Startup experiment with the newly installed lower tungsten divertor of KSTAR

Abstract

Plasma startup experiments after the KSTAR upgrade of a lower divertor from carbon to tungsten have been studied during the KSTAR 2023 plasma campaign. In-vessel eddy currents, especially those induced on the divertor supporting structure in the toroidal direction, are significantly altered by the upgrade of the lower divertor configuration. These changes could affect the poloidal field configuration of the plasma startup phase, which is essential for the reliable breakdown and burn-through of deuterium neutral gas and the stable rise of the plasma current. Here, we present the development process of a new startup scenario that considers these upgrades. The vacuum magnetic field design code was utilized to estimate the changes in the poloidal field due to the new divertor configuration. Modifications to the startup scenario were prepared to compensate for the different effects of eddy currents, and experimental validation and optimization were performed during the initial phase of the KSTAR 2023 campaign. The final version of the startup scenario and plasma startup data are presented, and the reliability of the new startup scenario was also confirmed throughout the remainder of the KSTAR campaign.