Abstract
Low level A. C. loss mechanisms in superconducting niobium strips, set back-to-back and carrying antiparallel current at either 30 Hz or 120 Hz, have been studied. Compensated waveforms, corresponding to the time rate of change of flux penetrating in from the inner surfaces of the strips, far from the edges of the strips, of the voltages induced in a nearby pick-up coil were obtained for various conditions of the bulk and inner surfaces of the strips. The results were corrected for edge effects which were examined experimentally and theoretically. The physical state of the strips was examined by various means and used in interpreting the results. No significant difference was found in the behaviour of the flux penetration at the two frequencies. The results were used to make a quantitative test of a theory, proposed by Melville 1, which attributes low level A.C. loss in type II superconductors to surface roughness combined with flux pinning by defects within the peaks of the surface, and of an extension of the calculations based on this theory dealing with flux penetration into the region below the surface roughness. It was found that the theory gave a reasonable semi-quantitative account of the main features of flux penetration particularly for abraded surfaces. However, the theory did not predict the single peaks per half cycle of compensated waveform that were observed experimentally at low current amplitudes. An investigation was carried out to ascertain the reason for these single peaks. It was concluded that possibly fairly sharp peaks in the inner surfaces of the strips, protruding above the rest of the surface, were involved in causing these single peaks per half cycle of compensated waveform.