Formation of ammonia during nitrogen-seeded discharges at ASDEX Upgrade

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. (paper)