IONIZATION COEFFICIENTS IN HELIUM, NEON AND HELIUM-NEON MIXTURES

The spatial growth of pre-breakdown ionization currents at values of E/N (E is the electric field and N the gas number density) within the range from 2·24 × 10-16 to 11·2 × 10-16 v cm2 was measured for helium, for neon and for eight different helium-neon mixtures with fractional concentrations of neon ranging from 0·05 to 0·95. For neon and all the mixtures the ionization growth was found to be accurately represented by the generalized Townsend equation. Analysis of the results in terms of this equation gave values for the primary ionization coefficient α for neon, which agreed with previously published results, and values of α/N for the mixtures, which showed that as the concentration of helium in neon increased α/N at a given E/N first increased and then passed through a maximum value. The maximum value of α/N can be typically 50% above the value for pure neon, and the concentration at which it occurs depends on the value of E/N. The secondary ionization coefficient ω/α for neon and for mixtures with large fractional concentrations (> 0·56) of neon decreased with increasing E/N, while ω/α for mixtures with small fractional concentrations of neon increased with increasing E/N. The measured ionization growth in helium showed systematic deviations from a Townsend growth, and the implications of these deviations for the determination of α from measurements of pre-breakdown currents in helium are briefly discussed.