Nb3Sn: Macrostructure, microstructure, and property comparisons for bronzeand internal Sn process strands

The variation in irreversibility field, B*(T), with temperature has been me asured for Nb3Sn superconducting strands manufactured for ITER using vibrat ing sample and SQUID magnetometers. The high performance strands were devel oped for both high transport critical current density, J(c), and low hyster esis loss. Despite a wide variety of designs and components, the strands co uld be split into two distinctive groups, based on the extrapolated irrever sibility 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 intrins ic critical current density of the superconductor, J(c(sc)), and the specif ic pinning force of the grain boundaries, Q(gb), were evaluated using the m easured J(c), and image analysis of the macro- and micro- structures. A bro nze-processed Nb(-Ta)(3)Sn was found to have a higher J(c(sc)) but lower Q( gb) than Nb3Sn produced from Nb filaments. This work shows that the maximiz ation of J(c) is both an intrinsic flux pinning issue and a quantitative is sue of how much Nb3Sn can be put into the composite package. The results fo r the ITER strands are compared to a high J(c) (but also high hysteresis lo ss) internal Sn strand. The high J(c) strand had a much higher J(c(sc)) and Q(gb) than any of the ITER strands.