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.