Long time constants of irregular AC coupling losses in a large superconducting coil

A large superconducting coil wound with Cable-in-Conduit (CIC) conductor caused an irregular AC loss that cannot be estimated from short conductor sample test results. It was confirmed that the irregular AC loss was generated by long current loops in the CIC conductor. We proposed a mechanism forming the long loops. The CIC conductor is composed of several staged sub-cables. 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. We measured cross over point contact resistance between two strands making the long loop. The calculated time constants of the long loops were shorter than the observed ones. 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 triplet were widely displaced from their original positions, and thereby their contacting lengths became longer than cross over ones to form line contacts. This fact can make the time constant of the loop longer and hence can explain the observed long time constants.