Repeatability of critical-current measurements on Nb3Sn and Nb-Ti wires

A varying degree of repeatability has been observed in critical-current (I-c) measurements of Nb3Sn and Nb-Ti wires as a function of the number of thermal cycles from room temperature to 4 K. The increase of I-c between the first and second thermal cycle can be 1% to 2% at 12 T for Nb3Sn wires. This was observed on a Nb3Sn 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-c changes beyond the error limits, it increases fairly monotonically with thermal cycling until it eventually saturates. In contrast, the I-c of a Nb-Ti wire is very repeatable with thermal cycling. This suggests that the effect on the Nb3Sn 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-c measurements using other mandrel materials. The increase in I-c of Nb3Sn wire could enhance the performance of some applications. However, the lack of repeatability in I-c measurements on Nb3Sn wires is a limitation in precise interlaboratory comparisons.