Measurement of Townsend's Ionization Coefficients and Attachment Coefficients in Oxygen

Pre-breakdown currents and breakdown potentials have been measured in oxygen over a pressure range of 60-600 mm Hg and an E/p range of 35 to 50 v cm -1 (mm Hg) -1 . From the semi-logarithmic plots of current against electrode separation, the Townsend ionization coefficient α/ p , and attachment coefficient η/ p and a generalized secondary coefficient γ (where present) have been evaluated employing the modified Townsend current growth equation. From the values of α/ p and η/ p so obtained and from the earlier measurements, the mean cross section for ionization and attachment have been evaluated and by comparison with those obtained from low pressure (single collision) experiments, the electron attachment in O 2 under high pressure swarm conditions has been interpreted in terms of the dissociative attachment process. Lastly, from the measured values of α/ p , η/ p and γ, values of the breakdown gap d s have been evaluated for the E/p range 40-50, employing the modified Townsend breakdown criterion and by comparison with the measured values, the uniform field breakdown in O 2 at threshold has been interpreted in terms of the Townsend build-up processes up to a pd of approximately 250mm Hg cm.