The seeding of gaseous impurities will be mandatory for ITER to protect the tungsten divertor from local heat loads. A promising candidate for radiative cooling seems to be nitrogen, but reactions with hydrogen may influence tokamak operation. In particular, the formation of ammonia can become a serious issue for gas plants, cryo pumps and wall conditioning. Therefore, the residual gases of discharges with and without nitrogen seeding at ASDEX Upgrade were investigated by mass spectrometry. For the deconvolution of the measured spectra a method was developed that takes into account different protium concentrations in different compounds. The applied absolute calibration of the mass spectrometers allowed a quantitative analysis. Significant formation of ammonia was observed during nitrogen-seeded H-mode discharges. Up to 8% of the seeded nitrogen atoms were detected in the form of ammonia molecules. Ammonia was present not only in the residual gas of the nitrogen-seeded discharge itself, but also in the residual gases of subsequent unseeded discharges. For calibration purposes ammonia was injected into the plasma vessel of ASDEX Upgrade without plasma operation. A significant part of the ammonia was retained in the vessel. The simultaneous observation of partly deuterated ammonia strongly indicates an interaction between metal walls and ammonia.

The medium size divertor tokamak ASDEX Upgrade (major and minor radii 1.65 m and 0.5 m, respectively, magnetic-field strength 2.5 T) possesses flexible shaping and versatile heating and current drive systems. Recently the technical capabilities were extended by increasing the electron cyclotron resonance heating (ECRH) power, by installing 2 × 8 internal magnetic perturbation coils, and by improving the ion cyclotron range of frequency compatibility with the tungsten wall. With the perturbation coils, reliable suppression of large type-I edge localized modes (ELMs) could be demonstrated in a wide operational window, which opens up above a critical plasma pedestal density. The pellet fuelling efficiency was observed to increase which gives access to H-mode discharges with peaked density profiles at line densities clearly exceeding the empirical Greenwald limit. Owing to the increased ECRH power of 4 MW, H-mode discharges could be studied in regimes with dominant electron heating and low plasma rotation velocities, i.e. under conditions particularly relevant for ITER. The ion-pressure gradient and the neoclassical radial electric field emerge as key parameters for the transition. Using the total simultaneously available heating power of 23 MW, high performance discharges have been carried out where feed-back controlled radiative cooling in the core and the divertor allowed the divertor peak power loads to be maintained below 5 MW m−2. Under attached divertor conditions, a multi-device scaling expression for the power-decay length was obtained which is independent of major radius and decreases with magnetic field resulting in a decay length of 1 mm for ITER. At higher densities and under partially detached conditions, however, a broadening of the decay length is observed. In discharges with density ramps up to the density limit, the divertor plasma shows a complex behaviour with a localized high-density region in the inner divertor before the outer divertor detaches. Turbulent transport is studied in the core and the scrape-off layer (SOL). Discharges over a wide parameter range exhibit a close link between core momentum and density transport. Consistent with gyro-kinetic calculations, the density gradient at half plasma radius determines the momentum transport through residual stress and thus the central toroidal rotation. In the SOL a close comparison of probe data with a gyro-fluid code showed excellent agreement and points to the dominance of drift waves. Intermittent structures from ELMs and from turbulence are shown to have high ion temperatures even at large distances outside the separatrix.

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