Effects of ion temperature anisotropy on an electrostatic turbulent transport in tokamak plasmas

The impacts of ion temperature anisotropy on the linear frequency and the nonlinear transport driven by the electrostatic instability are investigated using the gyrokinetic code NLT in the core region of a deuterium plasma under the cyclone base case parameters. The anisotropy factor T⊥/T∥ is introduced to describe the temperature anisotropy in the equilibrium distribution function, with T∥ and T⊥ denoting the parallel and perpendicular temperatures, respectively. These results from linear simulations illustrate that, in the situations with T⊥/T∥<2.0, the ion temperature gradient (ITG) instability can be significantly suppressed by decreasing T⊥/T∥, while the trapped electron mode (TEM) is slightly destabilized. In the situations with T⊥/T∥>2.0, effects of T⊥/T∥ on both the ITG instability and the TEM become less pronounced. The peak around T⊥/T∥≈2 may depend sensitively on the choice of parameters such as the density and temperature gradients. The nonlinear simulations reveal that the turbulent energy and particle fluxes increase at first and then decrease as the increment of T⊥/T∥, the peak values appear around T⊥/T∥=2.0. The poloidal spectrum of the perturbed potential in the nonlinear saturated stage peaks within the ITG-dominant region, indicating the predominant role of the ITG instabilities in driving the turbulent transport. As a comparison, an isotropic temperature scenario presented by a Maxwellian equilibrium distribution with equal energy to the anisotropic distribution is considered. It is found that the growth rate of ITG instability, as well as the ITG-driven energy and particle fluxes, are overestimated in the effective Maxwellian scenario compared to the temperature anisotropy scenario in both T⊥/T∥<1.0 and T⊥/T∥>2.0 regions. Especially, when T⊥/T∥ is far away from 1.0, the discrepancy becomes more significant.