Abstract
The variation of the density profile shape of H-modes in ASDEX Upgrade is governedby time constants much larger than τE. A method to determine these time constantsis presented. Especially high density discharges with moderate heating and constant gas puffshow very long phases of increasingly peaking density profiles which may not reach a steady state, althoughthe H-mode pedestal remains constant during the peaking process.If heated more centrally with ICRH the peaking is strongly reduced and a steady state is reached. The behaviour can be well modelled using D≈0.1χ and the neoclassicalWare-pinch. Strongly peaked density profiles lead to an earlier onset of neoclassical tearing modes (NTMs) in agreement with theoretical predictions of the strong influence of ∇n/n on the bootstrap current. Also for impurities an effect of the heat flux profile is observed: central heating with ECRH leads to a reduction of heavy ions in the plasma core.
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