Reducing transport via extreme flux-surface triangularity

Based on a gyrokinetic analysis of and extrapolation from TCV discharges with large negative and positive triangularity delta, the potential of extreme | delta | in reducing turbulent transport is assessed. Linearly, both positive and negative delta can exert a stabilizing influence, with substantial sensitivity to the radial wavenumber k(x) . Nonlinear fluxes are reduced at extreme delta in a trapped-electron-mode regime, whereas low-amplitude ion-temperature-gradient turbulence is boosted by large negative delta. Focusing on the former case, nonlinear fluxes exceed quasilinear ones at negative delta, a trend that reverses as delta > 0. A change in saturation efficiency is the cause of these features: the zonal-flow residual is boosted at delta > 0, reducing fluxes compared with the linear drive as delta is increased, and a shift towards larger zonal-flow scales occurs with increasing delta due to finite-k(x) modes weakening with delta.