Analysis of slow temperature and current ramps on the central solenoid insert coil

Abstract

An analysis procedure for modelling non-uniform current development with time in large superconducting cables has been applied to the central solenoid (CS) insert coil. This is a single layer wound 40 kA conductor (length about 140 m) which is tested inside the bore of the CS Model Coil. The conductor is a 1152-strand cable-in-conduit type, with six final stage bundles of 192 strands twisted around a central cooling channel. It is extensively instrumented and particularly suitable for investigations into non-uniform current development. Although current sharing measurements on this coil were performed with very slow (2 h) temperature or current ramp-ups to quench, the analysis suggests that the current distribution in the two cases is different, one being dominated by the inductively determined current distribution and the other by the resistive distribution at the joints. The inductive current distribution between the strands in the cable is obtained after current ramp-up, and slowly diffuses to the resistive distribution. Data from all the coil voltage taps show a very early voltage development in the case of the temperature ramp but not the current ramp. This can be matched by the analysis if a substantial degree of joint contact resistance non-uniformity at the level of the final substage is assumed. It is difficult to identify other effects that could cause such a widespread effect over the conductor length, and the deduced level of current non-uniformity is supported by measurements of cable current centre movement close to the joints from magnetic field sensors mounted around the conductor.