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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 (∼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.

JT-60SA is one of the largest superconducting tokamaks in the world, and its commissioning is ongoing to achieve the first plasma. The power supply (PS) for superconducting coils consists of ac/dc converter, switching network unit, and quench protection circuit. These subcomponents are required for each coil, and therefore, multiple subcomponents are used in JT-60SA. For plasma discharge, these many subcomponents need to be operated simultaneously, and this commissioning for system integration is an important issue. In the commissioning so far, the toroidal field (TF) coils were energized up to the rated current of 25.7 kA, and plasma was generated by the electron cyclotron range of frequency (ECRF) injection. In TF coils, fast discharge for quench protection was successfully achieved at a high current of 18 kA, which is 70% of the rated current. PF coils have been energized one by one up to ±5 kA, which is 25% of the rated current, and a high voltage of ±5 kV required for inductive plasma initiation was applied to the coils. The commissioning strategy for system integration of the coil PSs and these initial results are described in this article.