ac-Induced voltages in superconducting wires
Taylor, Henry Fuller
Houston, William V.
Master of Arts
The purpose of this work is to attempt an explanation of the energy loss in superconducting wires carrying varying current in terms of some theoretical model. In particular the model developed independently by Bean and by H. London (and henceforth referred to as the Bean model, since he published it first) is applied to predict voltages induced in a wire carrying alternating current. Bean's model, which prescribes the distribution of current in a sample as a function of present conditions and history, has been applied by other experimenters to accurately forecast the magnetization behavior of various non-ideal Type II superconductors. A basic premiss of the model is that the magnitude of the current density J is determined only by the magnitude of the local magnetic induction B. We have taken J(B) to be of the form deduced by Kim from magnetization measurements: We have also assumed that the depth of current penetration is much less than the wire's radius. Relations for voltage as a function of current are derived for ac and ac superimposed dc. Theoretical V vs. I curves are found to agree fairly well with oscilloscope tracings obtained for a defectsaturated Nb wire. Before each observation a canceling coil was set to make the measured voltage V= 0 for small alternating currents. We assume that these small currents are confined to the wire's surface, so that B 0 within the wire. If this is correct, then for large ac amplitude where wire is the flux contained within the material of the wire. The relation V has been assumed for the theoretical derivations.