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

Abstract 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 (N 2 ) seeded discharges on the inner walls of magnetic nuclear fusion devices. The experiments with pure deuterium (D 2 ) plasma irradiation on a cold rolled tungsten sheet (previously irradiated with pure N 2 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 D 2 plasma) with N 2 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 N 2 -H 2 -(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.