Role of nitrogen inventory and ion enhanced N-H recombination in the ammonia formation on tungsten walls. A DC glow discharge study

• Published in: Vacuum Vol. 151; pp. 66 - 72
• Main Authors: de Castro, A., Tabarés, F.L.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-05-2018
• Subjects: Ammonia; Glow discharge plasmas; Helium bombardment; N-H surface recombination; Nitrogen inventory; Tungsten; Ammonia; Glow discharge plasmas; Helium bombardment; Tungsten; N-H surface recombination; Nitrogen inventory
• ISSN: 0042-207X; 1879-2715
• DOI: 10.1016/j.vacuum.2018.02.004

Dedicated studies using Glow Discharge plasmas on tungsten (W) walls have been carried-out, trying to understand the surface chemistry and the underlying processes behind the mechanism of the ammonia formation that takes place during nitrogen (N2) seeded discharges on the inner walls of magnetic nuclear fusion devices. The experiments with pure deuterium (D2) plasma irradiation on a cold rolled tungsten sheet (previously irradiated with pure N2 plasma) at 150 °C showed the formation of deuterated ammonia as a result of the interaction between the trapped nitrogen atoms and the impinging deuterium on the surface. Conversely, the symmetric experiment based on the irradiation of the tungsten wall (previously irradiated with D2 plasma) with N2 plasma did not show significant ammonia production, thus suggesting that the presence of nitrogen on the surface is the first and crucial step in the ammonia formation mechanism. The study was completed with experiments that employed N2-H2-(Helium) mixed plasmas trying to understand the role of the helium bombardment (unavoidable in a Deuterium-Tritium fusion reactor) in the N-H recombination. An enhancement in ammonia generation up to a 45% was observed with increasing helium plasma contents.
•Ammonia is produced in tokamak N2 seeded discharges that are employed to mitigate the heat load and erosion onto the W divertor.•The interaction between nitrogen and tritium would originate “tritiated” ammonia, thus constituting a radioactive safety risk.•The presence of nitrogen (inventory) on the tungsten surface has been experimentally demonstrated as the fcrucial step in the formation mechanism.•Bombardment of the surface with helium in a reactor environment must be taken into account as its presence will be unavoidable in a D-T fusion device.•The enhanced N-H recombination induced by the energetic He bombardment seems to be related with the W surface (and associated chemistry) modification.