Magnetic losses in superconductors at high frequencies

Magnetic losses in superconductors at radio frequencies are analyzed in terms of a simple model for energy dissipation at local flux-trapping centers. A mathematical development shows that the effective surface resistance is proportional to frequency, the field penetration depth, the density of trapping sites, and a threshold function of the maximum RF field. Experimental data are obtained through RF pulse measurement of the quality factors of superconductive resonant circuits, having resonant frequencies near 100 Mc/s. Decay curves for the energy output of these circuits show a marked break or increase in quality factor at low-field levels. This is taken to correspond to the local critical field predicted in the model. The abruptness of the break leads to the conclusion that all of the trapping sites are of a similar nature, and are probably Abri-kosov fluxoids. If so, these measurements constitute the first direct observation of the transition of the fluxoid core.