Dissociative recombination process of ammonium for HN-MAR in high density D-N plasmas

Ammonia molecules formed in a nitrogen-seeded D-fueled plasma are shown to catalyze volumetric recombination of hydrogen ions. This Hydronitrogen Molecular Assisted Recombination (HN-MAR) process occurs by charge and ion exchange reactions with ammonia (ND ₃ ), leading to subsequent volumetric recombination with ND ₃ ⁺ and ND ₄ ⁺ molecular ions. When the plasma density is raised to the point where volumetric recombination dominates plasma losses to the wall [n _e > 10 ¹⁸ m ⁻³ ], a calibrated electrostatic quadrupole plasma analyzer shows that the ND ₄ ⁺ ammonium ion density fraction drops from 0.55 to 0.11. We show that this is consistent with a 0-d kinetics model that includes the HN-MAR process with rates that are proportional to the electron density. The results suggest that direct injection of ammonia would provide a more efficient pathway to achieve divertor detachment in future tokamak experiments.