a.c. losses in filamentary superconductors

Experiments have been undertaken to measure calorimetrically losses in small coils of superconducting multifilamentary composite conductor subjected to small alternating fields between 5 and 50 Hz superimposed upon a constant bias. It is found that the currents induced in the outer layers screen the wire and the loss is mainly due to these currents. The variation of the loss with the bias, hitherto a somewhat neglected subject, is found to be considerable, with losses increasing greatly as the bias is increased, together in one case with increasing losses when bias field strength is reduced below 1 Tesla. Comparison with losses calculated using the best available theoretical models show considerable agreement; with the exception of the increased losses at low bias fields agreement generally to within 20% of measured results has been obtained. It is found that the magnetoresistance of the matrix material is the most significant factor in determining the dependence of losses upon the bias, leading at high fields to losses more than twice as great as would otherwise occur. (author)