Behaviour of density profiles of H-mode discharges in ASDEX Upgrade

The variation of the density profile shape of H-modes in ASDEX Upgrade is governed by time constants much larger than τE. A method to determine these time constants is presented. Especially high density discharges with moderate heating and constant gas puff show very long phases of increasingly peaking density profiles which may not reach a steady state, although the 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 neoclassical Ware-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.