Z dependence of the core impurity transport in ASDEX Upgrade H mode discharges

The dependence of core plasma impurity transport on the Z number has been i nvestigated for ASDEX Upgrade H mode discharges. For the elements Ne, Ar, K r and Xe the diffusion coefficient in the centre is D less than or equal to 6 x 10(-2) m(2)/s and rises with the radial distance from the centre. With increasing Z number the transport becomes strongly convective with inward directed drift velocities that produce very peaked impurity densities for h igh Z. The inward drift decreases with decreasing deuterium density gradien t. Neoclassical transport of the impurities has been calculated numerically . The calculated diffusion coefficient and drift velocity are close to the experimental values for the lower-Z elements Ne and Ar. However, for high Z , the calculated diffusion coefficient is smaller by a factor of up to 2.5 and the inward drift velocity is too small by a factor of 10. Toroidal rota tion of the plasma that leads to an increased impurity density on the outbo ard side of the flux surfaces is not taken into account by the neoclassical calculations. Inboard/outboard asymmetries are not present for Ar and Ne w ith toroidal Mach number M-tor around 1. However, for heavier elements than Kr with M-tor approximate to 2 and an outboard/inboard ratio of approximat e to 1.5, poloidal variation of the impurity density is important and might account for the discrepancy between the measured, and calculated transport coefficients.