Fluid and Gyrokinetic Simulations of Impurity Transport at JET

Impurity transport in the Joint European Torus is studied using interpretative transport analysis of dedicated impurity injection experiments. Impurity transport coefficients due to Ion-Temperature-Gradient (ITG) mode and Trapped-Electron (TE) mode turbulence are calculated using a multi-fluid model which is compared with quasi-linear and nonlinear gyrokinetic simulation results obtained with the code GENE. The sign of the impurity convective velocity (pinch) and its various contributions are discussed. The dependence of the impurity transport coefficients and impurity peaking factor -nZ/nZ on plasma parameters like impurity charge number Z, ion logarithmic temperature gradient, collisionality, E¥B shearing, and charge fraction are investigated. It is found that for the studied ITG dominated JET discharges, both the fluid and gyrokinetic results show an increase of the impurity peaking factor for low Z-values followed by a saturation at moderate values of impurity peaking, much below the neoclassical predictions, for large values of Z. The results are in qualitative agreement with the experimental trends observed for the injected impurities (Ne, Ar, Ni) whereas for the background carbon species the observed flat or weakly hollow C profiles are not well reproduced by the simulations.
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