Dynamics of flux motion and ac losses in the superconducting sheath state

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Abstract
The temperature and rate dependence of the penetration of flux through the superconducting sheath is studied in the Pb–In alloy system. The waveform of the response of the sheath to a small sinusoidal magnetic field is investigated over temperature and frequency domains from 0.15Tc to Tc and 50 Hz to 1 kHz, respectively. The measured waveform is discussed in terms of a simple flux‐creep model, a viscous flow model, and a combined flux‐creep—flux‐flow model. The flow model assumes a distribution of pinning forces and an effective viscosity coefficient η, both of which are taken to be independent of frequency. No distribution of pinning forces is considered in either the flux‐creep or combined models. Computer analysis is employed to fit the models to the observed waveforms and to study the variation in the model parameters as a function of frequency and temperature. The simple flux‐creep model does not lead to the observed frequency dependence of the sheath response, while the flow model and the combined creep‐flow model are more satisfactory. However, the viscosity coefficient, η, of the latter models is found to decrease with increasing frequency.