HTS coils for applications such as superconducting magnetic energy storage (SMES), rotating machines, and magnetic resonance imaging (MRI) are wound with electrically insulated HTS wires, because in such applications, coil currents need to be changed rapidly and frequently. However, coils wound with insulated wires, especially YBCO coated wires with high current density, are easily damaged by quenches, and it is necessary to take proper measures to prevent such damage. Once the coil is damaged, its quenching current is degraded and it cannot be reused. The most probable cause of quench damage is overheating at a hot-spot of the coil wire during the quench protection sequence. When hot-spot temperature THS (the highest temperature of the coil wire during the quench protection sequence) exceeds a certain limit THSs, the coil is damaged. THSs together with THS are dependent on the impregnation methods. HTS coils are impregnated usually by epoxy resin to make the coil mechanically solid and improve thermal conduction between the wire layers. It was shown by the authors group that THSs of YBCO epoxy-impregnated coils is around 300 K and much lower than the temperature around 500 K where the critical current of the YBCO wire is thermally degraded. The reason for this is considered that the YBCO wires of the coil are damaged mechanically by local thermal expansion and excess compressive stress causing local bending of the wires at the hot-spot area.
The authors group are investigating property of ionic liquid as impregnation material of the HTS coil. The ionic liquid is in solid state at cryogenic temperature and well fixes the HTS wires and has high thermal conductivity compared with epoxy. The ionic liquid in the solid state melts at the hot-spot area of the coil, and it is expected the stresses to the wires caused by temperature rise at the hot spot are released. Therefore, the HTS coil impregnated with the ionic liquid is expected to have good durability against the quench damage. In this study, quench protection characteristics of are experimentally investigated using YBCO small scale test coils impregnated by ionic liquid and are compared with those of an epoxy-impregnated coil.
This work was supported by JSPS KAKENHI Grant Number 22K04075.