Variable mutual inductance in critical-current measurements

We have investigated the mutual inductance observed on pairs of voltage taps that are soldered to superconducting wires in order to measure their voltage-current, V-I, characteristics and determine their critical current. Measurements on a Nb-Ti superconductor are reported that are consistent with measurements on other Nb-Ti and Nb3Sn wires. The mutual inductance varies systematically with current, sweep direction, magnetic field, and whether the specimen was quenched in the previous measurement. Under certain conditions, the mutual inductance is independent of current ramp rate. The decay time of the inductive voltage signal, after ending the current ramp, is longer for the first current sweep after the specimen was quenched. This longer decay time can cause voltage errors even in a ramp-and-hold method for acquiring the V-I characteristic. The proposed mechanism for variable mutual inductance is magnetic hysteresis and a variable rate of flux entry due to shielding currents that change with transport current. Variable mutual inductance becomes a concern in superconductor characterization when currents or current ramp rates are high or when low voltage curves need to be extrapolated for measurements on marginally stable conductors.