Study of Z-Dependence of Impurity Transport at JET
The development of an ITER scenario will be an optimisation between confinement, stability and pollution by impurities. It will have to take into account the transport of He, with a charge of 2, product of the D-T reaction but also the transport of impurities originating from the plasma edge resulting from the sputtering of the wall and divertor material, i.e. Be (Z = 4), C (Z = 6) and W (Z = 74). The reduction of the divertor power load will also require seeding impurity such as Ar (Z = 18). Clearly, the understanding of the Z dependence of impurity transport is, if not necessary, a beneficial tool for the optimisation of ITER scenarios. In addition, the study of the Z dependence of impurity transport gives insight into mechanisms at play, in particular how micro-instabilities drive particle transport, and highlights the regions influenced by neoclassical transport. Initial results on the Z dependence of impurity transport in H-mode discharge were reported and compared with theoretical predictions. During the 2006-2007 campaign, experiments have specifically focused on clarifying the Z dependence of trace impurity transport in various plasma conditions. L-mode, H-mode discharges have been probed. The aim is to characterise the variation in the Z dependence of impurity transport across the plasma radius, and also to document for which plasma conditions an effect of the neoclassical transport is observed.