Accumulation of impurities in advanced scenarios

Impurity behaviour in advanced scenarios at ASDEX Upgrade and JET has been investigated. For ASDEX Upgrade improved H-mode plasmas at δ=0.35, accumulation of tungsten and metallic impurities was observed inside r/a=0.3 in cases with weak central heating. Central ECRH suppressed the accumulation by increasing the anomalous diffusion and by flattening the profile of the main plasma density which reduces neoclassical inward convection for the impurities. These effects are well described by impurity transport simulation and calculated v/D values agree with the measurements within a factor of 2. In JET reversed shear discharges with internal transport barriers, metallic impurities accumulate in cases with too strong peaking of the density profile. The peaking increases with the impurity charge and is low for the low-Z elements C and Ne as expected from neoclassical convection