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 REBCOcoated 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 x 10-12 omega m2. 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.