Recent Progress in Compact, Robust, and Superior Field-Tolerant QMG Current Leads Using RE–Ba–Cu–O Bulk Superconductors

High-temperature superconducting (HTS) bulk materials of RE-Ba-Cu-O (RE: Y or rare-earth elements) exhibit excellent Jc properties, even at 77 K, leading to current leads with large current capacity and superior magnetic field tolerance. The thermal conductivity of HTS bulk materials is two orders of magnitude smaller than that of the conventional current lead material of copper, and Dy-Ba-Cu-O has particularly low thermal conductivity in the RE-Ba-Cu-O family. Unlike HTS wires, HTS bulks have no metal sheath such as silver or silver alloys with high thermal conductivity, resulting in the reduction of heat leak through the current lead, even in the compact size. Although they are a kind of brittle ceramic or oxide, HTS bulks can be transformed into robust current leads by reinforcing them with the support cover made of glass-fiber-reinforced plastics with low thermal conductivity and high strength. In addition to excellent mechanical stress tolerance, HTS bulk current leads also have a robust structure that minimizes thermal stress, which was experimentally demonstrated by a rapid cooling test to liquid nitrogen over 100 cycles. Due to distinctive advantages such as compactness, robustness, and superior field tolerance, more than 500 pieces of HTS bulk current leads have been successfully provided so far.