A mechanism causing an additional AC loss in a large CICC coil

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 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 were calculated and their results can explain the data measured in a large SMES coil. The proposed mechanism is effective for estimating the additional AC loss in the coil.