Nonlocal effects in negative triangularity TCV plasmas

Abstract

Global gradient driven GENE gyrokinetic simulations are used to investigate TCV plasmas with negative triangularity. Considering a limited L-mode plasma, corresponding to an experimental triangularity scan, numerical results are able to reproduce the actual transport level over a major fraction of the plasma minor radius for a plasma with δ LCFS = −0.3 and its equivalent with standard positive triangularity δ. For the same heat flux, a larger electron temperature gradient is sustained by δ < 0, in turn resulting in an improved electron energy confinement. Consistently with the experiments, a reduction of the electron density fluctuations is also seen. Local fluxtube simulations are used to gauge the magnitude of nonlocal effects. Surprisingly, very little differences are found between local and global approaches for δ > 0, while local results yield a strong overestimation of the heat fluxes when δ < 0. Despite the high sensitivity of the turbulence level with respect to the input parameters, global effects appear to play a crucial role in the negative triangularity plasma and must be retained to reconcile simulations and experiments. Finally, a general stabilizing effect of negative triangularity, reducing fluxes and fluctuations by a factor dependent on the actual profiles, is recovered.