H-mode inhibition in negative triangularity tokamak reactor plasmas

Abstract: Instability to high toroidal mode number ($n$) ballooning modes has been proposed as the primary gradient-limiting mechanism for tokamak equilibria with negative triangularity ($\delta$) shaping, preventing access to strong H-mode regimes when $\delta\ll0$. To understand how this mechanism extrapolates to reactor conditions, we model the infinite-$n$ ballooning stability as a function of internal profiles and equilibrium shape using a combination of the CHEASE and BALOO codes. While the critical $\delta$ required for avoiding $2^\mathrm{nd}$ stability to high-$n$ modes is observed to depend in a complicated way on various shaping parameters, including the equilibrium aspect ratio, elongation and squareness, equilibria with negative triangularity are robustly prohibited from accessing the $2^\mathrm{nd}$ stability region, offering the prediction that that negative triangularity reactors should maintain L-mode-like operation. In order to access high-$n$ $2^\mathrm{nd}$ stability, the local shear over the entire bad curvature region must be sufficiently negative to overcome curvature destabilization on the low field side. Scalings of the ballooning-limited pedestal height are provided as a function of plasma parameters to aid future scenario design.