Possible causes of the premature voltage gradient of the CS insert coil [for ITER]

The CS (Central Solenoid) insert coil is configured as a single layer winding inside the 2 m bore of the CSMC (CS Model Coil), with a long uniform field over the center up to 13 T. Tcs (ramped temperature) and Ics (ramped current) measurements have been performed. The coil showed a premature voltage development in the Tcs test. After 10 000 current cycles of the insert, there was a further drop of 0.4 K in the measured current sharing temperature, associated with individual quenches during the cycling. In this paper, the possible causes for the premature voltage gradient are assessed. The process includes application of the single strand data to assess the expected performance of the insert coil and consideration of the impact of variations of strand current and field across the cable cross-section, using a new analysis program for nonuniform current diffusion in long multistrand cables.