Effect of negative triangularity on SOL plasma turbulence in double-null L-mode plasmas

TL;DR

This study uses advanced simulations to show that negative triangularity in double-null plasmas suppresses turbulence, improves confinement, and alters heat flux distribution, with implications for fusion reactor design.

Contribution

It provides the first detailed simulation-based analysis of how negative triangularity affects boundary plasma turbulence and heat flux in double-null configurations.

Findings

NT plasmas suppress interchange-driven instabilities.

NT plasmas have smaller blobs and slower propagation velocities.

Power load asymmetry is reduced in NT plasmas.

Abstract

The effects of negative triangularity (NT) on boundary plasma turbulence in double-null (DN) configurations are investigated using global, nonlinear, three-dimensional, flux-driven two-fluid simulations. NT plasmas exhibit suppressed interchange-driven instabilities, resulting in enhanced confinement and lower fluctuation levels compared to positive triangularity (PT) plasmas. This reduction in interchange instability is associated with the weakening of curvature effects in the unfavorable region, caused by the stretching of magnetic field lines at the outer midplane. The magnetic disconnection between the turbulent low-field side (LFS) and the quiescent high-field side (HFS) results in most of the heat flux reaching the DN outer targets. In NT plasmas, the power load on the outer target is reduced, while it increases on the inner target, indicating a reduced in-out power asymmetry compared PT plasmas. Furthermore, the analysis of power load asymmetry between the upper and lower targets shows that the up-down power asymmetry is mitigated in NT plasmas, mainly due to the reduced total power crossing the separatrix. The reduction of interchange instabilities in NT plasmas also affects the blob dynamics. A three-dimensional blob analysis reveals that NT plasmas feature smaller blob sizes and slower propagation velocities. Finally, an analytical scaling law for blob size and velocity that includes plasma shaping effects is derived based on the two-region model and is found to qualitatively capture the trends observed in nonlinear simulations.