AC losses with long time constants in a large CIC superconductor

A large superconducting coil wound with Cable-in-Conduit (CIC) conductor caused an additional AC loss which cannot be estimated from short conductor sample test results. It was confirmed that the additional AC loss was generated by long current loops in the CIC conductor. We propose a mechanism forming the long loops. If one strand on the surface of a sub-cable contacts with the other strand on the surface of the adjacent sub-cable, the two strands must encounter each other again at LCM (Least Common Multiplier) distance of all staged cable pitches and thereby result in forming a pair of a long loop. The long time constants of long loops were calculated. We orderly labeled all strands in a real CIC conductor, disassembling carefully the cable after peeling the conduit. It was found that the strands in a triplex were widely displaced from their original positions, and thereby their contacting lengths became longer than calculated ones. This fact makes the time constant of loop longer and hence can explain the observed long time constants.

AC loss

coupling loss

cable-in-conduit

twist pitch

long time constant