Theory of Flux-Flow Noise Voltage in Superconductors

Abstract

A theory of the time-dependent measured voltage and its associated power spectrum is developed for conditions of magnetic flux flow in the mixed and intermediate states of superconductors. Expressions for the measured voltage, appropriate for flux flow, are given and discussed. The case of a semi-infinite super-conductor containing a single moving fluxoid is then examined as an illustrative model, and potentials and fields are calculated from a London-model description. The resulting time-dependent voltage pulses are computed and shown to depend upon the spatial configuration of the measuring circuit leads. The measured time-dependent flux-flow voltage and the power spectrum arising from an array of fluxoids are formulated and found to depend upon the measuring-circuit configuration, the internal flux distribution, the effects of pinning, and the modes of flux motion. Previous experimental results by van Gurp are interpreted as lending support under certain conditions to a description in terms of flux-line dislocation dipoles proposed by Kramer. New experiments are suggested as tests of the theory.