Comparison of avalanche-like quenches between NbTi and Nb 3Sn cables

Abstract

Superconductors carrying currents larger than several 10 kA are required for magnetic confinement fusion systems. A cable-in-conduit-conductor (CICC), which has about 1000 of twisted thin superconducting strands in a ridged conduit, is a promising candidate for such large and high magnetic field superconducting system. A specific phenomenon in the CICC is so called avalanche-like quench (AQ). When some strands become resistive due to current saturation at critical current and/or a mechanical disturbance, the current re-distributes from the strands into neighboring ones, which are electro-magnetically coupled more strongly than the others. This causes current saturation in the neighboring strands. Finally, the whole cable becomes normal. We call this phenomenon an avalanche-like quench. Small multi-strand cables were tested by induced-current methods. Quench behaviors of both Nb 3Sn and NbTi cables are observed and compared. In the experiment, for the Nb 3Sn cable, the critical current measurement was stably carried out at high field without quench. In contrast, for the NbTi cable, we observed a number of AQs during inductive energization. Consequently, we suspected that the reason is based on the difference of critical temperature ( T C) between Nb 3Sn and NbTi. The difference of T C is several Kelvins, however, their macroscopic behaviors differ significantly.