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

• Published in: Journal of plasma physics 2021-06, Vol.87 (3), Article 905870311
• Main Authors: McKinney, I.J., Pueschel, M.J., Faber, B.J., Hegna, C.C., Ishizawa, A., Terry, P.W.
• Format: Article
• Language: English
• Subjects: 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Equilibrium; Fluid flow; fusion plasma; Geometry; Heat transport; Heliotrons; Ion temperature; Magnetic fields; Magnetohydrodynamic turbulence; Magnetohydrodynamics; Plasma; plasma instabilities; plasma nonlinear phenomena; Plasma physics; Plasma pressure; Shear; Simulation; Temperature gradients; Turbulence; Wavelengths
• ISSN: 0022-3778; 1469-7807
• DOI: 10.1017/S0022377821000581

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 $\beta ^{\rm KBM}_{\rm crit}$ that is an order of magnitude smaller than the magnetohydrodynamic (MHD) ballooning limit $\beta ^{\rm MHD}_{\rm crit}$ when a strong ion temperature gradient (ITG) is present. However, $\beta ^{\rm KBM}_{\rm crit}$ increases and approaches the MHD ballooning limit as the ITG tends to zero. For these configurations, $\beta ^{\rm KBM}_{\rm crit}$ 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 $\beta ^{\rm KBM}_{\rm crit}$. Despite large KBM growth rates at long wavelengths in HSX, saturation of KBM turbulence with $\beta > \beta _{\rm crit}^{\rm KBM}$ 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.