Effects of light impurities on zonal flow activities and turbulent thermal transport

Abstract

Nonlinear effects of light impurities on the zonal flow activities and electrostatic ITG (ion temperature gradient) mode are investigated with gyrokinetic simulations. For the investigation, a new multiple-ion-species gyrokinetic Poisson solver is implemented numerically. Benchmark tests for the new solver show good agreements with theoretical and previous simulation results. Nonlinear ITG simulations with and without light impurities are compared. To isolate nonlinear effects of impurities on ITG, simulation parameters are set to exhibit approximately identical spectra of linear growth rates for the admixed and pure deuterium cases (i.e., the cases with and without the impurities). With an intermediate safety factor (∼1.4), the admixed case shows smaller heat transport and more robust E × B staircase structures than the pure deuterium case. The locations of the transport suppression and staircases are strongly correlated, which indicate that light impurities have stabilizing effects on ITG by enhancing the staircase-like E × B shearing. Especially, the radial correlation length of the fluctuations is significantly reduced for the admixed case. On the other hand, the stabilizing effect of impurities is weakened with a high safety factor (∼5). In those cases, strong geodesic acoustic mode activity is observed, and the electric field is dominated by oscillating components instead of stationary staircases.