Flux pinning for In situ Nb 3Sn superconducting wire

Abstract

Critical currents and upper critical fields have been evaluated for in situ prepared Nb 3Sn-Cu multifilamentary superconducting wire in order to determine which aspects of filament morphology are most important for flux pinning. Wire specimens were prepared with a wide range of Nb 3Sn filament sizes as controlled by the as-cast dendrite size, the degree of wire drawing and the degree of filament coarsening. Tape specimens were examined as a function of orientation in the magnetic field. The Nb 3Sn filament and grain morphology were examined with SEM and TEM techniques. In all cases the pinning force was found to follow an h ∗ 1 2 (1-h ∗) 2 dependence where h ∗ is the reduced magnetic field, H/H ∗ c2 and the data consistently yielded parallel lines on a Krämer plot. Neither the perfect lattice shear model of Kramer nor the surface pinning model of Dew-Hughes adequately describe in situ data. Analysis of the critical current results and the Nb 3Sn filament and grain morphology indicates that flux pinning in these in situ materials is not dominated by either the Nb 3Sn/Bronze interfaces or the Nb 3Sn grain boundaries, but is probably due to a combination of pinning at both interfaces.