The measurement of ionization coefficients in hydrogen and nitrogen

Abstract

Pre-breakdown ionization currents have been measured in hydrogen and nitrogen by the Townsend method for E/p0 in the ranges 27-46 and 54-99 v cm−1 torr−1. The ionization coefficients have been deduced by applying both the Townsend equation and also an equation derived by Lucas in which allowance is made for the loss of secondary electrons due to loss of photons out of the gap. A digital computer technique was used for the deduction of the coefficients and the comparison of the validity of the equations.The degree of fit of the experimentally measured currents is better by the Lucas equation than by the Townsend equation over the region studied. The values of the electron ionization coefficient α in hydrogen as a function of E/p0 were in excellent agreement with the values summarized by Rose in 1956 when the pre-breakdown currents were analysed by the Townsend equation. This confirms the accuracy of the experimental apparatus. When the same currents were analysed by using the Lucas equation the α values were lower by no more than 7%. The main advantage of the Lucas equation is its ability to isolate the γI and δp coefficients for ions and non-resonant photons, respectively, instead of the obscure generalized secondary coefficient of Townsend. γI decreased to zero with decreasing E/p0, whilst δp increased with decreasing E/p0 and showed a marked pressure dependence satisfying the Stern-Volmer formula which expresses the destruction of excited states in collision with ground-state molecules.Similar results and conclusions were obtained for the values of the coefficients α, γI and δp for nitrogen.