Description
Tokamak plasmas with sufficiently strong negative triangularity (NT), in contrast to the conventional positive triangularity (PT) shaping, have been shown to suppress the transition to H-mode while still achieving high confinement regimes comparable to H-mode plasmas, but without harmful for plasma facing components (PFC) Edge Localized Modes (ELMs). Comparative modelling of Ion Temperature Gradient and Trapped Electron Modes (ITG/TEM) turbulence in NT and PT plasma shapes was performed using the nonlinear global gyrokinetic electrostatic particle code JOREK-GK in realistic X-point tokamak geometry including the Scrape Off Layer (SOL) for TCV, DIII-D and WEST tokamaks parameters. The numerical scans of upper (up and lower (low) triangularities at the same plasma profiles demonstrated the existence of the averaged negative triangularity threshold 0.5(uplow) < avth , showcasing the beneficial stabilizing effect of NT compared to PT on ITG/TEM turbulence. For the WEST parameters tested here the threshold was about avth < -0.25. Existence of longer correlation length of density fluctuations in PT compared to NT was demonstrated similar to experimental measurements on DIII-D. Stronger and more sheared zonal flows are generated in NT compared to PT. These factors are stabilizing for TEM/ITG turbulence and lead to smaller heat fluxes and heat conductivities in NT compared to PT plasmas. Bohm-like confinement scaling with normalized ion gyro-radius * was obtained both in NT and PT, however with better confinement for NT compared to PT which could be a favorable factor for future reactor size machines operation in negative triangularity without triggering ELMs.