Collisional kinetics of non-uniform electric field, low-pressure, direct-current discharges in H2

Abstract

A model of the collisional kinetics of energetic hydrogen atoms, molecules and ions in pure H2 discharges is used to predict Hα emission profiles and spatial distributions of emission from the cathode regions of low-pressure, weakly ionized discharges for comparison with a wide variety of experiments. Positive and negative ion-energy distributions are also predicted. The model developed for spatially uniform electric fields and current densities less than 10−3 A m−2 is extended to non-uniform electric fields, current densities of 103 A m−2 and electric field to gas density ratios E/N = 1.3 MTd at 0.002–5 Torr pressure. (1 Td = 10−21 V m2 and 1 Torr = 133 Pa.) The observed far-wing Doppler broadening and spatial distribution of the Hα emission is consistent with reactions among H+, H2+, H3+ and H− ions, fast H atoms and fast H2 molecules, and with reflection, excitation and attachment to fast H atoms at surfaces. The Hα excitation and H− formation occur principally by collisions of fast H, fast H2 and H+ with H2. Simplifications include using a one-dimensional geometry, a multi-beam transport model, and the average cathode-fall electric field. The Hα emission is linear with current density over eight orders of magnitude. The calculated ion-energy distributions agree satisfactorily with experiment for H2+ and H3+, but are only in qualitative agreement for H+ and H−. The experiments successfully modeled range from short-gap, parallel-plane glow discharges to beam-like, electrostatic-confinement discharges.