Kinetic ballooning mode turbulence in low-average-magnetic-shear equilibria

Kinetic-ballooning-mode (KBM) turbulence is studied via gyrokinetic flux-tube simulations in three magnetic equilibria that exhibit small average magnetic shear: the Helically Symmetric eXperiment (HSX), the helical-axis Heliotron-J and a circular tokamak geometry. For HSX, the onset of KBM being the dominant instability at low wavenumber occurs at a critical value of normalized plasma pressure β KBM crit that is an order of magnitude smaller than the magnetohydrodynamic (MHD) ballooning limit β MHD crit when a strong ion temperature gradient (ITG) is present. However, β KBM crit increases and approaches the MHD ballooning limit as the ITG tends to zero. For these configurations, β KBM crit also increases as the magnitude of the average magnetic shear increases, regardless of the sign of the normalized magnetic shear. Simulations of Heliotron-J and a circular axisymmetric geometry display behaviour similar to HSX with respect to β KBM crit. Despite large KBM growth rates at long wavelengths in HSX, saturation of KBM turbulence with β>β KBM crit is achievable in HSX and results in lower heat transport relative to the electrostatic limit by a factor of roughly five. Nonlinear simulations also show that KBM transport dominates the dynamics when KBMs are destabilized linearly, even if KBM growth rates are subdominant to ITG growth rates.