Thermal stability of Nb3Sn rutherford cables for accelerator magnets

Abstract

In large scale superconducting applications, like bending magnets in particle colliders, thermal stability is an important issue. A relatively small perturbation of about 100 µJ in a single point can create a small normal zone in the superconductor, which will experience sever joule heating. If the heating exceeds the cooling the normal zone will expand, and vice versa it will collapse. The thermal stability of a superconductor defines the amount of thermal energy required for a point disturbance to initiate an irreversible transition to the normal state. There is little empirical data available on this for the current generation Nb3Sn Rutherford cables. This work aims to provide an overview of the thermal stability in such cables. With the aid of point heaters, the thermal stability is probed at various positions on the cable surface under various operating conditions. It is shown that regardless of the high current density and poor cooling, these cables can recover from a quenched strand by current redistribution when the test conditions are far from the cable limitations. This transition point, where the cable can recover from a single normal strand, is determined for test current and applied magnetic field as close as possible to the typical operational conditions of a magnet coil within the experimental window. In the center part of a Rutherford cable, the thermal stability is significantly better than at the cable edges. This is due to a reduced RRR and degraded critical current at the thick and thin edges of the cable. The thermal stability decreases when Nb3Sn is operated at a lower temperature, i.e. 1.9 K. Two numerical models and one analytical model are compared to the empirical observations. It is found that none of the models predicts all features of the thermal stability curves with high accuracy.