Magnetic shear effects on ballooning turbulence in the boundary of fusion devices

TL;DR

This study investigates how magnetic shear influences ballooning turbulence in fusion device boundaries, revealing a transition from scale-separated to comparable radial and poloidal eddy sizes with varying shear levels.

Contribution

It combines nonlinear simulations, linear analysis, and analytical models to elucidate the impact of magnetic shear on turbulence structure and mode behavior in fusion edge plasmas.

Findings

High magnetic shear leads to scale separation with kx << ky.

Low magnetic shear results in kx ~ ky, resembling stellarator turbulence.

Strong magnetic shear enhances poloidal mode coupling, affecting mode structure.

Abstract

The effect of magnetic shear on ballooning-driven plasma edge turbulence is studied through nonlinear simulations complemented by linear numerical and analytical investigations. Nonlinear, 3D, global, flux-driven simulations using the GBS code show that the scale separation between radial, x, and poloidal, y, size of turbulent eddies, kx << ky , considered by Ricci et al. (2008) and extensively used to predict pressure gradient lengths, SOL width, particle and heat fluxes, is observed with high magnetic shear. In contrast, for low magnetic shear, kx ~ ky is observed, with fluctuation properties resembling those shown by recent low-shear stellarator simulations reported in Coelho et al. (2024a). Global linear investigations of the ballooning mode qualitatively captures the transition in mode structure with varying magnetic shear, showing that kx << ky is achieved with sufficiently strong poloidal mode coupling enhanced by increasing magnetic shear, resistivity, toroidal mode number, and equilibrium gradient scale length. This is confirmed by an analytical study considering a dominant poloidal mode and its sidebands, which highlights that the poloidal mode structure is determined by curvature and k parallel effects