Turbulence and Transport in Negative and Positive Triangularity

Triangularity δ is investigated via gyrokinetic simulations of ion-temperature-gradient-driven (ITG) modes. Large negative δ < −0.5 leads to a reduction in growth, as local magnetic shear dis-aligns modes from the region of bad curvature. At large positive δ > 0.5, the local shear forces the mode to finite radial wavenumber, reducing transport efficacy. This is confirmed by nonlinear simulations, which show that while zonal flows are stronger at δ > 0, their impact is comparable regardless of δ sign due to higher saturation efficiency at δ < 0. Analysis of extreme-δ experiments on TCV with δ ≈ ±0.6 shows to which degree such desirable effects can be achieved in experiment, and which properties apply to trapped-electron-mode (TEM) turbulence.

In another set of TCV discharges, δ < 0 geometry results in lower transport. It is found that both the δ > 0 and the δ < 0 discharges are near-marginal to decorrelation of field lines: a moderate increase in β leads to a substantial boost in field-line stochasticity and ensuant zonal-flow erosion, resulting in a non-zonal transition and extreme profile stiffness. Adding resonant magnetic perturbations (RMPs) to the mixed ITG-TEM state in TCV – with a substantial level of zonal-flow activity – shows insensitivity to RMPs in terms of fluxes.