Effect of Copper Resistivity and Filament Size on the Self-Field Instability of High-Jc Nb3Sn Strands

Nb3Sn strands with large filaments and high-J(c) exhibit instabilities due to magnetization flux-jumps at low fields in changing magnetic fields. In addition, at intermediate fields of 5 to 7 T, these strands quench prematurely at currents well below the critical current. Current-voltage measurements are typically used for critical current determinations, and the premature quenching observed is driven by current redistribution within the strand as the current is increased and is termed self-field instability. This instability is exacerbated as the temperature is lowered from 4.2 K to 2 K superfluid helium. A previous study examined wires in the quasi-adiabatic limit, where dynamic heat transfer mechanisms are suppressed. In this paper, we report on measurements in the temperature range of 4.2-2 K on high-J(c) RRP strands with varying copper stabilizer resistivities and Nb3Sn filament diameters. These measurements show that the residual resistivity ratio, RRR, of the copper stabilizer plays an important role in mitigating this instability. Also for strands with similar RRR, we find that the stability improves with decreasing filament diameters, although the improvement is not very dramatic.