Long Time Constants of irregular AC Losses in a Large Superconducting Coil

Abstract

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. Magnetic field decays of the loops with various long time constants were observed through Hall probes. We propose 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 numerically traced each strand in the CIC according to a method that the sub-cables at all sub-stages rotate around a center of inertia. The long time constants of long loops were calculated and a little bit 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 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. The proposed mechanism is effective for estimating the long loops causing additional AC losses in the coil