Gyro-Landau-fluid simulations of impurity effects on ion temperature gradient driven turbulence transport

The effects of impurities on ion temperature gradient (ITG) driven turbulence transport in tokamak core plasmas are investigated numerically via global simulations of microturbulence with carbon impurities and adiabatic electrons. The simulations use an extended fluid code (ExFC) based on a four-field gyro-Landau-fluid (GLF) model. The multispecies form of the normalized GLF equations is presented, which guarantees the self-consistent evolution of both bulk ions and impurities. With parametric profiles of the cyclone base case, well-benchmarked ExFC is employed to perform simulations focusing on different impurity density profiles. For a fixed temperature profile, it is found that the turbulent heat diffusivity of bulk ions in a quasi-steady state is usually lower than that without impurities, which is contrary to the linear and quasi-linear predictions. The evolutions of the temperature gradient and heat diffusivity exhibit a fast relaxation process, indicating that the destabilization of the outwardly peaked impurity profile is a transient state response. Furthermore, the impurity effects from different profiles can obviously influence the nonlinear critical temperature gradient, which is likely to be dominated by linear effects. These results suggest that the improvement in plasma confinement could be attributed to the impurities, most likely through adjusting both heat diffusivity and the critical temperature gradient.