Interstrand Resistance Measurements on the Conductor Terminations of TFPRO2 and JATF3 SULTAN Samples

A number of ITER TF cable-in-conduit samples that have been tested in the SULTAN facility performed significantly below the single strand expectations. Although performance degradation related to transverse electromagnetic loads plays a role in Nb₃Sn CICCs, a similar effect can occur when the current in the sample is non-uniformly distributed among the strands, driving part of the strands into saturation already at low currents and temperatures. A sufficiently homogeneous current distribution at low electric field level (less than 10 ??V/m) requires low interstrand resistances in the sample terminations and/or a homogeneous distribution of contact resistances between strands and the joint´s copper sleeve. To evaluate the influence of the joint design on qualification testing, we performed post-mortem interstrand resistance measurements on the conductor terminations of two different joint design concepts: the TFPRO2 and JATF3 SULTAN samples. The interstrand resistances and their distribution were determined on a large number of strand combinations. Accordingly, comparisons were made on how the sample performed in SULTAN. From the different design aspects, full solder filling of the terminations appears to have the strongest effect on the interstrand resistance reduction.