Dependence of AC loss in superconducting niobium above and below H_c1on surface and bulk condition

Abstract

Waveforms of rates of change of flux have been recorded over a wide range of fields above and below Hc1 in superconducting niobium strips with a variety of degrees of surface roughness. The waveforms are compared with those predicted by Melville's theory (1971-2) that attributes AC loss below Hc1 to pinning within the peaks, and derived losses are also compared. Although the predicted double maxima of the waveforms per half cycle are observed at most fields, their development near Hc1 is more gradual than expected, and at fields well below Hc1 single maxima are observed that appear to be associated with discrete protuberances. It is confirmed that dissolved oxygen is liable to be a more serious practical cause of high loss than moderate roughness.