There is renewed interest in negative triangularity (or reversed triangularity) tokamaks after the TCV and DIII-D experiments have shown that the low-mode confinement in the negative triangularity configuration can achieve the high-mode-level confinement without edge localized modes. This particularly addresses the divertor heat load issue for tokamak high beta confinement. Previous investigations have found, in agreement with experimental observations, that the beta limit for low-n (macroscopic) modes is somewhat lower in the negative triangularity case than in the positive one. In this work, we study higher-n (mesoscopic) modes. Using equilibria modeling of those used in the DIII-D negative triangularity experiment, the numerical results show that the intermediate-n modes with the resistive wall are actually more stable for the negative triangularity case than the positive triangularity case. This is consistent with the experimental observations of the lower level of turbulence transport in the frequency range of resistive wall modes in the negative triangularity case and can also be explained physically.
The effect of triangularity on fluctuations in a tokamak plasma