Kinetic Ballooning Mode turbulence in low-average-magnetic-shear equilibria*

Optimizing for turbulent transport in high-β stellarators requires understanding of electromagnetic (EM) turbulence in 3D magnetic geometries. In this work, we report studies of kinetic ballooning mode (KBM) turbulence in low-average-magnetic-shear (s) equilibria, namely HSX, Heliotron-J, and a circular tokamak. EM flux-tube simulations of HSX using the gyrokinetic code GENE show that the onset of KBM instability at low k_y occurs at a value of normalized plasma pressure β^KBM that is nearly an order of magnitude smaller than the MHD ballooning limit β^MHD. Both Heliotron-J and a low-s tokamak exhibit behavior similar to HSX with respect to β^KBM. Regardless, saturation of nonlinear simulations of HSX with β^KBM<β<β^MHD is achievable and results in lower heat fluxes than the electrostatic case, even though KBMs contribute significantly to the nonlinear state.

A fluid model is also introduced that extends the electrostatic model for 3D equilibria [C.C. Hegna et al., PoP 2018] to include finite-β effects. This fluid model retains the physics of ITG-KBM saturation in stellarators, which is dominated by the transfer of energy from unstable to stable modes at similar scales via nonlinear coupling. The model will be used to ascertain the relationship between magnetic geometry and β^KBM.