compensation methods. In this paper, in order to improve the trapped magnetic field homogeneity, the HTS bulk magnet with enlarged inner diameter was proposed and studied as the functions of size and shape of HTS bulk. The analytical study based on FEM was carried out to optimize the shape of HTS bulk magnet with enlarged inner diameter.

DOI： 10.1088/1742-6596/871/1/012088

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Language：English   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

This paper proposes a new method of achieving electrical stability and controllability of a high-temperature superconductor (HTS) coil. The proposed method applied a material with a metal-insulator transition (MIT) property to the turn-to-turn resistance of an HTS coil. As a result, the turn-to-turn resistance remained at the insulator level in the normal state, thus removing delay during charging and discharging. During transient state, the turn-to-turn resistance dropped to the metal level, and current bypass occurs similar to that in a noinsulation coil, thus preventing the coil from burning out. The validity of the MIT characteristic and the proposed method were verified through experiments.

DOI： 10.1109/tasc.2017.2655262

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Language：English   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Recently, multiple-stacked pancake coils wound with rare-earth barium copper oxide (REBCO) tapes are expected to apply for nuclear magnetic resonance (NMR)/MRI magnets. Since REBCO tapes are very expensive, REBCO magnets should be optimally designed so as to minimize the winding volume. Surely, a highly homogeneous magnetic field is also required for NMR/MRI magnets. To achieve the accurate homogeneity of REBCO magnets, it is necessary to compute the contribution of all REBCO layers one by one, because currents carry over very thin area compared with a whole magnet cross section. However, NMR/MRI magnets generating high-magnetic field usually consist of multiple-stacked REBCO pancake coils, and each pancake coil has > 300 turns. To evaluate the field homogeneity contributed by every REBCO layer, therefore, it is necessary to repeatedly compute more than ten thousand times as one magnet shape. In a conventional optimization algorithm, it is necessary to iterate more than one million times for the optimal design of an NMR or MRI magnet. Such an iterative computation is not realistic. Based on the backgrounds, we present a fast computation method using the shift operator of spherical harmonics. In addition, to confirm the validity of the proposed method, it was applied to the shape optimization of 1.5-T MRI magnet. In this paper, the optimization result and computation time are also shown.

DOI： 10.1109/TASC.2016.2637937

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DOI： 10.2221/jcsj.52.11

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DOI： 10.2221/jcsj.52.11

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Language：English   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We have been investigating three-dimensional actuators using bulk high-temperature superconductors (HTS) as transportation devices that move in free space. The large flux-pinning force of HTS allows motion without contact, even when the levitation force is not aligned against gravity, which could make them useful in clean or dangerous environments, such as clean rooms for semiconductor fabrication or in tunnels containing flammable gas. An actuator prototype, consisting of an HTS bulk with trapped magnetic field as a mover and a two-dimensional array of electromagnets as a stator, has been developed. In a previous study, the dynamic characteristics for traveling over floors were examined. In this study, horizontal, vertical, and oblique motions along vertical walls have been investigated. Movement characteristics have been studied for HTS bulks of two diameters and related to measurements of the axial components of their trapped magnetic fields. Maximum vertical displacements of 5.80 and 11.5 mm were achieved for HTS bulks of 60- and 46-mm outer diameters, respectively.

DOI： 10.1109/TASC.2016.2535665

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Language：English   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We have been studying the compact magnet for NMR device that consists of a stacked high-temperature superconducting (HTS) GdBCO bulk annuli trapped by a field cooling method. It was difficult to trap the uniform magnetic field above 4.7 T (200-MHz-class NMR magnet) and field homogeneity under 0.01 ppm/cm(3) at liquid nitrogen temperature (77.4 K) because of the low Jc-B characteristics of present HTS bulks. On the other hand, the strength and homogeneity of the magnetic field required for NMR relaxometry device are 1.5 T and 150 ppm/cm(3), respectively. Therefore, we have been investigating the development of the compact magnet for NMR relaxometry device. In our previous work, we obtained the trapped magnetic field over 1.5 T at 77.4 K using the stacked HTS bulk magnet with 80-mm height, and 150-ppm/cm(3) field homogeneity was obtained using the fabricated field compensation methods. However, the asymmetric problem of the magnetic field uniformity occurs by the different superconducting characteristics of HTS bulks when we produce the compact NMR magnet by stacking a large number of HTS bulks. Therefore, in this study, the field homogeneity of HTS bulk magnet by the arrangement of the degraded HTS bulk was investigated using 3-D FEM analysis. We examined the effects of the stacking position of degraded HTS bulk using various Jc-B characteristics.

DOI： 10.1109/TASC.2016.2521381

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Language：English   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We have studied a non-contact magnetic levitation system using ring-shaped high-temperature superconducting (HTS) bulks and ring-shaped permanent magnets (PMs). Because the HTS bulks were magnetized by PMs fixed between the HTS bulks, this PMs levitated with strong restoring force. We used a stator coil to rotate the iron shaft combined with PMs. In order to rotate the shaft stable, restoring the force of radial direction is very important. Therefore, the ring-shaped PMs magnetized in the radial direction were fitted to the shaft. In this rotating system, we focused on two parameters, i.e., the rotation and the control. In the rotation, we measured the maximum rotating speed and the axial vibration due to the rotating speed. The three types of stator coils were designed by electromagnetic analysis, and these stators were tested experimentally.

DOI： 10.1109/TASC.2016.2518021

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Language：English   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

High-temperature superconductor (HTS) bulks can trap high magnetic fields due to strong flux pinning. We have been investigating 3-D superconducting actuators using HTS bulks for transportation applications. The actuator consists of an HTS bulk as a mover and 2-D arranged electromagnets (EMs) as a stator. However, many EMs and power supplies would be needed to produce a 3-D actuator with a large moving area; therefore, a new type of 3-D actuator based on a 2-D array of permanent magnets (PMs) in combination with a small number of EMs was designed and tested. In this new type of actuator, the speed control of the HTS bulk mover is very important to develop a reliable noncontact transportation device. In this paper, the dynamic movement characteristics of the HTS bulk mover in the new type of actuator were experimentally investigated as a function of the gap distance between PMs and the drive current patterns.

DOI： 10.1109/TASC.2016.2536440

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Language：English   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The low temperature superconducting (LTS) coils are used in NMR devices, and liquid helium is used for their cooling. It is possible to reduce the operating costs by manufacturing the superconducting magnet that is consisted of the high temperature superconducting (HTS) coil using liquid nitrogen cooling. We have been developing the NMR relaxometry devices using the HTS double pancake coils and operated in the persistent current mode at liquid nitrogen temperature. The required strength and homogeneity of the magnetic field of proposed NMR relaxometry devices are 1.5 T and 150 ppm/cm(3), respectively. To achieve the target field strength of 1.5 T, it is necessary to increase the numbers of pancake coils and winding turns of each coil, and the large operating current is required as much as possible. In this paper, I-c - B properties of HTS wire and superconducting joint are measured to determine the operating current (load line) for the proposed NMR relaxometry devices. Moreover, 3-D finite-element-model-based analysis was carried out to design the stacked HTS magnets for compact NMR relaxometry with downsizing and low cost.

DOI： 10.1109/TASC.2016.2518487

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Language：English   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

This paper presents our analysis of the output characteristics of a high-temperature superconductor (HTS) coil depending on the turn-to-turn contact resistance. Important design factors were investigated by taking into account the turn-toturn contact resistance based on the analysis results. Analyses were performed on five self-fabricated coil models with different turn-to-turn contact resistances. To consider only the turn-to-turn contact resistance, we used a first-generation HTS BSCCO wire with simpler architecture and mechanical properties than the second-generation HTS wire. From this study, it was confirmed by experiment that as the turn-to-turn contact resistance decreases, the electrical stability increases but the output efficiency decreases compared with the input.

DOI： 10.1109/TASC.2016.2540162

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Language：English   Publisher：ELSEVIER SCIENCE BV  

We have been studying the compact magnet for NMR device that consists of a stacked high temperature superconducting (HTS) GdBCO bulk annuli trapped by a field cooling (FC) method. It was difficult to trap the uniform magnetic field above 4.7 T ( 200 MHz-class NMR magnet) and field homogeneity under 0.01 ppm/cm(3) at liquid nitrogen temperature (77.4 K) because of the low Jc-B characteristics of present HTS bulks. On the other hand, the strength and homogeneity of the magnetic field required for NMR relaxometry device are 1.5 T and 150 ppm/cm(3) respectively. Therefore, we have been investigating the development of the compact magnet for NMR relaxometry device. In our previous study, we obtained the trapped magnetic field over 1.5 T at 77.4 K using the stacked HTS bulk magnet with 80 mm height, and 150 ppm/cm(3) field homogeneity was obtained using the fabricated field compensation methods on inner diameter of 20 mm HTS bulk magnet. However, it is expected that the asymmetric problem of the magnetic field uniformity will be occurred by the crack in the HTS bulks with long-term operation. Therefore, in this study, the field homogeneity of HTS bulk magnet by the arrangement of the HTS bulk with crack was investigated using 3-D FEM analysis. We examined the effects of the stacking position of HTS bulk with the crack and the allowable range of degradation of HTS bulk to achieve the target field homogeneity of 150 ppm/cm(3). (C) 2016 The Authors. Published by Elsevier B.V.

DOI： 10.1016/j.phpro.2016.04.042

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Language：English   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).

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Language：English   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Compact NMR magnets consisted of the stacked HTS bulk annuli trapped by a field cooling (FC) method have been suggested and developed. The strength and homogeneity of the magnetic field required for the NMR relaxometry device are 1.5 T and 150 ppm/cm(3) respectively, and these values are much lower than the conventional NMR device. It is relatively easy to generate a magnetic field over 1.5 T at 77.4 K using the stacked HTS bulk annuli, but it is still hard to obtain 150 ppm/cm(3) field homogeneity with the conventional superconducting magnet (SCM) as the magnetizing magnets. In this paper, to improve the trapped magnetic field homogeneity and to obtain the enlarged sample space of HTS bulk magnet for compact NMR relaxometry, the HTS bulk magnet with 10 mm gap length in the center region of HTS bulk magnet (we call it " split coil-shaped HTS bulks") were proposed and studied as the functions of size and shape of HTS bulk. It was studied using 3D FEM based electromagnetic analysis. The improved field homogeneity was obtained by using hybrid models consisted with the HTS bulk and attached small size HTS coil.

DOI： 10.1109/TASC.2014.2374337

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Language：English   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The compact magnet for NMR device that consists of a stacked high temperature superconducting (HTS) Gd-Ba-Cu-O bulk annuli was trapped by a field cooling method. However, it was difficult to trap the uniform magnetic field above 4.7 T (200 MHz-class NMR magnet) and field homogeneity under 0.01 ppm/cm3 at liquid nitrogen temperature (77.4 K) because of the low Jc-B characteristics of present HTS bulks. On the other hand, the strength and homogeneity of the magnetic field required for NMR relaxometry device are 1.5 T and 150 ppm/cm3 respectively. It is easily to generate the trapped magnetic field over 1.5 T at 77.4 K using the stacked HTS bulks, but it is still hard to obtain 150 ppm/cm3 field homogeneity with the conventional superconducting magnet (SCM) as the magnetizing magnets. Therefore, we have studied the compensation methods to improve the field homogeneity. In this paper, we proposed the packaged HTS bulk magnet consisted of HTS bulks annuli and various iron rings as the field compensation, and investigated it in analysis and experiment.

DOI： 10.1109/TASC.2014.2372896

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Language：English   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

High-temperature superconductor (HTS) material can trap high magnetic fields due to its strong flux pinning force. Herein, based on our belief that such a device would be useful as a transporter in clean rooms where silicon wafers are manufactured, we report on the development of a three-dimensional (3-D) superconducting actuator using HTS bulk material in a noncontact transportation device that will move freely in 3-D space. The proposed actuator consists of a trapped HTS, which provides the mover, along with two-dimensionally arranged copper-wound iron-core electromagnets that form the stator. The current and the polarity of each electromagnet can be individually controlled via the power supply. In our previous experiments, a singlecomponent mover was observed to overshoot during operation, which led us to conclude that a more powerful levitation force was needed. In this study, single-component HTS movers (inner diameters of 46 and 60 mm) and multicomponent HTS movers were used to increase the levitation force and dynamic stability.

DOI： 10.1109/TASC.2015.2393055

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Language：English   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

In this study, a method for controlling the magnetomotive force using the current bypass of no-insulation magnets was developed, and the characteristics of 1G high-temperature superconductivity (HTS) no-insulation magnets were elucidated through experiments. The experiments were performed using a magnet designed with bismuth strontium calcium copper oxide HTS wires. Film-type heaters were used to control the MMF. The analysis of the experimental results was carried out in conjunction with an analysis using finite elements method.

DOI： 10.1109/TASC.2014.2365549

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High magnetic fields are used in many branches of modern medicine. So, the techniques for magnetic field concentration are very important, and the novel electromagnets and trapped HTS bulk magnets have been fabricated and used in the cell culture and the bulk magnetic drug delivery system. In general, although superconducting magnet can generate high magnetic fields, ON/OFF magnetic switching is impossible because of its large inductance. Therefore, not only the generating of magnetic field but also controlling of magnetic field including ON/OFF switching are very important issues. So, we have been studied the control methods of magnetic field using HTS bulks. In this study, the magnetic field control system consisted of the various shaped HTS bulks and magnetic sources were proposed to achieve the ON/OFF field control and field amplification. The shape of HTS bulk to develop the magnetic control system was optimized by FEM based on analysis, and the abilities of its field switching and amplification were confirmed by experiments. We found that the HTS bulk with slit did control the applied magnetic field, and amplified magnetic fields were obtained at slit parts of HTS bulks.

DOI： 10.1109/TASC.2014.2379946

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Language：English   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Magnetic drug delivery system (MDDS) is a key technology effects in the medical applications, and also the magnetic force control is a very important issue in MDDS. In general, the high magnetic field along the axial direction and high magnetic field gradient along the longitudinal direction are very useful for MDDS. So, we proposed the new magnetic force control system consists of superconducting magnet, high temperature superconductors (HTS) bulks or tapes and irons. In this system, the shielding currents in HTS bulks and tapes due to the diamagnetism of the superconductor generate the magnetic field with same direction of external magnetic field, and we can control the magnetic field gradient along the longitudinal direction by the arrangement of the superconductors and irons. In this study, the stacked GdBCO tapes were used in experiments and electromagnetic analysis based on finite-element method was carried out to optimize the arrangement of the superconductors and the irons. It was confirmed the control of magnetic field gradient along the longitudinal direction was possible by arranging the HTS bulks, tapes and irons on the superconducting magnet. In analysis, the magnetic field of 164.6 mT and field gradient of 1.89 T/m were obtained.

DOI： 10.1109/TASC.2014.2376992

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Language：English   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We have studied a noncontact magnetic levitation system using the ring-shaped high-temperature superconducting (HTS) bulks and ring-shaped permanent magnets (PMs). Because HTS bulks were magnetized by PMs fixed between the HTS bulks, these PMs levitated with strong restoring force. We used the stator coil to rotate the iron shaft that was combined with PMs. In order to rotate the shaft stable, a restoring force of the radial direction was important. Thus, we placed ring-shaped PMs that magnetized in the radial direction with the inner part of the HTS bulks. In this rotating system, we focused on two parameters, the rotation and the control. In the rotation, we measured the maximum rotational speed and the axial vibration caused by increasing of rotational speed. In the control, we measured slippage from the convergence position when rotated by 60 degrees. In addition, we compared the slippage by changing the conditions.

DOI： 10.1109/TASC.2014.2383411

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Language：English   Publisher：ELSEVIER SCIENCE BV  

We develop the compact NMR/MRI device using high temperature superconducting (HTS) wires with the persistent current mode operating. So, the joint techniques between 2G wires are very important issue and many studies have been carried out. Recently, the K.JOINS, Inc. has developed successfully the high performance superconducting joints between 2G wires by partial melting diffusion and oxygenation annealing process [1]. In this study, the current bypassing properties in a loop-shaped 2G wire are measured experimentally to develop the permanent current switch (PSC). The current bypassing properties of loop-shaped test coil wound with 2G wire (GdBCO) are evaluated by measured the self-magnetic field due to bypassed current by Hall sensors. The strain gauge was used as heater for persistent current switch, and thermal properties against various thermal inputs were investigated experimentally. (C) 2015 The Authors. 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.2015.05.088

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Language：English   Publisher：MYU, SCIENTIFIC PUBLISHING DIVISION  

Flexible electrodes for use in recording neural activity were fabricated in this study, and their use was demonstrated. By using the mechanical force generated by a magnetic field, an electrode can be inserted into a tissue, and the position of the electrode can be controlled. The position and insertion path of the electrode can be controlled accurately with a high degree of freedom. No stiffener materials are required for the insertion process. A headstage amplifier, which is indispensable for low-noise recording, was hybrid-integrated with a flexible electrode. A miniaturized main amplifier that served as a second-stage amplifier was also fabricated. The magnetic controllability of the fabricated electrode was demonstrated using a phantom of a tissue. An in vivo experiment was also performed for the functional validation of the recording function. Local field potentials were recorded successfully as a result of the experiment.

DOI： 10.18494/SAM.2015.1145

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The magnetic drug delivery system (MDDS) is a key technology to reduce the side effects in the medical applications. We proposed the new magnetic force control system that consists of superconducting magnet, high temperature superconductors (HTS) tapes and iron blocks. In this study, it was confirmed that the control of magnetic field gradient was possible by arranging the HTS tapes and iron blocks on the superconducting magnet. In addition, the multiple racetrack HTS magnets were proposed in order to generate the high field gradient along the longitudinal direction. (C) 2015 The Authors. 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.2015.05.132

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The electric device applications of a high temperature superconducting (HTS) bulk magnet having stable levitation and suspension properties due to their strong flux pinning force have been proposed and developed. We have been investigating the three-dimensional (3-D) superconducting actuator using HTS bulk to develop a non-contact transportation device.
Probably, the cost of the manufactory will be increased to install the 2-D arranged electromagnets (EM) in a large area because many EMs are needed to cover the area. Therefore, we have been trying to find the method for reducing the number of EMs. In this study, all the EMs except for rotation were replaced in the 2-D arranged permanent magnets (PM), and gap length between PMs were experimentally investigated to improve the dynamic behavior of the mover and to reduce the cost of the manufacturing. As a result, we have succeeded in conveyance of the bulk and reduce the convergence time and maximum overshoot. (C) 2015 The Authors. Published by Elsevier Ltd. 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.2015.05.141

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The electric device applications of a high temperature superconducting (HTS) bulk having stable levitation and suspension properties due to their strong flux pinning force have been proposed and developed. We have been investigating the three-dimensional (3-D) superconducting actuator using HTS bulk to develop the transportation device with non-contact and moves in free space. It is expected that our proposed 3-D superconducting actuator to be useful as a transporter used in clean room which manufactures the silicon wafer where dislikes mechanical contact and dust. Proposed the actuator consists of a field-cooled HTS bulk for mover and two-dimensional arranged multiple electromagnets as a stator. In our previous study, the dynamic characteristics on the floor traveling of the HTS bulk mover had been studied. Therefore in this study, a system for the wall traveling was proposed to use the limited space effectively. It is expected that the wall traveling system is very useful to apply the transporter used in the tunnels which flammable gas may be produced. In this paper, the optimal angle between electromagnets located at floor and wall for moving to the wall traveling from the floor traveling was investigated experimentally. The position displacement on the rotating of the HTS bulk during the wall traveling was measured. As a result, the bulk with initial gap of 2 mm could move to the wall traveling from the floor traveling. Also, the position displacement on the height direction during the rotating of the HTS bulk was 18 mm from the initial position because of its gravity, but it was possible to rotate by changing the pattern NSNS of the trapped magnetic field in near the initial position. (C) 2015 The Authors. Published by Elsevier Ltd. 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.2015.05.138

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In the case of motors and generators, the benefits of using high temperature superconducting (HTS) coils can be represented by the reduction of 50% in both losses and sizes compared to conventional machines. However, it is hard to establish quench detection and protection devices for the HTS coils applied to the rotors of motors and generators. So, the stability of the coils is lower than for the quiescent coils applied to NMR, MRI and so on. Therefore, it is important to improve the self-protection ability of HTS coils. We have studied the methods to improve the self-protection ability of HTS coils by removing the layer-to-layer insulation and inserting metal tape instead of the electrical insulation. The operating current in the non-insulated HTS coil was bypassed into the transverse direction by the generated normal region because of their electrical contact among the winding. In this study, we examined the method to control the current bypassing on layer-to-layer for controlling the inductance of the non-insulated HTS coil. The current bypassing properties on non-insulated HTS coil wound with 2G wires will be discussed. (C) 2015 The Authors. 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.2015.05.129

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We have been studying the compact magnet for NMR device that consists of a stacked high temperature superconducting (HTS) GdBCO bulk annuli. We can generate the trapped magnetic field over 1.5 T at 77.4 K and 150 ppm/cm(3) on inner diameter of 20 mm HTS bulks using field compensation methods. However, it is necessary to enlarge the inner diameter of the HTS bulk magnet because the diameter of commercial NMR probe is larger than 20 mm. In this paper, we studied an optimal shape of the stacked HTS bulk magnet to obtain the enlarged inner diameter using 3-D FEM based analysis. We was able to enlarge the inner diameter of the HTS bulk magnet from 20 mm to 34 mm remaining magnetic field strength of 1.5 T and magnetic field homogeneity of 666 ppm/cm(3) by proposed passive field compensation method. (C) 2015 The Authors. Published by Elsevier Ltd. 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.2015.05.135

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The magnetic drug delivery system (MDDS) is a key technology to reduce the side effects in the medical applications, and the magnetic force control is a very important issue in MDDS. In general, the high magnetic field along the axial direction and high magnetic field gradient along the longitudinal direction are very useful for MDDS. So, we proposed the new magnetic force control system that consists of superconducting magnet, high temperature superconductors (HTS) bulks or tapes and ferromagnetic substances. In this new system, the shielding currents in HTS bulks and tapes due to diamagnetism of the superconductor generate the magnetic field with same direction of external magnetic field, and we can control the magnetic field gradient along the longitudinal direction by the arrangement of the HTS bulks, tapes and ferromagnetic substances. In this study, the stacked GdBCO tapes without stabilizer were used in experiments and electromagnetic analysis based on finite element method (FEM) was carried out to optimize the arrangement of the superconductors and the irons. It was confirmed that the control of magnetic field gradient along the longitudinal direction was possible by arranging the HTS bulks, tapes and irons on the superconducting magnet and then it was possible to obtain a magnetic field higher than the external field. Crown Copyright (C) 2014 Published by Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2014.08.001

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The compact NMR magnets consisting of stacked HTS bulk annuli trapped by a field cooling (FC) method have been developed. However, it is difficult to trap a uniform magnetic field above 4.7 T (200 MHz-class NMR magnet) and field homogeneity under 0.01 ppm/cm(3) at liquid nitrogen temperature (77.4 K) because of the low Jc-B characteristics of present HTS bulk magnets. On the other hand, the strength and homogeneity of the magnetic field required for the NMR relaxometry device are 1.5 T and 150 pprn/cm(3) respectively; these values are much lower than the conventional NMR device. It is relatively easy to generate a magnetic field over 1.5 T at 77.4 K using the stacked HTS bulk annuli, but it is still hard to obtain 150 pprn/cm(3) field homogeneity with the conventional superconducting magnet (SCM) as the magnetizing magnets. In this paper, we proposed that the homogeneity had been improved by utilizing the remagnetization process.

DOI： 10.1109/TASC.2013.2290591

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In this study, the noncontact magnetic levitation system using combined high temperature superconducting (HTS) hulk annuli and ring-shaped permanent magnet (PM) was proposed. In the suggested levitation system, the HTS bulk annuli were magnetized by ring-shaped PMs fixed between the HTS bulk annuli, so there was no more need to apply source for magnetization. In this paper, the levitation forces on the radial and axial directions were measured using 1 PM + 2 bulks (1 unit) and 2 PM + 3 bulks systems to study the optimization of levitation system. To improve the levitation property of proposed levitation system, the shape of PM and materials of the rotor shaft were studied experimentally.

DOI： 10.1109/TASC.2013.2284021

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In the case of motors and generators, the benefits of using high-temperature superconducting (HTS) coils can be represented by the reduction of 50% in both losses and sizes compared to conventional machines. However, it is hard to establish quench detection and protection devices for the HTS coils applied to the rotors of motors and generators. Therefore, the stability of the coils is lower than for the quiescent coils applied to NMR, MRI, and so on. Therefore, it is important to improve the self-protection ability of HTS coils. We have studied the methods to improve the self-protection ability of HTS coils by removing the turn-to-turn insulation and inserting metal tape instead of the electrical insulation. When the coils are energized, there are voltages generated by sweep rate of the current (di/dt) and inductance of the coil. Because noninsulated HTS coils have electrical contacts along the transverse direction, a bypass current is generated along the transverse direction and noninsulated HTS coils have a risk of Joule heating in the stabilizer and metal substrate. Furthermore, the electrical contact resistance of noninsulated HTS coils was changed by a winding torque. In this paper, we measured the electrical contact resistance along the transverse direction with various winding torques for noninsulated HTS coils.

DOI： 10.1109/TASC.2013.2283103

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When a magnet generating highly homogeneous magnetic field is designed, a shimming is required. The shimming is usually performed so that the magnetic field only in the axis direction is compensated by iron pieces and/or coils. The compensation is commonly achieved by eliminating the coefficients of the spherical harmonics expansion of the magnetic field generated by the main magnet. Some papers showed the coefficients of the spherical harmonics expansion in only the z-direction for passive shimming. However, recently some magnets generate a magnetic field tilted from the z-axis, such as a magic-angle-spinning nuclear magnetic resonance/MRI. It is, therefore, necessary to eliminate the coefficients of the spherical harmonics expansion in the x- and y-direction.

DOI： 10.1109/TMAG.2013.2276736

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We have been developing a new compact NMR magnet using stacked HTS bulks. In this paper, in order to improve the trapped magnetic field homogeneity and to obtain the enlarged sample space of HTS bulk magnet for compact NMR relaxometry, the HTS bulk magnet with 10 mm gap length in the center region of HTS bulk magnet (we call it split coil-shaped THS bulks) were proposed and studied as the functions of size and shape of HTS bulk using 3D FEM based electromagnetic analysis. The improved field homogeneity was obtained using notch coil shaped HTS bulk configuration and the field compensation by attached the coil instated of the bulks. (C) 2014 The Authors.Published by Elsevier B.V.

DOI： 10.1016/j.phpro.2014.09.072

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In this paper, the compensation method to obtain the high field homogeneity (150 ppm/cm(3)) of compact NMR magnet was studied analytically. In order to improve the trapped magnetic field homogeneity of HTS bulk magnet, we used the packaged HTS bulk magnet consisted of HTS bulks annuli and 3 iron rings for field compensation. We could confirm that the target homogeneity of trapped magnetic field of HTS bulk magnet was achieved by the proposed passive compensation method. (C) 2014 The Authors. Published by Elsevier B.V.

DOI： 10.1016/j.phpro.2014.09.074

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The electric device applications of a high temperature superconducting (HTS) bulk, having stable levitation and suspension properties due to their strong flux pinning force, have been proposed and developed. We have been investigating the three-dimensional (3-D) superconducting actuator using HTS bulk to develop a non-contact transportation device. In our previous works, the overshooting of the HTS bulk mover magnetized by 2-D arranged electromagnets were observed. Therefore, the new magnetization method using sandwiched electromagnets was purposed in order to solve the overshooting problems of HTS bulk mover. In this paper, the dynamic stabilities on the rotating and horizontal moving of the HTS bulk mover magnetized by sandwiched electromagnets was investigated experimentally, and the maximum moving displacements and convergence times during the overshooting state were reduced by proposed magnetization method with small magnetizing current. (C) 2014 The Authors. Published by Elsevier B.V.

DOI： 10.1016/j.phpro.2014.09.073

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We have been investigating the levitation system without any mechanical contact which is composed of a field-cooled ring-shaped high temperature superconducting (HTS) bulks [1]. In this proposed levitation system, the trapped magnetic field distributions of stacked HTS bulk are very important. In this paper, the spherical solenoid magnet composed of seven solenoid coils with different inner and outer diameters was designed and fabricated as a new magnetic source. The fabricated spherical solenoid magnet can easily make a homogeneous and various magnetic field distributions in inner space of stacked HTS bulk annuli by controlling the emerging currents of each coil. By using this spherical solenoid magnet, we tried to make a large magnetic field gradient in inner space of HTS bulk annuli, and it is very important on the levitation of magnetic substances. In order to improve the levitation properties of magnetic substances with various sizes, the external fields were reapplied to the initially trapped HTS bulk magnets. We could generate a large magnetic field gradient along the axial direction in inner space of HTS bulk annuli, and obtain the improved levitation height of samples by the proposed reapplied field method. (C) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2013.05.020

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In designing and manufacturing of a new type of compact nuclear magnetic resonance/magnetic resonance imaging magnet, which consists of a stacked high-temperature superconducting (HTS) bulk annuli, the spatial homogeneity and the temporal stability of the trapped magnetic fields are the key issues. On the other hand, there is an asymmetry problem for superconducting properties including the critical current density along the axial (thickness) direction in HTS bulks due to its manufacturing process using a seed crystal located at the top surface. Therefore, in this study, we find the nonuniformity of the critical current density of the HTS bulk along the axial direction, which probably comes from the quench-melt-growth process using a seed crystal. The measured axial positions of the maximum magnetic field were slightly shifted to the bottom direction lower than the axial center position. The critical current density and/or superconducting properties of HTS bulks are probably decreased in proportion to the distance from a seed crystal. Therefore, the superconducting properties along the thickness direction in the HTS bulk were represented as a variable J(c) - B and electrical conductivity.

DOI： 10.1109/TASC.2013.2250331

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In the case of motors and generators, the benefits of using high-temperature superconducting (HTS) coils could be represented by the reduction of 50% in both losses and sizes compared to the conventional machines of the same rating. However, it is hard to establish the quench detection and protection devices for the HTS coils applied to the rotors of motors and generators. Therefore, the stability of the coils is lower than the quiescent coils applied to NMR, MRI, and so on. Therefore, it is important to improve the self-protection ability of HTS coils. We have been developing a novel self-protection method, which removes the insulation turn-turn winding and inserts various metal tapes instead of the insulation. In our previous research, the higher self-protection ability for the thermal disturbances in the HTS coils removed the insulation and inserted Cu tape has been proved. This paper presents the results of an experimental study for the quench current using the insulated, the noninsulated, and the Brass tape inserted HTS coils in liquid nitrogen and background magnetic fields. Also, the measured quench currents of each coil will be discussed.

DOI： 10.1109/TASC.2012.2234492

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A compact high temperature superconducting (HTS) magnet, which consists of a stack of 500 HTS thin film annuli, was constructed and tested. Each thin film annulus, manufactured by the AMSC using the Rolling Assisted Bi-axially Textured Substrate (RABiTS) method, has a square cross-section of 40 mm × 40 mm with a thickness of 80 m. It has a 25-mm center hole created by machining. This paper reports a study on the anisotropic Jc issue due to the rolling procedure of the Ni substrate direction and its impact on field homogeneity. Also, three different stacking methods with rotation angles of 22.5°, 90°, and 0° against the rolling direction have been tested to study their impacts on strength, spatial homogeneity, and temporal stability of trapped fields. Finally, the 500-annulus magnet was tested at 21 K under a cryogen-free environment using a GM cryocooler. The spatial field homogeneity and temporal stability were measured at 21 K and compared with those obtained in a bath of liquid nitrogen at 77 K.

DOI： 10.1016/j.physc.2012.02.042

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The high temperature superconducting (HTS) coils have very low normal zone propagation velocity (NZPV) and complicated quench behaviors. Because of these reasons, it is difficult to expect the self-protection. In this paper, two methods are suggested to improve the self-protection ability of HTS coils. One is to remove the insulation between turn-turn winding in HTS coils to enhance the thermal and electrical contacts along transverse direction and to increase the whole thermal stability of them. The other is Cu or Brass tapes are wound with HTS tape wire instead of the insulation to improve the thermal, electrical and mechanical stability of the HTS coils. To clarify the quench behaviors of the suggested coils, the normal zone propagation properties of the fabricated non-insulated HTS pancake coils were shown by measured voltage and temperature traces during the quench as a function of the operating temperature. And the characteristics of the normal transition and amount of the shared current to adjacent layers were quantitatively measured by a Hall sensor located at the center of HTS coils to measure the change of the self-magnetic field by operating current. The current sharing behaviors in the non-insulated HTS coil and the coil with Cu tape inserted were observed. The minimum quench energy (MQE) of the HTS coil with Cu tape inserted was the largest in the tested coils. It means that the HTS coil with Cu tape inserted had the highest transient stability against the thermal disturbances among the tested coils and the improvement of self protection abilities was proved. (C) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2012.03.064

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We achieved stable levitation of cylindrical permanent magnets and irons using stacked ring-shaped high temperature superconducting (HTS) bulks with 20 mm ID, 60 mm OD and 50 mm height, and those were magnetized by field cooling method. The levitation characteristics of permanent magnets and iron samples located in the inner space of that levitation system were investigated experimentally. Iron samples with needle-shape and smaller than 1 mm diameter could not levitate stably. However, we found that the high strength of magnetized field was not necessary to levitate small needle-shaped irons. In order to levitate them, we need a uniform magnetic field in radial direction, so, a spherical solenoid magnet that can easily make a homogeneous magnetic field in inner space of HTS bulk annuli was developed. The spherical solenoid magnet, composed of seven solenoid coils with different inner and outer diameters, was designed by an electromagnetic analysis and fabricated. (C) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2012.02.030

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NMR/MRI magnets have a protection device to prevent the damages due to a quench. On the other hand, the protection device design of HTS coils or magnets are very difficult because it has a very low normal zone propagation velocity (NZPV) and complicate behaviors of quench. We have studied the methods to improve the self-protection ability of HTS coils by removing the turn-to-turn insulation and inserting the metal tape instead of insulation. In this paper, the improved transient stabilities and self-protection abilities of HTS coils by removing the insulation and inserting metal tapes will be presented by minimum quench energy (MQE). (C) 2013 The Authors. Published by Elsevier B.V.

DOI： 10.1016/j.phpro.2013.05.014

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Stable levitation of cylindrical iron and permanent magnet (PM) samples of various sizes has been achieved by using high temperature superconducting (HTS) bulk annuli which were magnetized by field cooling (FC) method. In this paper, we examined the forces acting on iron and PM samples levitating in the inner space of HTS bulk annuli. The levitation forces of the 3 stacked HTS bulk system (3 bulks system) have been compared withthe 2 stacked HTS bulk system (2 bulks system) having a gap between the bulks. In our experiments, the levitation force increases with increasing the magnetization field strength, the strength of magnetic flux density of PM, and the sample size. The levitation force of the 2 bulks system was better than the 3 bulks system, and we found that the levitation force using the field in magnetized HTS bulk systems strongly depends on the strength of the magnetic flux density of the sample and the magnetic field gradient in the levitating space. © 2002-2011 IEEE.

DOI： 10.1109/TASC.2013.2242513

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In fabrication of a compact NMR (Nuclear Magnetic Resonance) magnet which consists of a stacked HTS (High Temperature Superconducting) bulk annuli, generally there are three key issues: spatial homogeneity, temporal stability, strength of trapped magnetic fields. This paper presents a study on the effects of axial gap length between stacked HTS bulks on the three key issues of a bulk HTS magnet for compact HTS NMR applications. The HTS bulk magnet of which the ID and OD are 20 and 60 mm respectively has a 50 and 80 mm heights depending on the axial gap lengths between HTS bulks. The gap length between each HTS bulk varied from 0 mm to 10 mm and were used as parameters to optimize, analytically as well as experimentally, the overall field homogeneity of the HTS bulk magnet. The optimized axial gap length was obtained by analytical results, and the better magnetic field homogeneity and temporal stability of trapped magnetic field were achieved by lower magnetization field. The improved spatial homogeneity and strength of generated magnetic field by a new compact NMR magnet will be presented.

DOI： 10.1109/TASC.2011.2182170

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In the case of motors and generators, the benefits of using superconductors could be represented by the reduction of 50% in both losses and sizes compared to the conventional machines of the same rating. The high temperature superconducting (HTS) coils wound with HTS conductors have very low normal zone propagation velocity (NZPV) and complicated quench behaviors because the conductors have a low thermal conductivity and large specific heat. According to these reasons, in case of HTS coils and magnets, it is difficult to expect the self-protection like as low temperature superconducting (LTS) coils. In this paper, we suggest a method which inserts the Cu tape between turn-turn winding in the HTS coil instead of electrical insulation to improve the self-protection ability of HTS coils. Because the thermal and electrical contacts along the transverse direction are enhanced by inserted Cu tape instead of insulator and the whole thermal stability of HTS coils would be increased. To clarify the quench behavior of this suggested coil, we investigate the normal-zone propagation properties of the fabricated HTS pancake coils wound with 2G conductor and Cu tape under the cryocooled operation using GM cryocooler. The effects of inserted Cu tape and the improved the self-protection ability for the suggested HTS coil will be discussed.

DOI： 10.1109/TASC.2011.2174559

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The electric device applications of a high temperature superconducting (HTS) bulk magnet, having stable levitation and suspension properties according to their strong flux pinning force, have been proposed and developed. We have been investigating a three-dimensional (3-D) superconducting actuator using HTS bulks to develop a non-contact transportation device which moves freely in space. In our previous work, the unstable movement characteristics of HTS bulk were observed under DC operation, and the AC control system was proposed in order to solve these problems. In this paper, the horizontal movement and the field decay characteristics as a function of the gap length between electromagnet and HTS bulk and the frequency of driving current were investigated experimentally.

DOI： 10.1109/TASC.2011.2181472

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High temperature superconducting (HTS) coils wound with HTS wires have very low normal zone propagation velocity (NZPV) and complicated quench behaviors because the wires have a high thermal stability and large specific heat. According to these reasons, in case of FITS coils and magnets, it is difficult to expect the self-protection like low temperature superconducting (LTS) coils.
In this paper, we suggest a method which can remove the insulation among turn-to-turn in the coil to improve the self-protection property of HTS coils. Because the thermal and electrical contacts along transverse direction are enhanced by no turn-to-turn insulation, the whole thermal stability of HTS coils begins to increase. Furthermore, although a quench occurs in the coil, it is possible to realize a self-protection of HTS coil because the current path of the coil is modified in order to avoid quenching. To confirm a quantitative evaluation for quench behavior of the suggested coil, we carried out the measurements with the coils having variety types of turn-to-turn insulation. The experimental results and the self-protection ability about the tested coils will be presented. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2011.05.209

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The electric device applications of a high temperature superconducting (HTS) bulk magnet, having stable levitation and suspension properties according to their strong flux pinning force, have been proposed and developed. We have been investigating a three-dimensional (3-D) superconducting actuator using HTS bulks to develop a non-contract transportation device which moves freely in space. It is certain for our proposed 3-D superconducting actuator to be useful as a transporter used in a clean room where silicon wafers, which do not like mechanical contact and dust, are manufactured. The proposed actuator consists of the trapped HIS bulk as a mover and two-dimensionally arranged electromagnets as a stator. Up to now, the electromagnets consisted with iron core and copper coil were used as a stator, and each electromagnet was individually controlled using DC power supplies. In our previous work, the unstable movement characteristics of HTS bulk were observed under the DC operation, and the AC electromagnets driven with AC controlled current was proposed to solve these problems. In general, the trapped magnetic field in HIS bulk was decayed by a time-varying external magnetic field. Thus, it needs to optimize the shapes of AC electromagnets and operating patterns, the decay properties of the trapped magnetic field in the HTS bulk mover by the AC magnetic field should be cleared. In this paper, the influences of the frequency, the overall operating time, the strength of magnetization field and drive current against the decay of trapped magnetic field were experimentally studied using the fabricated AC electromagnets. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2011.05.220

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A compact nuclear magnetic resonance (NMR) magnet is one of the new applications after a technique to enhance maximum trapped field of an high temperature superconducting (HTS) bulk. In design of a compact NMR magnet which consists of the stacked HTS bulk annuli, the issues of strength, spatial homogeneity and temporal stability by trapped magnetic fields are very important. This paper presents a study on the effects of magnetization field strength and gap length between stacked bulks for the compact HTS bulk NMR applications. Four-stacked HTS bulk magnet with ID 20 mm and OD 60 mm was prepared to investigate the optimized configuration. The thickness of each HTS bulk is 5 mm, and the gap lengths from 0 mm to 10 mm were used as parameters in analysis and experiment, respectively. Four-stacked HTS bulk magnets with various gap lengths were tested at two different background magnetic fields of 0.5 T and 2 T at 77 K. The optimized axial gap length was found out by analytical results, and the better magnetic field homogeneity and temporal decay property of trapped magnetic field were obtained by lower magnetization field in this experiments. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2011.05.215

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A magnetic chromatography is a very useful system for an ion and/or fine magnetic particle separation. Recently, its performance has been enhanced using a superconducting magnet. The superconducting magnet can generate a high magnetic field and its strong magnetic field gradients in a very small flow channel. We have developed the magnetic chromatography system to separate the fine particles. However, its numerical simulation is difficult, since the scale of the fine magnetic particles in fluid is much different from the scale of the superconducting magnet generating the strong magnetic field. In order to accurately simulate the magnetic separation, it is necessary to develop the simulation code dealing with the multiscale. The performance of the developed magnetic chromatography is evaluated by the developed simulation tool, and it is clarified that it is unsuitable for magnetic particle separation. Therefore, a magnetic chromatography is newly designed and its performance is evaluated by the developed simulation tool.

DOI： 10.1109/TMAG.2011.2158989

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In design and manufacture of a compact NMR (nuclear magnetic resonance) magnet which consists of a stack of high temperature superconducting (HTS) bulk annuli, spatial homogeneity and temporal stability of trapped magnetic fields are key issues. This paper presents a study on optimized configuration of stacked HTS bulk annuli for the compact NMR application. A three-stack bulk magnet was designed and tested at two different temperatures of 20 and 77 K, and two different background fields of 3 and 5 T. The test results were compared with analytic results to verify the validity of the calculation. Then, various optimized patterns were investigated numerically to find out an optimized stack pattern of HTS bulk annuli in a given condition. The results prove that with the proposed optimized method trapped field strength as well as its homogeneity of a bulk-annulus stack can be improved.

DOI： 10.1109/TASC.2010.2089590

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Magnetic chromatography is a very useful system for ion and/or fine magnetic particle separation due to strong magnetic field gradients in a very small flow channel. We have developed the magnetic chromatography system to separate the ions and fine particles. It is, however, difficult to separate the ions or the magnetic particles using the developed magnetic chromatography. It makes the strong magnetic field gradients in the flow channel, but the alternate direction of the gradient is not suitable for magnetic separation. Therefore, we have newly designed a novel magnetic column with ferromagnetic nano-wires. In this paper, the new magnetic column is presented, and the magnetic field and its gradients in the flow channel are investigated.

DOI： 10.1109/TASC.2010.2090857

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The spatial field homogeneity and time stability of a trapped field generated by a stack of YBCO square plates with a center hole (square "annuli") was investigated. By optimizing stacking of magnetized square annuli, we aim to construct a compact NMR magnet. The stacked magnet consists of 750 thin YBCO plates, each 40-mm square and 80-mu m thick with a 25-mm bore, and has a empty set10 mm room-temperature access for NMR measurement. To improve spatial field homogeneity of the 750-plate stack (YP750) a three-step optimization was performed: 1) statistical selection of best plates from supply plates; 2) field homogeneity measurement of multi-plate modules; and 3) optimal assembly of the modules to maximize field homogeneity. In this paper, we present analytical and experimental results of field homogeneity and temporal stability at 77 K, performed on YP750 and those of a hybrid stack, YPB750, in which two YBCO bulk annuli, each empty set46 mm and 16-mm thick with a 25-mm bore, are added to YP750, one at the top and the other at the bottom.

DOI： 10.1109/TASC.2010.2103920

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Electric device applications of a high temperature superconducting (HTS) bulk magnet, having stable levitation and suspension properties due to their strong flux pinning force, has been proposed and developed. We have been investigating a three-dimensional (3-D) superconducting actuator using HTS bulks to develop a non-contract transportation device that moves freely in space. It is expected that our proposed 3-D superconducting actuator will be useful as a transporter used in a clean room where silicon wafers which dislike mechanical contact and dust are manufactured. The proposed actuator consists of an HTS bulk (trapped magnetic field by field cooling method) magnet for the mover and two-dimensional arranged multi-pole electromagnets as the stator. Up to now, the electromagnets consisted with ironed cores and copper coils were used as a stator, and each electromagnet was individually controlled using DC supplies. The movement of HTS bulk (mover) was not smooth because of the DC control. In this paper, the new electromagnets driven with AC control were proposed to improve the moving characteristics of the bulks. The characteristics of horizontal movement by newly proposed system will be presented.

DOI： 10.1109/TASC.2010.2090446

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Magnetic separation is a very useful tool in medicine manufacturing and sludge disposal, and we have developed the magnetic chromatography system, which separates the magnetic particles or the ions from fluid due to its strong magnetic field gradients in the very small flow channel. There are many fine ferromagnetic wires on the wall of the developed magnetic column. A superconducting magnet applies a strong magnetic field to the magnetic column, and fine ferromagnetic wires make strong magnetic field gradients. It was, however, impossible to accurately evaluate the performance of the developed magnetic column due to the absence of a good simulation method. In order to enhance the accuracy of the simulation, it is necessary to couple the fluid dynamics simulation with the magnetic field simulation. Therefore, we have developed a simulation code dealing with the fluid dynamics, solving Navier-Stokes, control volume, and magnetic field equations simultaneously, and compared the simulation result to the experimental one to verify the validity.

DOI： 10.1109/TMAG.2010.2072911

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We have investigated the stability of 2.0 GJ toroidal HTS-SMES, which consists of 90 element coils wound from YBCO tape. If one or more element coils are quenched, the storage energy reduces due to change of the magnetic field distribution. A series of quench for some element coils may occur, when the energy of the quenched coil cannot be consumed adequately. The simultaneous quench of some element coil may cause the system down. Therefore, it is necessary to investigate the simultaneous quench properties.

DOI： 10.4028/www.scientific.net/MSF.670.3

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Recently, the performance of high-temperature superconducting (HTS) bulks such as a critical current density, size, and mechanical strength has been improved. In consequence, various applications with HIS bulks such as motors, bearings, and flywheels are being investigated by many research groups; Compact nuclear magnetic resonance (NMR) magnet is one of the new applications after a technique to enhance maximum trapped field of an HIS bulk more than 11.7 T. 500 MHz (1)H NMR frequency, has been developed. This new compact NMR magnet out of FITS bulks is far less expensive than those conventional NMR magnets and expected to be widely used in food and drug industry. In design and manufacture of those compact NMR magnets, the issues of spatial homogeneity and temporal stability of trapped magnetic fields in HTS bulk annuli are very important. In this paper, the characteristics of the trapped magnetic fields in a stack of assembled HIS bulk annuli were investigated with various axial spaces between HTS bulks, experimentally and analytically. (c) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2010.05.197

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High temperature superconducting (HTS) motors electromagnetically consist of a rotator wound with HTS wires and an armature with conventional copper wires like Litz wire. The HTS rotor windings, as field coils, consist of a straight part and an end-ring part. Because a major rotation torque is induced by an interaction between magnetic fields and current-carrying conductors in the straight part, most of mechanical stresses in the motor occur at the straight part. An end-ring is placed in the edge of the straight part and used to connect to each adjacent straight-part coils. The magnetic fields by coil currents concentrate on the end-ring part, therefore, it is expected that the critical current of the entire coil, straight and end-ring, can be determined by the magnitude of the field in the end-ring. This paper deals with the overall J(c) degradation in the end-ring part by self-field generated from the coil. In addition to electromagnetic analyses, we have performed a numerical analysis in order to evaluate mechanical stresses in the straight part of field coil by armature reaction on steady-state operation. The analytical results will be presented in this paper. (c) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2010.05.201

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The magnetic chromatography is a very useful system for an ion and/or fine magnetic particle separation because it has strong magnetic field gradients even in a very narrow flow channel. We have not only developed the magnetic chromatography system to separate the fine particles and ions, but also the numerical analysis code based on the fluid dynamics and electromagnetism to investigate the separating characteristics and to optimize design of magnetic column. In this study, the simple experiments using a superconducting magnet with a large room-temperature-bore and a micro-scale magnetic column consisting of ferromagnetic wires were carried out to understand the ions separation. The cobalt chloride (CoCl(2)) and the nickel sulfate (NiSO(4)) were used as ions, and the magnetic field and length of magnetic column were used as a parameter in an experiment and an analysis. It can be expected that the ion mobility of a single and complex are quite different, and the ability of the separation will be improved by increasing the column length without external magnetic field. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2010.05.211

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The stability and the protection of superconducting applications are very important issues. High temperature superconductor (HTS) is stable than low temperature superconductor (LTS). However, due to the high stability of HIS, HIS coils have complicate behaviors of normal transition - such as a recovery into the superconducting state or thermal runaway - which is determined by the magnitude of disturbance, coil size, the structure of winding wire and cooling condition. In this paper, we propose the quench protection system based on the active power method which can detect the quench by measuring the generating instantaneous active power in HTS coils. And the quench detection circuit adopted a software circuit by LabVIEW which was added a new quench protection technique to the circuit in order to improve its detection performance. Furthermore, it can monitor interior states of HIS coils with acquiring the only coil voltages in real time. To confirm the validity of the suggested technique, the experiments were performed with the hardware circuit for quench detection simultaneously. The experimental results are presented and discussed in this paper. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2010.05.226

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This paper presents experimental and analytical results of trapped field characteristics of a stack of square YBCO thin film plates for compact NMR magnets. Each YBCO plate, 40mm X 40 mm x 0.08 mm, has a 25-mm diameter hole at its center. A total of 500 stacked plates were used to build a 40-mm long magnet. Its trapped field, in a bath of liquid nitrogen, was measured for spatial field distribution and temporal stability. Comparison of measured and analytical results is presented: the effects on trapped field characteristics of the unsaturated nickel substrate and the non-uniform current distribution in the YBCO plate are discussed.

DOI： 10.1109/TASC.2010.2043832

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The magnetic chromatography is a very useful system for an ion and/or fine magnetic particle separation due to its strong magnetic field gradients in a very small flow channel. We have developed the magnetic chromatography system to separate the fine particles and ions. However, its numerical simulation is difficult, since the scale of the ions or fine particles in fluid is much different from the scale of the superconducting magnet generating the strong magnetic field. In order to accurately simulate the magnetic separation, it is necessary to develop the simulation code dealing with the multi-scale problem.

DOI： 10.1109/TASC.2009.2039928

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This paper presents the quench detection method with using normal transition behavior for HTS coil. If the HTS coil starts to propagate a normal zone by an initial quench, it will be recovered the superconducting state or runaway thermally. These behaviors were mainly determined by variety parameters like the magnitude of a disturbance, HTS coil size, the structure of winding wire and cooling condition. However, the first step of detection or protection for coils should be started from detecting the presence of a non-recovering normal zone. This paper classifies the initial quench behaviors of HTS wires or coils as a model of 3-type tendencies, and suggests suitable detection techniques for each tendency with using the end-to-end voltage of coil. In additional, this method is able to detect a quench without a decisive criterion used at other quench detection methods. In order to confirm the validity of the method, we used the HTS sample coil wound with YBCO tapes. This paper presents these results and the quench protection properties with fabricated detection method.

DOI： 10.1109/TASC.2010.2043238

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Recently, the superconducting devices using cryogenic and superconducting technologies were widespread in the field of electric power. The 2G wires with YBCO superconducting layer, metal substrate and stabilizer layer are expected to be used in future superconducting applications, because it has high properties in high temperature and high magnetic fields. In order to operate safely at high transport currents, the thermal stability of the wounding wire should be addressed in terms of detection and protection against the quench. On the other hand, the understanding of the 3-dimensional normal zone expansion during a quench is very important to design the superconducting devices such as magnets and cables. This paper presents the results of an experimental study for normal zone propagation properties in the longitudinal and transverse directions using the pancake coil wound with 2G wires which have Cu and SUS stabilizers under the cryocooler cooling. And the minimum quench energy (MQE) of the pancake coils wound with two different kinds of stabilizers as functions of operating temperature and transport current will be described.

DOI： 10.1109/TASC.2010.2044652

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The magnetic separation is a very useful tool to manufacture medicine. We have developed the magnetic chromatography system, which separates the magnetic particles or the ions from fluid due to its strong magnetic field gradients in a very small flow channel. In order to simulate the magnetic separation, we developed the simulation code dealing with the multi-scale problem, and verified the results compared with the experimental ones. © 2010 IEEE.

DOI： 10.1109/CEFC.2010.5481745

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Recently, the performance of high temperature superconducting (HTS) bulks such as critical current density, size, and mechanical strength has been improved rapidly. So. various applications using HTS bulks such as motors, bearings and flywheels have been investigated by many research groups. A compact nuclear magnetic resonance (NMR) magnet is one of the new applications after a technique to enhance maximum trapped field of the HTS bulk more than 11.7 T (500 MHz (1)H NMR frequency) has been developed.
This new compact NMR magnet out of HTS bulks is cost-effective compared with conventional NMR magnets and then expected to be widely used in food and drug industry. In design and manufacture of the compact NMR magnets, spatial field homogeneity of the large trapped magnetic field in HTS bulk annuli is a crucial issue because the behavior of a trapped field is highly non-linear and, as a result, a technique to improve the field homogeneity such as active/passive shimming now becomes more challenging compared with that of the conventional counterparts.
This paper presents the magnetic field distributions in single and three assembled HTS bulk annuli measured by a 3-axis and multi-arrayed Hall sensor under two different cryogenic environments: (I) in a bath of liquid nitrogen (LN(2)) and (2) dry cooling by a cryocooler. The spatial homogeneity changes with various operating temperatures were investigated and the effect of critical current density enhancement by lowering the operating temperature on the field homogeneity improvement was discussed in detail. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.05.161

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To understand the effects of the hot pressing on the pinning behavior for the SiC doped MgB2 tape wire, magnetic field and temperature dependences on a critical current density J(c) and the pinning force have been studied. The MgB2 tapes were fabricated using an in situ powder-in-tube (PIT) method, the hot pressing process with nano-SiC powders and the carbon steel sheath. J(c) of the hot pressing processed wire is enhanced especially at low field compared with that of the no hot-pressed wire. It is suggested that the enhancement of J(c) is caused by the improvement of the MgB2 grain connectivity came from the hot pressing.
The pinning analysis using the pinning models of a surface pinning and a normal point pinning lead to that the scaled curve of the pinning force for the SiC-doped MgB2 tape with the hot pressing process showed anisotropy in a lower field. This result suggests that the pinning center of the hot pressing processed MgB2 wire forms into inequi-axial shape, which is different from that in the tape without the hot pressing having the equi-axial pinning center in our previous report. It was also found that the scaled pinning force of the SiC-doped MgB2 tape with the hot pressing is larger than the surface pinning model in a higher field. This suggests that the pinning force was enhanced by the hot pressing in a higher field. (C) 2009 Published by Elsevier B.V.

DOI： 10.1016/j.physc.2009.05.231

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The magnetic chromatography (MC) is a very useful system for an ion and/or fine magnetic particle separation due to its strong magnetic field gradients in a very small flow channel. We have not only developed the MC system to separate the fine particles and ions, we have also developed the numerical analysis computer code based on the fluid dynamics and electromagnetism to investigate the separating characteristics and to optimize design of magnetic column. In this paper, we studied MC technology for magnetic particles and ion separations using micro-scale magnetic column with ferromagnetic wires experimentally. The influences of the strength of magnetic gradient, flow velocity and coefficient of viscosity of solvent were studied by the developed computer code.

DOI： 10.1109/TASC.2009.2019143

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Recently, the superconducting devices using cryogenic and superconducting technologies were widespread in the field of electric powers. The quench protection is one of the important issues for safety operating of the superconducting devices, because the excessive joule heating brings about damage to the superconducting magnets when the quench occurs. However, if the HTS coil starts to transit normally by an initial quench, it will be recovered into the superconducting state or runaway thermally. These behaviors were mainly determined by the magnitude of a disturbance, HTS coil size, the structure of winding wire and cooling condition. Therefore, in order to detect the transition into the normal state at HTS coil more accurately, it needs to investigate these effects as parameters. This paper describes the electromagnetic and thermal behaviors of HTS during normal transition by the initial quench. The aim is the development of the quench detection method about HTS coil for 5 MW HTS motor after an initial quench. It adopted the hardware and the software that were able to estimate the quench by monitoring the voltage and temperature for the behavior between the thermal runaway and the recovery. This paper will be presented these results and the quench protection properties with fabricated detection method.

DOI： 10.1109/TASC.2009.2017695

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The minimum quench energy (MQE) and normal zone propagation velocity (NZPV) of three kinds of Nb3Al superconductors fabricated by the rapid heating, quenching and transformation (RHQT) process were measured under various conditions of applied magnetic field (10-14 T), temperature (7-11 K), and transport current (80-95% of the critical current), while cooled by a cryocooler for developing the over 20-T class cryogen-free magnet. As a result, MQE values were related to the critical current density (J(c)); high MQE was obtained for low J(c). It is assumed that J(c) has a stronger influence on the MQE than specific heat, thermal conductivity, resistivity, and other parameters of the composite superconductor including the matrix and the stabilizer. NZPV was mainly proportional to the transport current density varying with applied field and temperature. The second contribution to NZPV is assumed to be heat capacity depending on the wire configuration.

DOI： 10.1109/TASC.2009.2018149

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Recently, the performance of high temperature superconducting (HTS) bulks such as critical current density, size, and mechanical strength has been improved rapidly. So, various applications with HTS bulks such as motors, bearings, and flywheels have been investigated by many research groups. A compact nuclear magnetic resonance (NMR) magnet is one of the new applications after the technique to enhance maximum trapped field of the HTS bulk more than 11.74 T, a corresponding NMR frequency of 500 MHz, has been developed. This new compact NMR magnet out of HTS bulks is far less expensive than those conventional NMR magnets and expected to be widely used in food and drug industry. In design and manufacture of those compact NMR magnets, spatial field homogeneity of large trapped magnetic field in HTS bulk annuli is essential. This paper presents the magnetic field distribution in single and three assembled HTS bulk annuli, measured by a 3-axis hall sensor, and experimental results of its spatial homogeneity improvement by mounting an iron ring inside or outside of the HTS bulk annuli.

DOI： 10.1109/TASC.2009.2019083

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The resistive type fault current limiter based on YBCO thin film with high critical current density was proposed and developed because of its fast recovery characteristics. We have been developing a magnetic field measurement system with hall sensor to study the critical Current density distribution or magnetic properties of high-T-c superconductors such as thin film, tape conductor and coated conductors. In this study, the transport current distribution in straight line and U-corner region of meander type resistive fault current limiter using YBCO thin film was investigated experimentally. The both axial and radial self-magnetic field components according to the direct (DC) and alternating (AC) transport currents were measured by two-axis hall probe to estimate the current distribution. And also, the self-magnetic field at the corner of bispiral type fault current limiter was measured by two-axis hall probe with a high spatial resolution to compare with the meander type fault Current limiter. (C) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2008.05.264

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In an attempt to understand the effects of SiC addition on the pinning behavior for the in-situ PIT MgB2 tape wire, field and temperature dependencies on critical current density, J(c) and the pinning force were studied by considering applied magnetic field direction for the rectangular-shaped wire. As a result, it was clarified that the SiC-doped wire has the equiaxial pinning center and the un-doped wire has the inequi-axial pining center. Taking the pinning scaling law into account, it is found that the SiG-doped MgB2 wire has two kinds of the pinning center and the dominant pinning center changes with temperature, that is, the pinning centers are the grain boundary and the precipitates at high and low temperatures, respectively. The un-doped MgB2 wire is assumed to have anisotropy of the pinning mechanism.

DOI： 10.1109/TASC.2008.921871

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When the quenching occurs in a superconducting coil, excessive joule heating in normal area may damage the coil. It is necessary to detect quenching in the coil as soon as possible and discharge the magnetic energy stored in the coil. Therefore, we propose a superconducting coil protection system based on an active power method. The system is highly resistant to the noise and does not require cancel voltage taps, so it is useful for both AC and DC coils. We have presented the effectiveness of the system using some test coils cooled in LN2 or LHe. However, we have not discussed the effectiveness of the proposed system for helium-free cryocooled magnets, in which a larger temperature rise occurs after quenching than in liquid-cooled magnets. In this paper, we verify the effectiveness and practicality of the proposed system through coil protection tests for a cryocooled Nb3Al LTS coil. (C) 2008 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.cryogenics.2008.02.002

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DOI： 10.1109/ICEMS.2007.4412325

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We purposed and tested the contactless ac inductive method to estimate the critical current density of high temperature superconducting (HTS) films by detecting a third-harmonic voltage. In this study, a minimum diameter of the HTS film to estimate the correct critical current density was experimentally investigated using three miniature inductive coils with various sizes. It was clarified that the correct critical current density could be measured and estimated when the diameter of HTS films was about 1.2 times larger than outer diameter of each tested inductive coil. The accuracy of detection for micro-scale defects with various shapes in HTS films was studied using miniature inductive coils by the third-harmonic voltage method. And it was possible to detect the defects in HTS films by obtained the slope values of initial region at generated third-harmonic voltage traces. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.03.481

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The magnetic chromatograph (MC) system is one of the promising methods for ion separation because of its strong magnetic field gradients. We have been developing the MC system to separate the fine particles and ions, and have also developed the numerical analysis computer code based on fluid dynamics and electromagnetism to investigate the separating characteristics and optimization of MC system. We investigated MC technology for magnetic particles separation using superconducting magnet and micro-scale magnetic column with ferromagnetic wires experimentally. A channel height of magnetic column for ion separation was optimized by the developed computer code. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.04.305

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Recently, HTS coils have been developed for electric power apparatuses. In superconducting coils, local and excessive joule heating may give damage to the superconducting windings when a quench occurs and therefore it is essential that the quench is detected quickly and precisely so that the coils can be safely discharged. Resistive voltage measurement method is universally used for the quench detection, however, it is vulnerable to an electromagnetic noise which causes insufficient quench detection and at least needs a central voltage tap in windings. In a large superconducting coil, a lead-wire from the central voltage tap may cause a short-circuit when high voltage will be applied. In this paper, we present a quench protection system based on the active power method which detects a quench by measuring the instantaneous active power generated in a superconducting coil. The protection system based on this method is very strong against to the noise and no more needs a central voltage tap. The performance of system developed by us is confirmed by using test coil wound with Bi-2223 HTS tapes. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.02.047

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We have been developing a 3D superconducting actuator with HTS (high temperature superconducting) bulk and two-dimensionally arranged multiple electromagnets. The application of HTS bulk enables stable levitation and suspension due to its strong pinning force. The performance of the actuator is determined mainly by the spatial distribution of the generated magnetic field of the electromagnets. For the sake of improving the levitation force, we proposed to attach a sliced iron sheet (we called this cap core) on the top surface of the core in each electromagnet. The proposed cap core was expected to lead to an intensive concentration and abrupt gradient increase of the perpendicular component in the magnetic field through the HTS bulk of the electromagnets. We experimentally confirmed and measured the effect of the cap core. For the optimized design of the cap core, we calculated the spatial distribution of the magnetic field on the surface of the electromagnet using 3D FEM analysis for several cap cores and estimated the levitation force of the HTS bulk using the calculated result. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.05.029

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We have been developing a three-dimensional (3-D) superconducting actuator with HTS bulk and electromagnets. Proposed 3-D superconducting actuator consists of the stator constructed by two-dimensional arranged electromagnets and HTS bulk superconductor as a mover. To realize the 3-D superconducting actuator with high performance, we have to investigate the shape-dependences of electromagnets and HTS bulk on levitating characteristics. In this study, we fabricated the individually controlled eight-polarities electromagnets and the two kinds of bolted joints cap cores to improve the control ability and levitation characteristics. And also, we prepared the disk-shaped and ring-shaped (with 20 rum inner diameter) HTS bulk with 60 mm diameter, and evaluated how magnetic field distribution influences the levitating characteristics as functions of levitation force and height. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.03.496

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The new in situ MgB2 tapes with/without SiC addition and the hot pressing process have been fabricated using the carbon steel sheath. The critical temperature (T-c), critical current density (J(c)) and the AC transport current loss of those tapes have been studied and discussed in various conditions. The MgB2 tapes with the carbon steel sheath that improved the core hardness and homogeneity, resulting in high J(c), and the SiC addition, and the hot pressing brought narrow T-c transition width. As the carbon steel has ferromagnetism, it affected the loss of the MgB2 tape. The large losses that were two to three orders of magnitude higher than Norris' theoretical curve were obtained. It was clarified that the main loss of the MgB2 tape was the hysteresis loss of the carbon steel sheath. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.04.266

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In this paper, we report a basic study on the development of a two-axis Hall sensor array using epitaxial lift-off (ELO) film bonding technology. We developed a novel Hall sensor array fabricated on a Si substrate that can directly measure axial and radial magnetic field components (Bx and Bz) simultaneously. We investigated conditions of basic fabrication process and fabricated a basic test device. We also estimated basic characteristics of the Hall sensor and measured the DC self-magnetic field profiles of a YBCO strip. The results clarified that the ELO film bonding technology is suitable for fabricating two-axis Hall sensor array.

DOI： 10.1109/TASC.2007.899041

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We have been investigating a three-dimensional (3-D) superconducting actuator using HTS bulk to develop the transportation device with non-contact and move in free space. Proposed superconducting actuator consists of the stator constructed by two-dimensional arranged electromagnets and HTS bulk superconductor as a mover. The 2-D arranged electromagnets were individually controlled to move the HTS bulk. It is expected that our proposed 3-D superconducting actuator will be useful as a transporter used in clean rooms where silicon wafers are manufactured. Wafers must be protected from contact with dust and other contamination. To realize the superconducting actuator with HTS bulks, the electromagnetic characteristics of HTS bulk, such as trapped field strength and patterns, should be clarified and the optimized controlling system is also required. Therefore, in this study, to construct the optimal 3-D actuator system, the fundamental characteristics of the levitation for HTS bulks with individually controlled four and eight-polarities electromagnets were investigated experimentally. And two kinds of cap cores attached the top of the core to improve the control ability of proposed actuator were fabricated and tested.

DOI： 10.1109/TASC.2007.898974

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Pre-bending effects of Nb3Sn composite wires, enhancement in J(c) at the as-cooled condition and J(c) peak, J(cm), against applied tensile strain, are well known in especially Cu-Nb reinforced Cu stabilized Nb3Sn wire. In an attempt to understand the effects, three directional strain analyses using FEM were studied by considering thermally-induced residual strain due to temperature difference between the reaction temperature and the cryogenic temperatures including the pre-bending process at room temperature for three types of Nb3Sn composite wires. To evaluate effects of change in three directional strain, the von Mises strain was introduced. As a result, the minimum von Mises strain against the applied tensile strain corresponded to J(cm); low strain shows J(cm) enhancement by the pre-bending for Cu-Nb reinforced Nb3Sn wire. It was found that there is a direct correlation between the minimum von Mises strain and J(cm) related to the pre-bending.

DOI： 10.1109/TASC.2007.899597

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Many high-Tc superconducting (HTS) tape manufactures make an effort to improve the transport characteristics of HTS tapes. When AC or DC current flows in the HTS tape, AC or DC magnetic self-fields are induced around the conductor by the current and these self-fields affect on the AC loss and the critical current like the transport characteristics of HTS tapes. The spatial distribution and magnitude of the self-field are variable due to the neighboring materials on the HTS tape. In this paper, the relation between the transport characteristics and the self-field distribution of HTS tapes with paralleled ferromagnetic material (Ni tape) and/or diamagnetic material (BSCCO tape) is experimentally investigated. The self-fields according to the DC and AC transport currents were measured by hall probe with axial and radial magnetic field components. The critical currents of HTS tapes with paralleled the Ni tape are slightly decreased and the transport current losses markedly increased. However, the critical currents and transport current losses of HTS tapes with paralleled BSCCO tape showed more improved performance than those of single HTS tape. We explain these results using the change of the spatial distribution and magnitude of self-field on HTS tapes by the neighboring magnetic materials.

DOI： 10.1541/ieejpes.127.327

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DOI： 10.2221/jcsj.42.382

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We have been developing a new type of 3-D superconducting actuator using the strong pinning force of HTS (High Temperature Superconducting) bulk. The system includes HTS bulk and two-dimensionally arranged multiple electromagnets. The application of HTS bulk enables stable levitation and suspension, and its performance is determined mainly by the spatial distribution of the generated magnetic field of the electromagnets. To enhance the levitation force in the actuator, it is necessary to design the optimized geometry of the electromagnets. The target of the design is to lead to an intensive concentration and abrupt gradient increase of the perpendicular component in the magnetic field through the HTS bulk over the electromagnets. For the optimized design of the electromagnets, we calculated the spatial distribution of the trapped magnetic field and the levitation force of the HTS bulk using 3-D FEM analysis that is adopted the critical state model.

DOI： 10.1109/ICEMS.2007.4412287

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We have been developing a three-dimensional (3-D) superconducting actuator consists of a field-cooled HTS bulk for mover and two-dimensional arranged multiple electromagnets as a stator. The HTS bulk is magnetically connected with electromagnets and it can be moved in 3-D directions and rotates without upper side electromagnets and spatial restrictions. The current and the polarity of each electromagnet were individually controlled by the switching power supply. In this paper, we experimentally investigated the effects of the shapes of HTS bulks (disk shape with/without inner hole) and electromagnets (4 and 8 polarities) in points of stabilities and forces of levitation. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2006.06.026

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We have been developing a magnetic field measurement system with two-axis hall-probes to study the critical current density distribution or magnetic properties of high-T-c superconductors. In this study, the current distributions at corners of the meander type resistive fault current limiters were measured by hall probe with high spatial resolution. The both of axial and radial magnetic field components (B-x and B-z) according to the direct (DC) and alternating (AC) transport currents were measured by hall probe to estimate the current density distribution with high accuracy. And, it is confirmed that the measured radial components are very useful to estimate the current distribution. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2006.06.027

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Reducing AC loss in a superconducting apparatus is one of the most important issues, and the precise measurement and estimation of AC losses are essential to reduce them. The four-terminal method is universally used as an electric measurement method of AC losses for superconducting tapes and wires. In this method, noise and inductive voltage superposed on the terminal voltage of the superconductor are eliminated by a lock-in amplifier and cancel coil, respectively, and then measurement of very small resistive voltage is achieved. However, using this conventional method, a plurality of measuring instruments and apparatuses are needed, and therefore the measuring system becomes complicated and much time is consumed in the calibration process. In this paper, we present a simple and precise measurement system based on an active power detection method, which is proposed as a quench detection method. The proposed system consists of a small number of instruments and apparatus and is less susceptible to noise. Its usefulness is verified by comparing the proposed method and the conventional four-terminal method in measuring the AC transport current loss of a Bi2223/Ag tape. (C) 2006 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.cryogenics.2006.04.001

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Many high temperature superconducting (HTS) tape manufactures make an effort to reduce the transport current loss of HTS tapes. The knowledge of critical current and self-field in an HTS tape is very useful to compute the transport current losses. The spatial distribution and magnitude of self-field are variable due to the neighboring materials. In this paper, the critical currents and the transport current losses of BSCCO and YBCO tapes with paralleled magnetic material (Ni tape) and/or diamagnetic material (BSCCO tape) are experimentally investigated to improve the AC loss properties. The critical currents of HTS tapes with paralleled Ni tape are slightly decreased and the transport current losses are markedly increased. However, the critical currents and transport current losses of HTS tapes with paralleled BSCCO tape have not current carrying are more improved than single HTS tape.

DOI： 10.1109/TASC.2005.869674

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AC transport current loss (self field loss) of ex-situ-processed MgB2 superconducting tapes with Ni sheath was measured under different conditions of transport current value, transport current frequency, and temperature. Experimental results were compared with Norris' theoretical curves and previous measured losses of MgB2 tapes with CuNi and stainless-steel sheaths, which were larger than the theoretical ones and previous data. The loss vs. normalized transport current (transport current normalized by critical current) curves varied with temperature and the loss at 9 K was larger than that at 14 K. The measured loss approached Norris' curves in the high normalized transport current region. In order to gain insight into these origins, FEM analysis of magnetic flux concentration by Ni and estimations of hysteresis loss of ferromagnetic nickel-sheath and of homogeneity of the core were carried out. As a result, the hysteresis loss of Ni was very small based on the estimation and the loss, measurement of Ni. Main contributions to large loss are considered to be self-field enhancement due to magnetic flux concentration by the Ni-sheath and critical current density distribution of the MgB2 core caused by the inhomogeneity of critical current density and critical temperature.

DOI： 10.1109/TASC.2006.870448

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A superconducting actuator with multiple motions is proposed and experimentally investigated for the development of microelectrical machines. The disk-shaped YBCO bulk was located above the electromagnets and cooled down in the field-cooling method, which enabled the flux pinning. The YBCO bulk superconductor was driven in vertical, horizontal and/or self-rotation by PC-controlled coil currents using multiple electromagnets. The most suitable levitation stability was obtained when we used four electromagnets and NNSS trapped field type. It was confirmed that the levitation force and horizontal moving force strongly depended on the coil current and the air gap length, respectively. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2005.03.069

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Language：English   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The critical current density, J(c), of metal-organic deposition process using trifluoroacetates (TFA-MOD) based YBCO films was measured between 20 K and 77 K in magnetic fields up to 9.5 T as a function of the field direction. C-axis oriented YBCO films was deposited on LaAlO3 single crystalline substrates and also on CeO2 buffered IBAD (YSZ/Hastelloy) substrates by TFA-MOD method. The obtained Jc values at 77 K and 0 T were 7 and 1.7 MA/cm(2) for LaAlO3 and IBAD substrates, respectively. High Jc values over 10(5) A/cm(2) were maintained up to 4 T (B//c-axis) for the LaAlO3 substrate and up to 3 T (B//c-axis) for the CeO2 buffered IBAD substrate. These results imply that TFA-MOD with simple fabrication process is very promising for the practical YBCO coated conductor. And the dependences on temperature and angular of the Jc at various external magnetic fields for some different thicknesses YBCO films was reported.

DOI： 10.1109/TASC.2005.847757

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Language：English   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

It is known that current sharing effects of the stabilizer are limited in cryocooling conditions being different from poolboiling. Therefore we have studied stability under it, focusing on minimum quench energy (MQE) of Nb3Sn superconductors under various conditions of the applied magnetic field, temperature, and the transport current, for various volume fractions and configurations of Cu stabilizer. From the experimental results, the larger the external Cu volume fraction, the higher the MQE was obtained. All MQE data were summarized by only one line in characteristics of normalized MQE and normalized B (B/B-c2). The obtained MQE behavior was to be characterized by only one empirical equation. It is thought that the obtained formula is useful for a standard of designing the Nb3Sn superconductor and its coil under cryocooled condition.

DOI： 10.1109/TASC.2005.848946

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Language：Japanese   Publisher：The Institute of Electrical Engineers of Japan  

AE (Acoustic Emission) techniques are universally used for investigating and watching the deformation and crack of various materials because they enable the non-destructive and continuous measurement. However, it is not always easy to clarify the main cause or mechanism of AE signal generation by its analysis and an AE sensor is generally vulnerable to the noise, and therefore advanced measurement and estimation techniques of AE signal are needed. In this paper, the authors present time-frequency visualization of AE signal using the wavelet transform. The proposed method enables especially precise measurement of the frequency band and generation time of AE signal and gives a clue in advanced AE techniques.

DOI： 10.1541/ieejfms.125.1069

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DOI： 10.2221/jcsj.40.420

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Novel porous NdGaO3 (NGO) thin film was fabricated as a new seed layer for liquid phase epitaxy growth of YBa2Cu3O7-delta. The microstructure and crystallization mechanism of NGO new seed layer due to post-annealing temperature were revealed by high resolution SEM. Amorphous NGO layers on LaAlO3 single crystalline substrate obtained by pulsed laser deposition method at room temperature and this layer shows the very porous and columnar structure. And this amorphous NGO layers were annealed at wide temperature range below 1300 degreesC in air. Amorphous NGO layer begins to crystallize by post-annealed at 750 degreesC and its orientation gets better with increased annealing temperature. The thickness of NGO layers decrease with increased annealing temperature above 500 degreesC and the well-oriented NGO layers were obtained between 900 and 1300 degreesC. The NGO layers annealed at above 1100 degreesC show very dense film. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/S0921-4534(03)00862-1

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The microstructure and crystallization mechanism of NdGaO3 (NGO) new seed layer for liquid phase epitaxy (LPE) growth of YBa2Cu3O7-delta (YBCO) were revealed using high resolution SEM and TEM. NGO layer deposited by PLD at room temperature shows that very porous and columnar structure and this layer indicated amorphous by x-ray analysis. Amorphous NGO layer was crystallized with good in-plane and out of plane orientations keeping columnar structure by simple post annealing process. Biaxially aligned YBCO thick films were successfully fabricated on oriented novel porous NGO new seed layer: Although many cracks were observed for normally LPE grown YBCO films on NGO(110) single crystalline substrate; it was crack-free formed on porous NGO(100) film.

DOI： 10.1109/TASC.2003.811851

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Thick YBa2Cu3O7-delta (YBCO) films have been successfully formed by the liquid phase epitaxy (LPE) method on Ni-Cr alloy tape. A buffer layer of yttria stabilized zirconia and seed layer of YBCO were formed by the ion beam assisted deposition and by the pulsed laser deposition method, respectively. YBCO films have been grown from the solution of BaO-CuO-Ag-BaF2 at 820, 845 and 860 degreesC under low oxygen partial pressure of 2% to prevent the metallic substrate from reacting with the solution. Good YBCO films are obtained by the growth at 820 degreesC, and the best one shows high critical current density of 1.9 MA/cm(2). A cross-sectional observation by high resolution scanning electron microscopy shows neither reaction layer nor damaged region at the substrate[YBCO film interface. theta-2theta and pole figure X-ray diffraction of the seed layer and the YBCO LPE layer show that in-plane orientation distribution is highly improved by the LPE process. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4534(02)01579-4

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The chemical solution deposition (CSD) technique has been applied to prepare both YBa2Cu3O7-δ (YBCO) and buffer layers on a YSZ substrate. Y2O3 and CeO2 were employed as the buffer layers, and it was confirmed that they grew epitaxially by the CSD method. Cross-sectional transmission electron microscope observations revealed that Y2O3 grew up to the top of the film. Furthermore, the YBCO film on the Y2O3 or CeO2/Y2O3 layers were c-axis aligned and showed a good in-plane alignment. The transition temperature and the critical current density of the YBCO/Y2O3/YSZ multilayer were 89 K and 4 × 104 A/cm2 at 77 K, 0 T, respectively. © 2001 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4534(01)00448-8

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Metalorganic deposition process using trifluoroacetate (TFA-MOD) processed YBCO film on LaAlO3 and IBAD (YSZ/Hastelloy) substrate exhibited high critical current density, J(c), at 77.3 K. The obtained J(c) values at 0 T were 7 and 1.7 MA/cm(2) for LaAlO3 and CeO2 buffered IBAD, respectively. High J(c) values over 10(5) A/cm(2) were maintained up to 4 T (B parallel toc-axis) for the LaAlO3 substrate and up to 3 T (B parallel toc-axis) for the CeO2 buffered IBAD substrate. These results imply that TFA-MOD with simple fabrication process is very promising for the practical YBCO coated conductor. (C) 2001 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4534(01)00471-3

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