The onboard superconductive magnets of levitated railway systems are required to have reliability and high-level durability as well as a lightweight construction and energy-saving features. The use of high-temperature superconductive wires in manufacturing superconductive coils will enable them to be directly cooled to operating temperature by refrigerators without using liquid helium. This will cut the weight of coils and save power consumption by onboard refrigerators. The RTRI therefore selected a yttrium-based wire (which potentially features low cost and has excellent magnetic field characteristics) from among several high-temperature superconductive alternatives, and discussed its application to onboard superconductive magnets in levitated railways.
Using the characteristics of the yttrium-based selection (a rare earth-based wire), the RTRI analyzed the masses of coils and refrigerators at different cooling temperatures, and found that there was a cooling temperature that would minimize the total mass of coils and refrigerators (Fig. 1). The introduction of high-temperature superconductive wire is expected to realize a reduction of about 40% both in the total mass of superconductive coils and refrigerators and in the power consumption of refrigerators.
After manufacturing a small prototype superconductive coil (Fig. 2), the RTRI subjected it to a current-carrying test. The current-carrying capacity of the coil was found to be equal to that of the wire itself, demonstrating that there were no problems in coiling. By repeating this test at different temperatures, the RTRI plans to determine the most appropriate cooling temperature and develop a small superconductive magnet housed in an adiabatic vessel.