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

Abstract

ASDEX Upgrade results on crucial issues of H-mode operation arereported, i.e. density profile shape, onset conditions for neoclassicaltearing modes (NTM) and smoothing of the power exhaust without losingconfinement. Starting from experiments on density peaking at highdensity, a model for particle transport is developed using theneoclassical particle pinch and assuming that the particle diffusioncoefficient D is locally proportional to the heat conductivity χ.This assumption links the D profile to the heat-flux profile due tothe stiffness of the temperature profiles. This suggests that densityprofile shapes of a centrally heated reactor will be significantlyflatter than in a device heated by neutral beam injection (NBI). In thedischarges with density peaking (3,2)-NTMs develop at significantlysmaller values of βN than in H-modes with rather flatdensity profiles. Local analysis reveals that for such discharges thedensity gradient term of the bootstrap current is of the same size asthe temperature gradient term. For the experimental temperature (T)and density (n) profiles close to the q = 1.5 surface, the modelpredictions agree well with the observed onset of the NTM. Finally, wedescribe the H-mode operation with edge localized modes (ELMs) oftype-II in ASDEX Upgrade. This ELM type allows one to smoothen the powerexhaust significantly as compared to type-I ELMs without reducing theconfinement. Pure type-II ELMy H-modes are so far limited to safetyfactors q>4.2 at 95% poloidal flux (q95) and hightriangularity δ≈0.4, but the latest results indicate thatboth thresholds may be reduced if the current is increased and theconfiguration is kept very close to double null.