Development of a Superconducting Joint Resistance Evaluation System

We have developed an evaluation system for superconducting and/or ultra-low resistance joint resistances. The system evaluates the joint resistance (Rj) by a principle of a current decay in a LR closed circuit. To minimize the measurement time, a one-turn closed-loop coil, i.e., both ends of the conductor are joined together, is used for a sample. Typical inductance of the one-turn loop with a diameter of 100 mm is ~0.47 μH. Thus, measurement time, i.e., times to measure the current are ~1 minute and ~1 hour for Rj of 10
^-9
Ω and 10
^-12
Ω, respectively. Using this system, performance of the joint with resistance ranging 10
^-14
-10
^-8
Ω can be quantitatively evaluated as a function of injected current magunitude and temperature. Resistances of the Nb-Ti samples that were joined using Pb-Bi solder were successfully evaluated to be in the range of 10
^-14
-10
^-13
Ω at 4.2 K with sample current up to ~350 A.