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

Abstract

A large superconducting coil wound with a Cable-in-Conduit (CIC) conductor caused an additional AC loss, which cannot be estimated from the 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 for the formation of the long loops. The CIC conductor is composed of several staged subcables. If one strand on the surface of a subcable contacts the other strand on the surface of the adjacent subcable, the two strands must encounter each other again at the LCM (Least Common Multiplier) distance of all staged cable pitches and thereby form a pair of long loops. We numerically traced each strand in the CIC according to a method in which the subcables at all substages rotate around the center of inertia. The calculated long-time constants of the long loops were slightly shorter than the observed ones. We labeled all strands by order in a real CIC conductor, disassembling the cable carefully after peeling the conduit. It was found that the strands in a triplex were widely displaced from their original positions, so that their contacting lengths became longer than the calculated ones. This fact makes the time constant of the 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.