Development of the Protection System for JT-60SA Superconducting Magnet Against Vacuum Degradation

The JT-60SA project succeeded in commissioning test toward the first plasma in 2023 under a framework: the Satellite Tokamak Programme of the Broader Approach Agreement between EU and Japan (H. Shirai et al., 2016). JT-60SA is a tokamak type magnetic confinement device with a superconducting magnet system (Y. Koide et al., 2015) developed and operated at the Quantum Science and Technology institute (QST) in Japan. The magnet system consists of 18 Toroidal Field coils, 6 Equilibrium Field coils, and 4 modules of Central Solenoids, and all the coils are superconducting magnets. In the first commissioning, during 2020 - 2021, JT-60SA has experienced a discharge incident at the EF1 magnet (H. Shirai et al., 2024). As a conclusion of the investigation of the incident, it turned out that the insulation resistance of the magnet system was insufficient. Although the insulation was partly reinforced, the voltage holding under accidentally arising Paschen conditions was not improved enough for the recent operation (OP-1, 2023) (K. Hamada et al., 2024). To prevent the magnet system from discharging during vacuum degradation, a vacuum monitoring system must be prepared to detect vacuum degradation and ramp down the magnet current immediately in case of unexpected vacuum degradation. In this article, the development of the vacuum monitoring system, operational results, and the future plan of the system will be described.