The effect of plasma triangularity on turbulent transport: modeling TCV experiments by linear and non-linear gyrokinetic simulations

The effect of plasma shape on confinement has been experimentally explored in the TCV tokamak revealing that the core electron heat transport is significantly reduced by a negative triangularity configuration, which could indicate a (partial) stabilization of the microinstabilities at play in a conventional positive triangularity configuration. This work is a theoretical investigation of the effect exerted by triangularity on plasma turbulence. In particular, it compares the TCV experimental results with non-linear local gyrokinetic simulations performed on the basis of actual MHD equilibrium reconstructions. In both the linear and non-linear phases, negative triangularity is found to have a stabilizing influence on ion-scale instabilities, specifically on the so-called trapped electron mode (TEM) which is the dominant instability in the conditions of the TCV experiments considered; more specifically, the variation of the heat flux with triangularity calculated by the non-linear simulations is in fair agreement with the experimental results. The resulting stabilization is a result of a rather complex modification of the toroidal precessional drift of trapped particles exerted by negative triangularity.