Masahiko Takahashi

Development of magnetic refrigerator for room temperature application

This paper describes experimental results of a magnetic refrigerator operated at room temperature. Spheres of Gadolinium, 2.2 kg in weight and 0.3 mm in diameter, are used as a magnetic working material. The magnetic materials were divided into two vessels and reciprocated between high and zero magnetic fields. High magnetic field up to 4 T was applied by a cryocooler cooled superconducting magnet. Refrigeration capacity of 100 W was obtained with operating frequency of 0.167 Hz. Details of the magnetic refrigerator construction and experimental results are presented.

Development of MVA class HTS SMES system for bridging instantaneous voltage dips

A SMES system of MVA class for bridging instantaneous voltage dips has been developed using Bi-2212 wire. The Bi-2212 wire has high-performance conductive characteristics that do not deteriorate at a low temperature in high magnetic fields beyond 10 T. These characteristics enable a compact design of a SMES system of the Bi-2212 wire. In addition, coils of the Bi-2212 wire can be adequately insulated due to a high temperature margin. Therefore, the SMES system designed by using the coils has advantages to enhance dielectric strength and output power of the system. In our previous study, a SMES system consisting of 4 unit coils was constructed and the various properties were examined. Up to the present, the total 18 unit coils were stacked to make a coil system (outer diameter: 700 mm, height: 554 mm, stored energy: 984 kJ) and installed into a SMES system of 1 MVA for bridging instantaneous voltage dips. Also, the cooling system of the HTS SMES has been improved. The characteristics of the conduction cooled HTS coils of 1 MJ class were investigated in the operations of 1 MVA SMES system for bridging instantaneous voltage dips. Thermal reliability was verified during each operations of exciting, standby, bridging and current damping. Moreover, the repetitive bridging operations even worked out every 5 minutes. Advantages of the conduction cooled HTS coils for SMES were verified.

Development of 1 Watt Class 4 K Gm Refrigerator with Magnetic Regenerator Materials

This paper describes the experimental results of the 4 K GM refrigerator which obtained the maximum refrigeration capacity of 1.05 W at 4.2 K. The COP value for the 4 K GM refrigerator at 4.3 K was 1.92*10-4 and almost same as that for commercially used GM+JT refrigerator at 4.3 K. The technical points of this refrigerator were to adopt a hybrid structural regenerator and to optimize the intake/exhaust valve timing. The hybrid structural regenerator consisted of Ero.9Ybo.1Ni and Er3Co regenerator materials. Ero.9Ybo.1Ni has a large heat capacity at lower than 10 K. On the other hand, Er3Co has a large heat capacity at a higher temperature region. The intake/exhaust valve timing was also changed to improve the refrigeration capacity at 4.2 K.

Recent Progress in Magnetic Refrigeration Studies

After the 1985 Cryogenic Engineering Conference, two directions for the fundamental investigations on the magnetic refrigeration to expand the refrigeration range above ~15 K have been developed by our group. One is the improvement of the refrigeration characteristics able to refrigerate from ~20 K for the Carnot magnetic refrigerator and the other is the fundamental study of the Ericsson magnetic refrigerator. As for the former purpose, we used a new magnetic material, Dy3A15012, as the refrigerant in a reciprocating Carnot magnetic refrigerator instead of Gd3Ga5012. Consequentially, we succeeded in expanding the refrigeration range. As for the latter, we have established the method to make the refrigerant suitable for the ideal Ericsson cycle including two kinds of iso-magnetic field processes. Now, the investigation of the Ericsson magnetic refrigeration cycle and refrigerator is starting.

RandD Project on HTS Magnets for Ultrahigh-Field MRI Systems

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

Optimization of operational parameters for a 4K-GM refrigerator

This paper describes the experimental results of a 4 K-GM refrigerator, which uses magnetic regenerator materials. The technical point of this study is ascertaining the effects of operational parameters for improving the 4.2 K refrigeration capacity. Compressor capacity and displacer stroke are main parameters in this study. A larger compressor enables a larger refrigeration capacity at a smaller temperature difference in the second regenerator. A smaller compressor, however, is preferable to maintain 4.2 K, when temperature difference is large. Displacer strokes of 12, 20 and 32 mm are investigated. The optimum reciprocating speed for the 4.2 K refrigeration capacity strongly depends on the displacer stroke. The largest refrigeration capacity, however, are almost equal for each of the strokes. In this study, the optimized stroke is found to be 20 mm to obtain refrigeration capacities at the second stage (4.2 K) and the first stage (40 K) simultaneously.

Design and Test Results of a Fault Current Limiter Coil Wound With Stacked YBCO Tapes

This paper describes the design and test results of a 6.6 kV-class superconducting fault current limiter (FCL) coil wound with YBCO tapes. YBCO tapes, with stainless-steel lamination, were prepared for an experimental FCL coil. The main feature of this FCL coil is winding of multiple YBCO tapes in parallel in order to increase the rated current. To obtain a rated current up to several hundred amps, four tapes, electrically insulated, were wound in parallel to form a non-inductive coil. The coil specifications, such as winding pitch, number of turns, and transpositions, were carefully designed by numerical simulation to equalize the current distribution in each tape. The fault current limiting performance of the FCL coil was evaluated through over-current tests using a capacitor bank. The FCL coil successfully suppressed a fault current of 10.4 kA to below 2.1 kA.

Design and Experimental Results of Three-Phase Superconducting Fault Current Limiter Using Highly-Resistive YBCO Tapes

As one of the programs in the Ministry of Economy, Trade and Industry (METI) R&D project on coated conductors, we developed a three-phase 6.6 kV superconducting fault current limiter (SFCL) and conducted some evaluation tests. The developed SFCL mainly comprised a set of three-phase current-limiting coils installed in a sub-cooled nitrogen cryostat with a Gifford-McMahon (GM) cryocooler, circuit breakers, and a sequence control circuit. Two tapes were wound in parallel in each limiting coil to obtain a rated current of 72 A rms. AC characteristics of each coil were measured, and relevant performance metrics were obtained. The whole system was installed in a cubicle. Short circuit experiments were then conducted with a short circuit generator. In a three-line ground fault test, the SFCL successfully restricted a short circuit current of over 1.56 kA to about 840 A with an applied voltage of 6.6 kV. The system integration ability and the obtained data show the promise of this approach for practical implementation. The SFCL was ready for user field tests.

Conduction Cooling Test of a Splittable Quadrupole for ILC Cryomodules

A superconducting splittable quadrupole magnet was designed at Fermilab for use in ILC-style cryomodules, in which the magnet is to be assembled around the beam tube to avoid contaminating the ultraclean superconducting RF beam volume. This quadrupole was built and first tested in a liquid helium bath environment at Fermilab, where its quench and magnetic performance were characterized. The device is intended to be cooled by conduction when installed in cryomodules, so a separate test was made at KEK where an appropriate conduction cooling test facility exists. We present results of the thermal performance of the magnet in the conduction cooling mode, and discuss its excitation characteristics in this operating mode.

Evaluation of Magnetic Field Homogeneity of a Conduction-Cooled REBCO Magnet with a Room-Temperature Bore of 200 mm

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