Splice joint resistances of commercial REBCO-coated conductors and their reduction

The applications of high-Tc superconducting magnets comprising REBa2Cu3O7-x (REBCO)-coated conductors, such as nuclear magnetic resonance instruments, high-field magnets, high-energy particle accelerators, and industrial apparatuses, are being actively developed worldwide. A few of them are conduction-cooled magnets and are operated at temperatures above the critical temperature of low-Tc superconductors. To reduce the heat loads of cryocoolers and improve their energy efficiency, low resistance and handy joint technologies for REBCO-coated conductors are required. We evaluated the resistances of soldered joints of commercial REBCO-coated conductors in liquid helium. We soldered REBCO-coated conductors manufactured by Fujikura Ltd. and SuperPower Inc. over a silver protective layer using superconducting Pb0.45Bi0.45In0.1 solder. The joint resistance was dominated by the interfacial resistance between REBCO and the silver protective layers and was estimated to be 3 × 10−12 Ωm2. Further, a drastic reduction in the joint resistances could be realized by improving the electrical connectivity between REBCO and the silver layers. Assuming a conduction-cooled high-Tc superconducting magnet, we demonstrated quasipersistent operation of a loop specimen of REBCO-coated conductors with a soldered joint using two Gifford-McMahon cryocoolers. The exponential decay of induced current in the loop specimen was observed with a decay time constant that was dependent on the joint resistance.