Parametric dependences of impurity transport in tokamaks

Impurity accumulation has been observed in many tokamak plasma experiments and predicted by collisional transport theory. However in most experimental cases observed transport departs from the collisional predictions ('anomalous transport'), admittedly due to turbulent effects. Diffusion is used as a criterion to assess the relative contributions of collisions and turbulence to observed impurity transport in the published literature. In the ITER relevant confinement modes (H-mode and internal transport barrier scenarios) turbulence always contributes but with large variations. The predicted parametric dependences of impurity transport are reviewed when available. Information on turbulent diffusion is scarce. Predicted collisional and turbulent convection velocities can be directed either inwards or outwards. The collisional predictions match satisfactorily a number of observations. Theoretical predictions of the turbulent convection velocity including recent quasilinear gyrokinetic results are in qualitative agreement with a dedicated experiment. This is only a first step toward a complete validation of the turbulent impurity convection theoretical models and predictive modelling of impurity transport in tokamaks.