Paper

H-mode inhibition in negative triangularity tokamak reactor plasmas

Nelson, A O, Saarelma, S, Paz-Soldan, C

2022

Overview

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.

• Publisher: Cornell University Library, arXiv.org
• Subject: Aspect ratio / Ballooning modes / Curvature / Destabilization / Elongation / Parameters / Plasmas (physics) / Stability / Tokamak devices