Time constant measurement in technical superconductors

Time constant measurements are a useful method for characterising technical superconductors. The interpretation of measured pickup voltages may lead, however, to misleading results, because the signals are composed of an infi nite number of exponential functions. In time constant experiments coupling currents are induced in the sample by an external field variation. The com plete theoretical solution of the time dependence of induced voltage signal s is given using a slab model approach. We give explicit solutions for line ar, exponential and sinusoidal excitation fields. Experiments with a copper slab, with a stack of superconducting wires, with a high T-c tape, with a flat copper bar and with a Rutherford type cable were performed. The result s were found to be in good agreement with the calculation. We further discu ss the optimum conditions for the choice of parameters in a time constant m easurement. (C) 1998 Elsevier Science Ltd. All rights reserved.