Ac-loss and time constant measuring techniques of high-Tc tapes and stacks of tapes

Abstract

The article gives an overview of ac-loss and time constant measuring techniques of superconducting tapes and stacks of tapes at 77K. The widely used measuring technique is based on the measurement of the magnetisation loop, which is proportional to the dissipated energy. Since the measured signal depends on the geometry of the sample and of the pickup coil, calibration of the system is necessary for a precise measurement. The calibration technique is discussed. Induced coupling currents in a filamentary superconductor decay after the end of field change with a time constant τ. This coupling current time constant gives information about internal properties of the sample, e.g. about the transverse matrix resistivity within the superconducting composite, and it is a measure for the expected coupling losses. Furthermore the interfacial resistivity between filaments and matrix which may greatly reduce the coupling losses can be calculated from the measured time constant. In special cases, a second (shorter) time constant can be detected which reflects the decaying eddy currents in the normal conducting matrix. This allows the separation of the loss contributions of filamentary section and matrix. The shape of the induced pickup voltage during a magnetic field pulse allows calculating the effective permeability of the conductor.