Optimization of confinement, stability and power exhaust of the ELMy H-mode in ASDEX Upgrade

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Abstract
ASDEX Upgrade results on crucial issues of H-mode operation are reported, i.e. density profile shape, onset conditions for neoclassical tearing modes (NTM) and smoothing of the power exhaust without losing confinement. Starting from experiments on density peaking at high density, a model for particle transport is developed using the neoclassical particle pinch and assuming that the particle diffusion coefficient D is locally proportional to the heat conductivity χ. This assumption links the D profile to the heat-flux profile due to the stiffness of the temperature profiles. This suggests that density profile shapes of a centrally heated reactor will be significantly flatter than in a device heated by neutral beam injection (NBI). In the discharges with density peaking (3,2)-NTMs develop at significantly smaller values of βN than in H-modes with rather flat density profiles. Local analysis reveals that for such discharges the density gradient term of the bootstrap current is of the same size as the temperature gradient term. For the experimental temperature (T) and density (n) profiles close to the q = 1.5 surface, the model predictions agree well with the observed onset of the NTM. Finally, we describe the H-mode operation with edge localized modes (ELMs) of type-II in ASDEX Upgrade. This ELM type allows one to smoothen the power exhaust significantly as compared to type-I ELMs without reducing the confinement. Pure type-II ELMy H-modes are so far limited to safety factors q>4.2 at 95% poloidal flux (q95) and high triangularity δ≈0.4, but the latest results indicate that both thresholds may be reduced if the current is increased and the configuration is kept very close to double null.