Investigation of Ramp-Rate Limitation on the Superconducting Magnet for the Grenoble 40-T Hybrid System

The superconducting magnet for the Grenoble 40-T Hybrid system has been tested, up to its guaranteed value of 7 T, on its own and together with the resistive magnet. Although the superconducting magnet has been consistently operated at 7 T several times, it has shown ramp-rate limitations. In particular, when trying to ramp to field in less than 1 hour, premature quenches have been observed. Subsequent measurement of the voltages across the sections of the superconducting coil has revealed the presence of anomalous voltage spikes during ramp-up and ramp-down of the magnet. These spikes have been attributed to the presence of a shorted turn in the superconducting magnet. A theoretical model simulating the shorted turn has been developed and used to reproduce the observed magnet behavior. In particular, by analyzing the relative amplitude of the voltage spikes, we have determined the likely position of the short in the magnet and the amplitude of the current induced in the shorted turn during ramping. In this paper, both theoretical and experimental aspects of the analysis are presented. Finally, the plan of actions to fix the problem is reported