Experimental Electron Energy Distributions for Townsend Discharges in Helium

Abstract

The energy distribution of electrons in Townsend discharges in helium has been investigated experimentally for a range of $\frac{E}{N}$ between 1.65\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}15}$ and 6.32\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}15}$ V ${\mathrm{cm}}^{2}$, through the use of a retarding-field method. The effects of electron reflection and secondary emission from the surfaces of the retarding-field analyzer are considered; it is shown that these phenomena are relatively unimportant when low-reflectance materials are used to coat analyzer surfaces. Experimental energy distributions exhibit a curtailment of the distribution function near the first excitation potential of helium, except for $\frac{E}{N}=6.32\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ V ${\mathrm{cm}}^{2}$. The low-energy behavior of the experimental energy distribution indicates an increase of the most probable energy with increasing $\frac{E}{N}$.