Overcurrent Test of High-Temperature Superconducting Coils With Liquid Hydrogen Immersion Cooling

The operating temperature of a high-temperature superconducting (HTS) coil is desired to be 15-40 K from the viewpoint of specific heat of the material and the operating current. Development of relatively small-capacity coils with gas helium cooling or refrigerator cooling in this temperature range is underway. Here, refrigerant cooling is desirable for large-capacity coils and liquid hydrogen, which has 20.7 K atmospheric pressure saturation temperature, is a candidate refrigerant. However, due to the difficulty of handling hydrogen, there have been few studies on the HTS coil cooled by liquid hydrogen. And in the HTS coil, the heat generated in the normal conducting region causes a chain of temperature rise and decrease of the critical current, which leads to an irreversible increase in coil temperature beyond the balance with the cooling conditions, and so-called thermal runaway is a problem. In this study, we report on the observation of thermal runaway phenomena in Bi2223 coils under liquid hydrogen immersion cooling. The coils were energized twice at saturated condition of 950 kPa pressure and 30 K temperature. We observed thermal runaway in both energizations and the coil tap voltage before thermal runaway was several tens of mV. From this test, it could be said that the liquid hydrogen cooling has a potential to protect HTS coils.