Stuttgart 2012 – scientific programme

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P: Fachverband Plasmaphysik

P 12: Poster: Plasma-Wand-Wechselwirkung

P 12.3: Poster

Tuesday, March 13, 2012, 16:30–19:00, Poster.III

Formation of ammonia during nitrogen seeded discharges at ASDEX Upgrade — •Daniel Neuwirth, Volker Rohde, Thomas Schwarz-Selinger, and ASDEX Upgrade Team — Max-Planck-Institut für Plasmaphysik, EURATOM Association, Garching, Germany

The seeding of impurities will be mandatory for ITER to protect the tungsten divertor from local heat loads. Presently the best candidate seems to be nitrogen, but reactions with hydrogen are possible. The formation of ammonia is a serious issue for gas plants, cryo pumps and plasma control. Therefore the residual gases of discharges with and without nitrogen seeding at ASDEX Upgrade have been 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. A significant formation of ammonia was observed during nitrogen seeded H-mode discharges. Up to 8% of the seeded nitrogen atoms have formed ammonia molecules. Ammonia was present in the residual gas of the nitrogen seeded discharge itself, but also in the residual gases of subsequent non nitrogen seeded discharges. Futhermore a reduced partial pressure of methane during nitrogen seeded discharges was observed. Undeuterated 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 indicates an interaction between tungsten wall and ammonia.