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

Abstract

Nitrogen impurity seeding is a promising technique for increasing the radiative power dissipation
\nrate in the edge plasma of a fusion device. It will be required in future fusion devices such as ITER
\nto reduce the directed heat flux on the divertor strike-points to within erosion limits. However,
\nchemical reactions between nitrogen and fuel isotopes may complicate tritium control measures by
\nincreasing in-vessel retention and impacting the gas-handling plant. To gain insight into the
\nnitrogen–hydrogen plasma chemistry a volume-averaged (global) model is developed and
\ncompared with experimental measurements in the MAGnetised Plasma Interaction Experiment
\nplasma device. A set of 702 reactions is compiled and used to model the population dynamics of 51
\nrelevant neutral, ionic, electron, surface and metastable excited state species. Stable equilibrium
\nvalues are compared to results from an experimental investigation in which a combination of mass
\nspectrometry, Langmuir probe analysis and optical emission spectroscopy is used to determine
\nneutral and positive-ionic trends under the same conditions. The dominant ammonia production
\nmechanism is found to be the Langmuir–Hinshelwood reaction between adsorbed atomic hydrogen
\nand NH2s above 25% hydrogen concentration. For lower hydrogen proportions the Eley–Rideal
\nreaction between free atomic hydrogen and NH2s is found to dominate. The dominant loss
\nmechanism (for all compositions) is found to be electron impact dissociation into neutral fragments.