H-mode inhibition: Using shape to destabilize infinite-n ballooning modes in reactor conditions

By preventing the growth of pedestal gradients, edge instability to high toroidal mode number (n) ballooning modes can inhibit access to H-mode regimes in tokamak plasmas. In this work, we explore the potential for robust L-mode reactor operation through this mechanism by modeling infinite-n ballooning stability as a function of internal profiles and equilibrium shape using a combination of the CHEASE and BALOO codes. Negative triangularity is found to be a primary lever on preventing access to the 2nd stability region for high-n modes, though the critical triangularity necessary for L-mode operation is observed to depend in a complicated way on the equilibrium aspect ratio, elongation and squareness. In order to stabilize high-n ballooning modes, the local shear over the entire bad curvature region must be sufficiently negative to overcome curvature destabilization on the low field side, which is not possible below a critical triangularity. Extreme low or high squareness can also be employed to close access to the 2nd stable region. Scalings of the ballooning-limited pedestal height are provided as a function of plasma and machine parameters to aid future scenario design. These results suggest that negative triangularity reactors should maintain L-mode-like operation.