Following the incident on 9th March 2021 involving Equilibrium Field Coil No 1(EF1), the joints of the EF1 feeders were found to have been damaged during the coil energization test. Experts in Japan and in Europe have jointly investigated the root cause of the incident. The measures needed to recover have been identified and a revised schedule for the integrated commissioning has been prepared.
It was determined that the primary reason for the damage to the joints was insufficient insulation at the point where a quench detection (QD) cable emerged from the ground insulation around a joint. A short circuit occurred due to this insufficient insulation and the arc formed damaged the shells of the EF1 terminal joints. This in turn allowed a helium leak to the cryostat.
The insulation of the joints will be reinforced so as to ensure enough insulation capability. Moreover, other joints with a similar structure will be repaired as well in order to prevent recurrence elsewhere. Furthermore, other joints without a similar structure will also be reinforced in view of thorough prevention of recurrence. The repair on site will be carried out under the thorough quality management. After the repair work, the high voltage holding capability will be tested under locally-simulated worst-case conditions to confirm a sufficient insulation capability. To complete the repair and the test, the process to be used on site will be thoroughly qualified and it will be shared thoroughly among the workers.
Then in around February 2022, vacuum evacuation of the vacuum vessel and the cryostat will start and cooling down of the super-conducting coils will follow to resume the suspended integrated commissioning of JT-60SA. The experience and knowledge obtained from this incident is being widely disseminated to support the consolidation of knowledge necessary for ITER, DEMO and any future superconducting tokamak.
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.
While the JT-60SA was completing some final commissioning steps, last Tuesday, at about 9PM JST, during high voltage commissioning of one of the poloidal field coils there was an unexpected increase of current. The magnet is now being warmed from cryogenic temperature all the way to room temperature so that access in the cryostat can be gained, to verify the exact problem and then immediately set out remedial actions.
A root cause analysis will also be carried out to make sure such problem will not happen again and to transfer this experience to the ITER colleagues