Repeatability of critical-current measurements on Nb/sub 3/Sn and Nb-Ti wires

A varying degree of repeatability has been observed in critical-current (I/sub c/) measurements of Nb/sub 3/Sn and Nb-Ti wires as a function of the number of thermal cycles from room temperature to 4 K. The increase of I/sub c/ between the first and second thermal cycle can be 1% to 2% at 12 T for Nb/sub 3/Sn wires. This was observed on Nb/sub 3/Sn wire by all four laboratories that participated in a recent interlaboratory comparison conducted in the International Thermonuclear Experimental Reactor (ITER) project. These data indicate that if I/sub c/ changes beyond the error limits, it increases fairly monotonically with thermal cycling until it eventually saturates. In contrast, the I/sub c/ of a Nb-Ti wire is very repeatable with thermal cycling. This suggests that the effect on the Nb/sub 3/Sn wire is due to its strain sensitivity. Most of these data were taken with the sample on a Ti-6Al-4V measurement mandrel. This study also investigated the repeatability of I/sub c/ measurements using other mandrel materials. The increase in I/sub c/ of Nb/sub 3/Sn wire could enhance the performance of some applications. However, the lack of repeatability in I/sub c/ measurements on Nb/sub 3/Sn wires is a limitation in precise interlaboratory comparisons.