Non equilibrium velocity distributions of H- ions in H2 plasmas and photodetachment measurements

A theoretical study of the energy distribution function of the negative hydrogen ion H- in typical conditions for multicusp ion sources is presented. The case of H/H2 mixture is studied by a Monte Carlo solution of the kinetic equation for H- transport. A simple analytical model is obtained for the case of a fully dissociated H plasma and uniform reduced electric field (E/N). Results are in good agreement with the two-group distribution deduced from laser photodetachment experiments and explain the low energy group as an effect of the charge exchange collisions of negative ions with H atoms.