Tungsten behaviour in radiatively cooled plasma discharges in ASDEX Upgrade

In the all W ASDEX Upgrade impurities are injected routinely for radiative cooling. Amongst the different species used, nitrogen has been shown to provide the best cooling effect, while maintaining or even improving the energy confinement. Depending on the divertor plasma parameters a reduction or increase of the W sputtering yield compared to the unseeded phases is observed. Although usually the energy confinement is improved during N2 seeding, no increased W concentrations are observed. Obviously the particle confinement is not improved and there are no signs of central density peaking. Moreover the sustained regular ELM activity prevents a strong inward transport of W across the pedestal region. In contrast, when using Ar as seeding impurity the ELM frequency is strongly reduced and also a change of central particle transport is observed provoking peaking of the W density profile.