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

Abstract

A large superconducting coil wound with Cable-inConduit (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.

Monte carlo simulation of strand position in cic conductor

“…By using the calculated strand position various parameters such as contact stress, mechanical disturbances, and AC loss can be calculated [4]. One example of the derived parameter is shown in Fig.…”

Investigation of long time constants of magnetic fields generated by the JT-60SA CS1 module

“…In large superconducting coils wound with CIC conductors, there are two types of coupling currents. One is the 'regular' coupling current which can be estimated by the cabling pattern of a CIC conductor [13,14]. The other is the 'irregular' coupling current which is an unexpected phenomenon from the conductor design.…”

“…Fig 13. Self-magnetic field of the short straight conductor in the joint sample during the sample current of 20 kA.…”