Impurity pinch generated by trapped particle driven turbulence

Impurity turbulent transport in tokamak plasmas is investigated in the case of trapped ion mode (TIM) and trapped electron mode (TEM) using a non-linear bounce-averaged gyrokinetic model. In particular, we focus on the thermo-diffusive pinch and the curvature-driven pinch observed in non-linear simulations. Thermodiffusion is analyzed independently of curvature pinch by disabling the latter artificially. A parameter scan shows that the direction of thermodiffusion depends mainly on two factors. First, it depends on the sign of the turbulence phase velocity, and the resulting transport is inward in the case of TEM whereas it is outward in the case of TIM. Second, it depends on the sign of the impurity temperature gradient. Moreover, the magnitude of the thermodiffusion tends to decrease with respect to the impurity charge Z, while it is proportional to the intensity of background turbulence. Lastly, the direction of the curvature pinch is investigated as a function of the magnetic shear. The results of non-linear simulations are compared with quasi-linear theory. The Kubo number is calculated for our non-linear simulations to justify this comparison. Quasi-linear theory is found to be in qualitative agreement with our gyrokinetic numerical observations.