Lessons Learned From EF1 Electrical Short Incident During JT-60SA Integrated Commissioning Test

The superconducting tokamak fusion test device, JT-60SA, was constructed as the Satellite Tokamak project in the Broader Approach Program between Japan and EU. During the JT-60SA integrated commissioning tests of the Nb-Ti equilibrium magnetic field coil EF1 high voltage (similar to 5 kV) test, the EF1 power supply current surged suddenly and was stopped by the overcurrent interlock of the power supply. At the same time, the vacuum in the cryostat deteriorated, and the pressure in the magnet cooling loops increased. After the system warmed up, visual inspection revealed that discharges occurred between ground and both positive and negative terminal joints of EF1. Several holes, melted by the electrical short and identified as the source of the helium leak into the cryostat, were found in the cylindrical metal part of the joint. The data recorded at the time of the event were analyzed with a conclusion that the discharge first occurred between the positive terminal joint and its support bracket at ground potential and immediately afterward between the neighboring negative terminal joint and its support bracket. These two arcs created a short-circuited loop through which the coil discharged for about 1.4 s. The energy during the short circuit is estimated to be about 60 kJ.