The effect of triangularity on tokamak boundary plasma turbulence is investigated using global, flux-driven, three-dimensional, two-fluid simulations. The simulations show that negative triangularity (NT) stabilizes boundary plasma turbulence, and linear investigations reveal that this is due to a reduction of the magnetic curvature driven by interchange instabilities, such as the resistive ballooning mode (RBM). As a consequence, the pressure decay length Lp, related to the scrape-off layer (SOL) power fall-off length λq, is found to be affected by triangularity. Leveraging considerations on the effect of triangularity on the linear growth rate and nonlinear evolution of the RBM, the analytical theory-based scaling law for Lp in L-mode plasmas, derived by Giacomin et al (2021 Nucl. Fusion61 076002), is extended to include the effect of triangularity. The scaling is in agreement with nonlinear simulations and a multi-machine experimental database, which includes recent TCV discharges dedicated to the study of the effect of triangularity in L-mode diverted discharges. Overall, the present results highlight that NT narrows the Lp and considering the effect of triangularity is important for a reliable extrapolation of λq from present experiments to larger devices.
The effect of triangularity on fluctuations in a tokamak plasma