Temperature-dependent AC loss and time constant measurements in high- Tc superconductors

Abstract

A technique is described which allows an easy and precise loss measurement of superconducting high- T c tapes in an alternating magnetic field in the temperature range between 4.2 K and the critical temperature (∼115 K for Bi tapes). It combines a calorimetric measuring technique with a magnetisation measurement. While the magnetisation measurement is faster and more sensitive at 77 K, the calorimetric measurement which is performed in the same measuring run allows an exact calibration. A field orientation parallel or perpendicular to the broad face of the tapes can be chosen without warming up the sample. Measurements of coupling current decay time constants are performed in the same device. Coupling currents are induced in the tapes by exponential field decay or by half-cycle sinusoidal magnetic field pulses. Time constants can be measured over a wide range between ∼20 μs and hundreds of milliseconds. Measured voltage traces are well described by a theoretical model, if an average effective permeability μ ef is used as a fit parameter. This procedure therefore allows determination of the sample μ ef values.