Prospect of Spherical Tokamak towards a Power Reactor -Challenge towards the Lowest Aspect Ratio Tokamak- 2. How High is the Beta Limit for STs? 2.1 Where is the Upper Limit for the High-Beta ST Operation?

Abstract

High-beta ST research is motivated by its high normalized current I_p/aB_T0 achievable at a low aspect ratio and its direct access to second stability for ballooning modes. Key issues for the high-beta ST sustainment are profile control for ballooning/ current-driven kink stabilities and positioning of the conductive wall for mode suppression. ST termination by resistive wall modes (RWM) revealed that velocity shear is a useful tool for the better stability.