Transient losses in superconductors

Under steady state conditions, there are no losses in superconducting wires. However, when subjected to alternating or transient magnetic fields or transport currents, losses in type II superconductors can become significant. This report deals with hysteresis losses at 4 K measured by magnetization and complex magnetic susceptibility. The theoretical and experimental relationships between ac susceptibility and magnetization as functions of dc field were examined in terms of the critical state model. Minor loop hysteresis loss was shown to be obtainable by direct measurement of loop area, from the imaginary component of ac susceptibility, and from the reversible susceptibility plus the critical current density or full-penetration field. Hysteresis and transport losses measured simultaneously in a Nb-Ti superconducting coil were found to agree substantially with the predictions of Minervini's two dimensional model. Hysteresis loss measurements in a series of fine filament Nb3Sn superconductors showed that the effective filament diameter is a function of interfilament separation and local area ratio of matrix material to Nb. A review of internal fields in superconductors showed the importance of demagnetization factors and internal fields for the correct analysis of magnetic data. A theoretical method of calibrating ac susceptometers for cylindrical specimens, which is based on a mutual-inductance calculation, was developed.