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Variable mutual inductance in critical-current measurements



Loren F. Goodrich, Theodore C. Stauffer


We have investigated the mutual inductance observed on pairs of voltage tapes that are soldered to superconducting wires in order to measure the voltage-current (V-I) characteristics & determine the critical current. Measurements on a Nb-Ti superconductor are reported that are consistent with measurements on other Nb-Ti & NB3Sn wires. Under given conditions, the mutual inductance is independent of current ramp rate. However, the mutual inductance varies systematically with current, sweep direction, magnetic field, & whether the specimen was quenched in the previous measurement. In addition, 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 & variable rate of flux entry due to shielding currents that change with transport current. Variable mutal inductance becomes a concern in superconductor characterization when currents or current ramp rates are high or when V-I curves need to be extrapolated to higher voltages for measurements on marginally
Advances in Cryogenic Engineering


critical current, hysteresis, mutual inductance, Nb-Ti, ramp rate, voltage taps, voltage-current characteristic


Goodrich, L. and Stauffer, T. (2002), Variable mutual inductance in critical-current measurements, Advances in Cryogenic Engineering (Accessed June 15, 2024)


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Created December 31, 2001, Updated October 12, 2021