During the coil energization test of Equilibrium Field Coil No1(EF1) with the high voltage of 5-7 kV in March 2021, the coil current increased rapidly and interrupted by the interlock of over current in power supply. After few minutes, the pressure in the cryostat increased from 10-3 Pa to 7000 Pa. After the JT-60SA was safely stopped, the integrated commissioning was suspended.
To observe the current feeders inside the cryostat, the temperatures of superconducting coils have been increased from -269 ℃ to room temperature for 20 days, and vacuum vessel and cryostat were vented on 8th, April. Melted spots were observed with the marks of the discharge on the outer shell of the terminal joints of the EF1 coil, which is a part of the EF1 current feeders with He cooling channel. From the visual inspection and analyses, the discharge was considered to be caused by an insufficient voltage holding capability at the joint of the current feeders with nominal voltage of 10 kV. The pressure rise of cryostat was caused by the He leakage through the melted spots on the current feeders. No damage has been observed on the EF1 coil itself.
An analysis to identify the root cause is currently underway involving many experts in Japan and in Europe. This effort will allow to effectively repair the damaged terminal joint as well as make sure this will not happen in other similar areas. Key will also be to later widely disseminate such analysis to support the consolidation of knowledge necessary for ITER, DEMO and any future superconducting tokamak.