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 (R-j) 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 similar to 0.47 mu H. Thus, measurement time, i.e., times to measure the current are similar to 1 minute and similar to 1 hour for R-j of 10(-9) Omega and 10(-12) Omega, respectively. Using this system, performance of the joint with resistance ranging 10(-14)-10(-8) Omega 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) Omega at 4.2 K with sample current up to similar to 350 A.