Time constant measurement (decay of coupling currents) in high T/sub c/ tapes

Abstract

Coupling currents in high T/sub c/ Bi-2223 tapes were induced by a fast change of an external magnetic field. The measurements were performed using exponential field decays and half cycle sinusoidal excitation fields. The induced coupling currents, decaying after the end of the field variation, are detected using a pick-up coil surrounding the sample and a compensation coil. The measurements were performed at 77 K in perpendicular external field. An increase of the measured time constant /spl tau/ with the amplitude of external field variation was found. A theory presented recently based on a slab model describes the measured voltage traces reasonably well, if an average effective permeability /spl mu//sub ef/ is used as a fit parameter. It approaches unity for large external field amplitudes. /spl tau///spl mu//sub ef/ was found to be constant as expected. Time constants deduced from ac-loss measurements in the low frequency range are in good agreement with the directly measured values.