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

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. [“High- and low-confinement modes in simple magnetized toroidal plasmas,” Phys. Rev. Lett. 100, 225002 (2008)] and extensively used to predict pressure gradient lengths, scrape-off layer 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 Coehlo et al. [“Global fluid simulation of plasma turbulence in stellarators with the GBS code,” Nucl. Fusion 64, 076057 (2024)]. Global linear investigations of the ballooning mode qualitatively capture 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∥ effects.