Precision Measurements of Ionization Coefficients in Uniform Static Fields

Abstract

Recent work has shown that the growth of ionization currents in air and nitrogen up to high values of the parameter pd (where p is the gas pressure and d the gap distance) follows the well-known generalized Townsend equation and that the observed breakdown potentials are in agreement with those calculated from the Townsend breakdown criterion. Having established the nature of the general mechanism the next stage is to determine the ionization coefficients α and ω/α as accurately as possible. The present paper gives an analysis of the errors in the coefficients arising from experimental errors in the measurement of the ionization currents, and on the basis of the analysis defines the experimental conditions necessary for the accurate determination of the coefficients at high values of pd in hydrogen. The apparatus designed to satisfy these conditions is described, and the values of the ionization coefficients, determined for a range of pd from about 200 to 900 mm Hg cm in hydrogen, are given. Investigation of the effect of the state of the cathode surface on the secondary ionization coefficient showed that even at the comparatively high value of pd similar 350 mm Hg cm in hydrogen, the cathode played a significant role in the growth of ionization currents and thus in the setting of the breakdown criterion.