Critical current degradation of a NbTi-multifilament conductor due to heat treatment

In the framework of the development of superconducting coils for the stellarator Wendelstein 7-X, the reduction of the critical current due to thermal treatment was investigated. During production steps like the extrusion of the Al-jacket onto the superconducting cable and also during welding and soldering, heat flux into the NbTi-strands occurs. For optimized NbTi-strands a degradation of the critical current was expected. The results of measurements of the critical current of an annealed NbTi multifilamentary superconductor at 4.2 K in the field range from 2 T to 10 T are presented. The commercially available conductor has been annealed at various temperatures up to 600°C and times up to 120 s in order to simulate the conditions during such production steps. Special attention is paid to the shape of the voltage-current transition which is characterized on the one hand by a power law and on the other hand by an exponential function