Influence of the heat treatment of internal tin Nb 3Sn strands on the critical current and the magnetization losses

Abstract

Internal tin strands for the ITER Toroidal Field Model Coil have been systematically heat treated to form Nb 3Sn. Resulting superconducting cross-sections and elementary distribution were monitored by SEM and EDX. Variations of the heat treatments with respect to annealing conditions were reflected in the field dependence of the critical current values I c( B). Measurements were performed in fields up to 12 T at 4.2 K. Also hysteresis and coupling losses of the same wires have been measured at 4.2 K and ramp rates up to 500 mT/s. From these data variations of various critical parameters ( I c0, B c2) as well as of the effective time constant nτ of the strand could be deduced.