A volume-averaged model of nitrogen-hydrogen plasma chemistry to investigate ammonia production in a plasma-surface-interaction device

Nitrogen impurity seeding is a promising technique for increasing the radiative power dissipation rate in the edge plasma of a fusion device. It will be required in future fusion devices such as ITER to reduce the directed heat flux on the divertor strike-points to within erosion limits. However, chemical reactions between nitrogen and fuel isotopes may complicate tritium control measures by increasing in-vessel retention and impacting the gas-handling plant. To gain insight into the nitrogen-hydrogen plasma chemistry a volume-averaged (global) model is developed and compared with experimental measurements in the MAGnetised Plasma Interaction Experiment plasma device. A set of 702 reactions is compiled and used to model the population dynamics of 51 relevant neutral, ionic, electron, surface and metastable excited state species. Stable equilibrium values are compared to results from an experimental investigation in which a combination of mass spectrometry, Langmuir probe analysis and optical emission spectroscopy is used to determine neutral and positive-ionic trends under the same conditions. The dominant ammonia production mechanism is found to be the Langmuir-Hinshelwood reaction between adsorbed atomic hydrogen and NH_2s above 25% hydrogen concentration. For lower hydrogen proportions the Eley-Rideal reaction between free atomic hydrogen and NH_2s is found to dominate. The dominant loss mechanism (for all compositions) is found to be electron impact dissociation into neutral fragments.