Flux Trapping of rf Fields in Superconductors

Abstract

Measurements on superconductive resonant circuits in the frequency range of 10-1000 Mc/sec indicate that flux trapping of the rf magnetic fields is responsible for a residual loss, not predicted by surface-resistance theories. A simple model, based on flux trapping at local sites, predicts such a loss for both type I and type II materials. The surface resistance due to flux trapping is shown to be proportional to frequency, the density of trapping centers, the penetration depth, and a "threshold" function of the rf magnetic field. Experimental data are given for several materials, verifying the salient features of the model. It is also concluded that the flux-trapping centers are of a common nature.