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

Coupling currents in high T-c Bi-2223 tapes were induced by a fast change o f an external magnetic field. The measurements were performed using exponen tial field decays and half cycle sinusoidal excitation fields. The induced coupling currents, decaying after the end of the field variation, are detec ted using a pick-up coil surrounding the sample and a compensation coil. Th e measurements were performed at 77 K in perpendicular external field. An i ncrease of the measured time constant tau with the amplitude of external fi eld variation was found. A theory presented recently based on a slab model describes the measured voltage traces reasonably well, if an average effect ive permeability mu(ef) is used as a fit parameter. It approaches unity for large external field amplitudes, tau/mu(ef) was found to be constant as ex pected. Time constants deduced from ac-loss measurements in the low frequen cy range are in good agreement with the directly measured values.