The REBCO-coated conductor is one of the promising technologies for Maglev application in terms of its high operating temperature. Since REBCO coils are directly cooled using cryocoolers, liquid helium is unnecessary. However, the coil structure must be optimized for efficient conduction cooling. We propose a novel REBCO coil structure that has high thermal conductivity and eliminates the delamination due to epoxy impregnation. A REBCO coil is wound with the coated conductor and a polytetrafluoroethylene (PTFE) tape for insulation. The coil winding is then impregnated with epoxy resin. At the same time as the impregnation, heat transfer members (e.g., copper plates) are bonded to the top and bottom surfaces of the coil winding. This paper contains three topics. First one is the thermal resistance measurements of the impregnation materials. It was observed that thermal stress increases the thermal resistances of paraffin wax and cyanoacrylate resin. It is believed that impregnation materials with high adhesiveness are appropriate for cryocooler-cooled coils. Second is demonstration of the novel coil structure. The third topic is the experimental production of a REBCO coil that has a congruent racetrack shape like that of the Maglev onboard magnet. It was confirmed that the novel coil structure eliminates degradation of the full-scale racetrack REBCO coil.
Conventional 4K GM (Gifford-McMahon) cryocoolers, while providing highly reliable cooling solutions for largescale superconducting applications such as magnetic resonance imaging (MRI) and superconducting magnets, encounter several difficulties including physical size and temperature reached when being fitted into a superconducting electronic device system. To create a suitable cooling solution for such devices, we developed a new, compact 2K GM cryocooler. The heat exchanger and regenerator were optimized using a numerical simulation method developed for 4K GM cryocoolers. Although development is still in progress, we successfully reduced the total length of the expander cylinder to about 240 mm. A series of performance tests of the prototype unit have shown that, under no-load conditions, the lowest temperature at the second stage reached about 2.1 K and temperature oscillation was less than ±21 mK. With a 1 W / 20 mW heat load, the temperature was 44.4 K at the first stage and 2.23 K at the second stage, which we believe shows sufficient cooling capacity for small-scale superconducting electronic device cooling. As future work, we will focus on a compact design for not only the expander cylinder, but also the compressor and drive system, and finally provide a complete compact cooling system solution.
We synthesized HoxEr1-xN (x = 0, 0.25, 0.5) sphere materials using the hot isostatic pressing (HIP) method and studied the regenerator performance by measuring the cooling power obtained by a 4 K-GM cryocooler loaded with them. At 4.2 K, we observed a cooling power higher, though slightly, than loaded with HoCu2 spheres, which have been the standard commercial regenerator material. A higher cooling power, 25% increment to the observed, is expected by removing the defects occurring in the present samples which have degraded the performance.