Low-current electric discharges in H2-He mixtures

Tests are made of the ability of currently available theory to predict experimental ratios of the electric field to gas density E/N required for the steady-state operation of low-current, moderate-pressure discharges in hydrogen-helium mixtures. In the experiments the fractional concentration of H2 in He was varied from 1% to 4% and the discharge current, i.e., 40±10 μA, and total gas density, i.e., 1023–1025 atoms/m3, were chosen to avoid gas heating and striations. The experimental E/N values for the 4-mm-diam tube varied from 6×10−21 V m2 for 1% H2 in He at a gas density of 7×1024 m−3 to about 4×10−20 V m2 for 4% H2 in He at 1.5×1023 m−3. Theoretical steady-state E/N values versus gas density were calculated using (a) solutions of the Boltzmann equation for He metastable excitation, He ionization, and H2 ionization rate coefficients; (b) published theoretical calculations of the effects of a finite Debye length and a finite ion mean free path on the electron loss rate; and (c) models for the ion-molecule conversion and Penning ionization processes. The differences between theoretical and experimental values of E/N are less than 10% for total gas densities above 6×1023 m−3, but are 20–30% for a gas density of 1.6×1023 m−3. We conclude that except at low gas densities the simple theory gives a good description of low-current discharges in this gas mixture.