Development of diverted plasma discharge and plan for advanced divertor study in VEST

Abstract

Operational scenario for diverted plasmas has been developed in Versatile Experiment Spherical Torus (VEST) not only for improving plasma performance but also for preparing upcoming experiments on advanced divertor concepts such as snowflake and x-point divertor. Diverted discharge scenario is designed using an in-house simulation code and experimentally realized by adjusting turn-on time of two partial solenoid (PF2) coils located at both ends of the center stack. With the diverted scenario, we achieve the improvement in plasma duration by similar to 20% compared to the limited plasma operation, mainly due to the mitigation of impurity influxes from the center stack made of tungsten. Indeed, the impurity O-I line is observed to be decreased to 50% of the limited plasma operation. Fast camera images and equilibrium reconstruction based on magnetic diagnostics clearly shows the successful generation of diverted plasmas as well. This paper presents the first results on the diverted plasma operation in VEST and future plan for the experiments on the advanced divertor concept.