Validity limits of the passive treatment of impurities in gyrokinetic tokamak simulations

In gyrokinetic simulations of turbulent impurity transport, trace impurity species are often treated as passive species, in the sense that they are not included in Maxwell equations. This is consistent with the assumption that impurities with low enough concentrations are impacted by turbulence generated by electrons and main ions, but do not impact it significantly in return. In this work, we relax this assumption, and investigate the active impacts of impurity on impurity transport as a function of its concentration, in the presence of trapped-particle-driven turbulence. We focus on $W^{40+}$ tungsten, which is relevant for modern tokamaks, and adopt a reduced gyrokinetic bounce-averaged model for trapped particles in a simplified tokamak geometry. The impacts depend on the relationship between equilibrium density gradient and temperature gradient. When these gradients are equal, we observe that tungsten can be treated as a passive species for concentrations below $2\times 10^{-4}$. Above this concentration, the impurity significantly impacts both density and heat transport, essentially quenching them for concentrations above $10^{-3}$. This quenching occurs as electric potential fluctuations become in phase with impurity density fluctuations.