Local plasma shape modification to facilitate access to quasi continuous exhaust (QCE) regime

Abstract

The effects of a poloidally localized bulge at outer midplane are investigated with regards to the stability of ideal MHD peeling-ballooning modes (PBMs) and $n = \infty$ ballooning modes. The effect of the bulge is that the $n = \infty$ ballooning mode stability at the bottom of the H-mode pedestal is significantly degraded, while the PBM stability is only slightly affected. The ratio of separatrix to pedestal density at which the $n = \infty$ ballooning mode limit is reached before the peeling-ballooning limit, decreases with the bulge size. The reduction of the required separatrix density would extend the access to quasi-continuous exhaust (QCE) operating mode without large edge localised modes to a wide range of scrape-off layer conditions. The free boundary equilibrium calculation shows that a bulge required for a significant stability change for a Spherical Tokamak for Energy Production fusion reactor can be achieved with 600 kA current in a midplane shaping coil.

Keywords

• QCE
• pedestal
• ballooning mode
• ELM
• plasma shaping