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 ballooning modes. The effect of the bulge is that the 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 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.
This paper investigates how a localized bulge at the outer midplane affects the stability of ideal MHD peeling-ballooning and ballooning modes. The bulge significantly degrades the ballooning mode stability at the bottom of the H-mode pedestal, while only slightly affecting the peeling-ballooning mode stability. This reduction in required separatrix density could extend the access to the quasi-continuous exhaust (QCE) operating mode without large edge localized modes.