Influence of Cr plating on the coupling loss in cable-in-conduit conductors

The coupling current loss of Nb3Sn based Cable-In-Conduit (CIC) conductors can be controlled by the interstrand resistance (R-c) with a Cr coating on the strand surface. The R-c determines the amount of coupling loss that is a driving factor for the design of large cabled conductors operating under a time varying field, like the conductors for the ITER fusion project. Nine CIC specimens have been compared which are fully identical except for the Cr plating of the strand surface. The coupling losses of the 48 strands CIC specimens, can change up to a factor of four depending on the vendor of the Cr plating. The measurement results of coupling losses backed by direct interstrand R-c measurement, show a clear correlation between loss and Cr vendor, however, the quantitative correlation has a margin of error. In addition, the statistical piece-to-piece scattering in coupling loss, the possible influence of the Cr vendor on the value of the RRR and the dependence of the sample length on the R-c were s(c) studied. Moreover the strand crossovers are examined by micrography and surface roughness tests.