Plasma beta dependence of ion temperature gradient driven turbulence influenced by Shafranov shift

Abstract

Plasma β dependence of ion temperature gradient (ITG) driven turbulence is investigated using gyrokinetic simulations, where β is the normalized pressure. In our β scan, self-consistent magnetohydrodynamic (MHD) equilibrium state is numerically calculated for each value of β. It is found that the influence of the Shafranov shift cancels out the electromagnetic stabilizing effect on the ITG mode, and the growth rate of the ITG mode is accordingly unchanged as β increases. As a result, the turbulent energy transport does not decrease with β as suggested by the $s-alpha$ model (Ishizawa et al 2019 Phys. Rev. Lett.123 025003). A significant difference from the $s-alpha$ model is the increase of the energy transport with β. It is also found that the critical onset β value for the kinetic ballooning mode is significantly increased by the influence of the Shafranov shift. The cancellation of the electromagnetic stabilization by the Shafranov shift is explained by the decrease of magnetic drift frequency in the dispersion relation of electromagnetic ITG modes obtained by using a fluid approximation.