Influence of surface work function and neutral gas temperature on mechanism of H− production in negative hydrogen ion sources

Abstract

Negative hydrogen ion sources (NHISs) based on surface production with cesium (Cs) seeded can fulfill the demanded parameters for neutral beam injection systems for ITER. In this study, the Global Model for Negative Hydrogen Ion Source based on volume-produced H− ions is developed to include surface-produced H− ions and is validated against experimental data obtained in a planar inductively coupled plasma discharge used for study of Cs effect on H− production. The H− density predicted by the model decreases three times with surface work function from 2.1 to 4.5 eV, achieving good agreement with the experimental results, as surface conversion yield of particles to H− ions shows exponential decline with surface work function. The model predicts the rise in neutral gas temperature remarkably enhances surface production but reduces volume production of H− ions, because of increase in surface conversion yield of H atoms to H− ions and in electron temperature, respectively. The dependences of H− production on surface work function and neutral gas temperature are analyzed by evaluating creation rates of the H− ions from different reaction pathways. The developed model can be applied for prediction of H− production in NHISs and ultimate parameter optimization of negative ion beams for fusion reactors.