Overview of ASDEX Upgrade results

Abstract

The closed ASDEX Upgrade Divertor II, `LYRA', is capable of handling heating powers of up to 20 MW or P/R of 12 MW/m, owing to a reduction of the maximum heat flux to the target plates by more than a factor of 2 compared with the open Divertor I. This reduction is caused by high radiative losses from carbon and hydrogen inside the divertor region and is in agreement with B2-EIRENE modelling predictions. At medium densities in the H mode, the type I ELM behaviour shows no dependence on the heating method (NBI, ICRH). ASDEX Upgrade-JET dimensionless identity experiments showed compatibility of the L-H transition with core physics constraints, while in the H mode confinement, inconsistencies with the invariance principle were established. At high densities close to the Greenwald density, the MHD limited edge pressures, the influence of divertor detachment on separatrix parameters and increasing edge transport lead to limited edge densities and finally to temperatures below the critical edge temperatures for H mode. This results in a drastic increase of the H mode threshold power and an upper H mode density limit with gas puff refuelling. The H mode confinement degradation approaching this density limit is caused by the ballooning mode limited edge pressures and `stiff' temperature profiles relating core and edge temperatures. Repetitive high field side pellet injection allows for H mode operation well above the Greenwald density; moreover, higher confinement than with gas fuelling is found up to the highest densities. Neoclassical tearing modes limit the achievable β depending on the collisionality at the resonant surface. In agreement with the polarization current model, the onset β is found to be proportional to the ion gyroradius in the collisionless regime, while higher collisionalities are stabilizing. The fractional energy loss connected with saturated modes at high pressures is about 25%. A reduction of neoclassical mode amplitude and an increase of β have been demonstrated by using phased ECRH and ECCD in the O point of islands. Advanced tokamak operation with internal transport barriers for both ions and electrons has been achieved with flat shear profiles and q0 > 1 or with reversed shear and qmin > 2. With flat shear a stationary H mode scenario was maintained for 40 confinement times and several internal skin times with βN = 2 and HITERL-89P = 2.4, where fishbones keep q0 at 1. βN is limited by either neoclassical tearing modes in the case of flat shear or kink modes with reversed shear.