Influence of magnetic shear on kinetic ballooning modes and electromagnetic stellarator turbulence

The influence of magnetic shear on kinetic ballooning modes (KBMs) and electromagnetic turbulence is reported in the Wendelstein 7-X (W7-X) and Heliotron-J (H-J) stellarators. Gyrokinetic simulations at finite normalized plasma pressure β reveal that sub-threshold KBMs (stKBMs) are present in all configurations, i.e., KBMs that are resonantly destabilized far below the iMHD limit. A reduced KBM model shows that boosting magnetic shear and weakening bad curvature stabilizes (st)KBMs in stellarator geometry. In nonlinear simulations, instead of undergoing nonlinear electromagnetic stabilization, turbulent fluxes in W7-X increase with β when stKBMs are destabilized. Lower-magnetic-shear configurations of W7-X are found to produce lower transport, due to the generation of stronger zonal flows. In H-J, turbulent fluxes reduce with β – despite hosting stKBMs – due to a boosted zonal-flow response at higher β. This highlights the importance of accounting for both linear and nonlinear dynamics when aiming to improve reactor performance.