Impurity dynamics in the presence of transport barriers in tokamaks

Impurity transport in tokamak core plasmas is investigated with a three-dimensional global fluid code. The diffusion coefficient and the pinch velocity of impurity transport in tokamaks are studied using the fluid model for ion temperature gradient and trapped electron mode driven turbulence in tokamak plasmas. It is shown that in the presence of an internal transport barrier created by a reversed magnetic shear configuration or external E x B shear flow, a reversal of impurity pinch velocity is obtained, which changes from inward direction to outward direction. This scenario is favorable for expelling impurities from the central region and decontaminating the core plasma. The pinch reversal is attributed to a change of sign of the curvature pinch velocity. This modification is mostly due to the reversal of magnetic shear for the hollow q profile. When a strong E x B shear flow is externally imposed, it is rather due to a change of the turbulence mean phase velocity. (C) 2010 American Institute of Physics. [doi:10.1063/1.3481462]