Nb/sub 3/Sn: macrostructure, microstructure, and property comparisons for bronze and internal Sn process strands

The variation in irreversibility field, B*(T), with temperature has been measured for Nb/sub 3/Sn superconducting strands manufactured for ITER using vibrating sample and SQUID magnetometers. The high performance strands were developed for both high transport critical current density, J/sub c/, and low hysteresis loss. Despite a wide variety of designs and components, the strands could be split into two distinctive groups, based on the extrapolated irreversibility fields, which lie about 10% lower than the upper critical field. "Bronze-process" strands exhibited consistently higher B*(T) (28 T to 31 T) compared with "internal Sn" process (24 T to 26 T) conductors. The intrinsic critical current density of the superconductor, J/sub c(sc)/, and the specific pinning force of the grain boundaries, Q/sub gb/, were evaluated using the measured J/sub c/, and image analysis of the macro- and micro-structures. A bronze-processed Nb(-Ta)/sub 3/Sn was found to have a higher J/sub c(sc)/ but lower Q/sub gb/ than Nb/sub 3/Sn produced from Nb filaments. This work shows that the maximization of J/sub c/ is both an intrinsic flux pinning issue and a quantitative issue of how much Nb/sub 3/Sn can be put into the composite package. The results for the ITER strands are compared to a high J/sub c/ (but also high hysteresis loss) internal Sn strand. The high J/sub c/ strand had a much higher J/sub c(sc)/ and Q/sub gb/ than any of the ITER strands.