Measurement of ionization and attachment coefficients in humid air in uniform fields and the mechanism of breakdown

Abstract

Measurement of pre-breakdown currents and breakdown potentials in humid air in the E/p range of 30 to 40 v cm-1 (mm Hg)-1 at total pressures of 150 and 300 mm Hg with partial pressures of water vapour in the range 2.5 to 15 mm Hg indicate a pronounced increase in attachment compared with conditions in dry air. From the semi-logarithmic plots of current against electrode separation, Townsend's ionization coefficient α and an attachment coefficient η have been obtained for humid air employing the modified Townsend equations for the growth of current. From the measured breakdown potentials, values of Townsend's secondary coefficient γ have been calculated using the modified Townsend breakdown criterion. Further the percentage increase in breakdown potential in humid air has been plotted as a function of the partial pressure of water vapour. From similar measurements in pure water vapour at pressures of 10 and 20 mm Hg in the E/p range of 30 to 50 v cm-1 (mm Hg)-1, values of α/p and η/p have been obtained for water vapour. A mean cross section for attachment has been computed for various values of mean electron energies assuming a Maxwellian distribution and employing the earlier measurements of drift and agitation velocities.