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

Plasma beta dependence of ion temperature gradient (ITG) driven turbulence is investigated using gyrokinetic simulations, where beta is the normalized pressure. In our beta scan, self-consistent magnetohydrodynamic (MHD) equilibrium state is numerically calculated for each value of beta. 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 beta increases. As a result, the turbulent energy transport does not decrease with beta 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 beta. It is also found that the critical onset beta 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.