Overview of ASDEX Upgrade results

The closed ASDEX Upgrade divertor II 'Lyra' is capable to handle heating powers up to 20 MW or P/R of 12 MW/m due to a reduction of maximum heat flux to the target plates by more than a factor of two compared to 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 from 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, influence of divertor detachment on seperatrix parameters and increasing edge transport lead to limited edge densities and finally temperatures below the critical edge temperatures for H-mode. This results in a drastically 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, and 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 gyro-radius in the collisionless regime, while higher collisionalities are stabilizing. The fractional energy loss connected with saturated modes at high pressures is about 25%. Reduction of neoclassical mode amplitude and increase of β has been demonstrated by using phased ECR heating and current drive in the islands O-point. Advanced tokamak operation with internal transport barriers for both ions and electrons have 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 an HITER-89P= 2.4, where fishbones keep the q0 at one. βN is limited by either neoclassical tearing modes in case of flat shear or kink modes with reversed shear. (author)