Irregular AC losses with long time constants in large cable-in-conduit conductors

A large superconducting coil wound with Cable-in-Conduit (CIC) conductor causes both irregular AC losses that cannot be estimated from short conductor sample test results, and regular AC losses that are proportional to cable twisting pitch squared. We proposed a mechanism forming loops that generated the irregular losses. The CIC conductor is composed of several stages of sub-cables. If one strand on the surface of a sub-cable contacts another strand on the surface of the adjacent sub-cable, the two strands must encounter each other again at the LCM (Least Common Multiplier) distance of all staged cable pitches, and thereby a long loop is formed. We orderly labeled all strands in CIC conductors for the SMES and the LHD. It was found that strands in a triplet were widely displaced from their original positions on one cross section, but contacted each other tightly on the other cross section. This fact suggests that the loop with the large displaced strand links irregularly with external field so that the loops cause the irregular AC losses. Moreover, it indicates that a contacting length of the large displaced strands can be quite long, giving rise to a low contact resistance for the loop, and leading to the long time constants. It is believed that the widely displaced strand are inherent in a CIC conductor. It was demonstrated that the strand surface coated with CuNi was effective to suppress the irregular AC losses.