Gyrokinetic simulations of the kinetic electron effects on the electrostatic instabilities on the ITER baseline scenario

Abstract
The linear and nonlinear simulations are carried out using the gyrokinetic code NLT for the electrostatic instabilities in the core region of a deuterium plasma based on the International Thermonuclear Experimental Reactor (ITER) baseline scenario. The kinetic electron effects on the linear frequency and nonlinear transport are studied by adopting the adiabatic electron model and the fully driftkinetic electron model in the NLT code, respectively. The linear simulations focus on the dependence of linear frequency on the plasma parameters, such as the ion and electron temperature gradients \kappa_T_\rmi,e\equiv R/L_T_\rmi,e , the density gradient \kappa_n\equiv R/L_n and the ion–electron temperature ratio \tau=T_\rme/T_\rmi . Here, R is the major radius, and T_\rme and T_\rmi denote the electron and ion temperatures, respectively. L_A=(\partial_r\ln A)^1 is the gradient scale length, with A denoting the density, the ion and electron temperatures, respectively. In the kinetic electron model, the ion temperature gradient (ITG) instability and the trapped electron mode (TEM) dominate in the small and large k_\theta region, respectively, where k_\theta is the poloidal wavenumber. The TEMdominant region becomes wider by increasing (decreasing) \kappa_T_\rme ( \kappa_T_\rmi ) or by decreasing \kappa_n . For the nominal parameters of the ITER baseline scenario, the maximum growth rate of dominant ITG instability in the kinetic electron model is about three times larger than that in the adiabatic electron model. The normalized linear frequency depends on the value of \tau , rather than the value of T_\rme or T_\rmi , in both the adiabatic and kinetic electron models. The nonlinear simulation results show that the ion heat diffusivity in the kinetic electron model is quite a lot larger than that in the adiabatic electron model, the radial structure is finer and the time oscillation is more rapid. In addition, the magnitude of the fluctuated potential at the saturated stage peaks in the ITGdominated region, and contributions from the TEM (dominating in the higher k_\theta region) to the nonlinear transport can be neglected. In the adiabatic electron model, the zonal radial electric field is found to be mainly driven by the turbulent energy flux, and the contribution of turbulent poloidal Reynolds stress is quite small due to the toroidal shielding effect. However, in the kinetic electron model, the turbulent energy flux is not strong enough to drive the zonal radial electric field in the nonlinear saturated stage. The kinetic electron effects on the mechanism of the turbulencedriven zonal radial electric field should be further investigated.

