Overview of ASDEX Upgrade results

Abstract

Ion and electron temperature profiles in conventional L and H mode on ASDEX Upgrade are generally stiff and limited by a critical temperature gradient length ∇T/T as given by ion temperature gradient (ITG) driven turbulence. ECRH experiments indicate that electron temperature (Te) profiles are also stiff, as predicted by electron temperature gradient turbulence with streamers. Accordingly, the core and edge temperatures are proportional to each other and the plasma energy is proportional to the pedestal pressure for fixed density profiles. Density profiles are not stiff, and confinement improves with density peaking. Medium triangularity shapes (δ<0.45) show strongly improved confinement up to the Greenwald density nGW and therefore higher βvalues, owing to increasing pedestal pressure, and H mode density operation extends above nGW. Density profile peaking at nGW was achieved with controlled gas puffing rates, and first results from a new high field side pellet launcher allowing higher pellet velocities are promising. At these high densities, small type II ELMs provide good confinement with low divertor power loading.In advanced scenarios the highest performance was achieved in the improved H mode with HL-89PβN ≈ 7.2 at δ = 0.3for five confinement times, limited by neoclassical tearing modes (NTMs) at low central magnetic shear (qmin ≈ 1). The T profiles are still governed by ITG and trapped electron mode (TEM) turbulence,and confinement is improved by density peaking connected with low magnetic shear. Ion internal transport barrier (ITB) discharges - mostly with reversed shear (qmin>1) and L mode edge - achieved HL-89P ⩽ 2.1 and are limited to βN ⩽ 1.7 by internal and external ideal MHD modes. Turbulence driven transport is suppressed, in agreement with the E × B shear flow paradigm, and core transport coefficients are at the neoclassical ion transport level, where the latter was established by Monte Carlo simulations. Reactor relevant ion and electron ITBs with Te ≈ Ti ≈ 10 keV were achieved by combining ion and electron heating with NBI and ECRH, respectively.In low current discharges full non-inductive current drive was achieved in an integrated high performance H mode scenario with [`n]e = nGW, high βp = 3.1, βN = 2.8 and HL-89P = 1.8,which developed ITBs with qmin ≈ 1. Central co-ECCD at low densities allowsa high current drive fraction of >80%, while counter-ECCD leads to negative central shear and formation of an electron ITB with Te(0)>12 keV.MHD phenomena, especially fishbones, contribute to achieving quasi-stationary advanced discharge conditions and trigger ITBs, which is attributed to poloidal E × B shearing driven by redistribution of resonant fast particles. But MHD instabilities also limit the operational regime of conventional (NTMs) and advanced (double tearing, infernal and external kink modes) scenarios. The onset βN for NTM is proportional to the normalized gyroradius ρ*. Complete NTM stabilization was demonstrated at βN = 2.5 using ECCD at the island position with 10% of the total heating power. MHD limits are expected to be extended using current profile control by off-axis current drive from more tangential NBI combined with ECCD and wall stabilization.Presently, the ASDEX Upgrade divertor is being adapted to optimal performance at higher δ's and tungsten covering of the first wall is being extended on the basis of thepositive experience with tungsten on divertor and heat shield tiles.