Characteristic current degradation of commercial NbTi-multifilamentary superconductors due to annealing

In the framework of the development of superconducting coils for the stellarator physics experiment Wendelstein 7-X (W 7-X) the reduction of the critical current, better defined as characteristic current due to annealing was investigated. The characteristic current level was defined using the 0.1 μV/cm criterion. The technical superconductor for the magnets of this experiment is a cable-in-conduit conductor with a co-extruded Al-alloy jacket. During the co-extrusion of this jacket onto the superconducting cable heat influx to the NbTi-strands occurs. This is also true for subsequent working steps like welding, brazing, and soldering. Therefore, a degradation of the characteristic current is expected for optimized NbTi-strands. In order to clarify the behaviour of the NbTi-strands at temperatures which occur during the co-extrusion process and/or the working steps mentioned above a study was performed where the annealing temperature and annealing time was varied, the temperature up to 600 C and the time up to 120 s. The NbTi multifilamentary superconductor is commercially available. The results of the measurements of the characteristic current at 4.2 K in the field range from 2 T to 10 T are presented. A generalization of the results is made in order to point out the tendencies of the degradation of the characteristic current. These results were used to predict the behaviour of the W 7-X conductor cable itself. (orig.)