Angular Dependence of Critical Currents in Plasma-Plated Nb3Sn Strip Superconductors

Abstract

In many instances it appears that the limiting performance of a superconductive device is determined by low-field instabilities rather than by the high-field current density. If the device is made of flat-strip superconductors, a further complication may arise from the fact that in the low-field regions the magnetic field usually is oriented at some nonzero angle to the surface of the strip. To see how these conditions affect the performance of plasma-plated superconductors, we have measured critical currents in flat strips of Nb3Sn in transverse applied field, varying the angle θ between the surface of the strip and the magnetic field. When transport current is applied first and the field increased until the specimen undergoes a normal transition, the relationship Hc(IT) = H0(IT)+H1(IT) cos2θ is roughly obeyed, where IT is the transport current and H0 and H1 are found to be of comparable magnitude. When the magnetic field is applied first and the transport current increased to its critical value, virtually no angular dependence is observed. We tentatively explain this result by describing the plasma-plated material as a matrix of miniature hollow cylinders aligned with the magnetic field. Because of the anisotropy of the plating process the effective wall thickness of the ``cylinders'' varies with the specimen orientation. When the field is applied first, the transport current can find paths around these ``cylinders'' which will not result in locally exceeding the critical current density of the material until nearly all the specimen is critical. However, when the transport current is applied first, the additional shielding currents induced by the magnetic field are forced to travel in paths which are already carrying transport current, and locally the critical current density may be exceeded, precipitating a normal transition before the entire specimen is critical. Further tests are being carried out to determine the effect of heat treatment and strip dimensions on this angular dependence of the critical current. These results should discriminate between our proposed explanation and the possibility that the effect is simply a demagnetization phenomenon.