Experimental Electron Energy Distributions for Townsend Discharges in Helium

The energy distribution of electrons in Townsend discharges in helium has been investigated experimentally for a range of E/N between 1.65×{}10^-15 and 6.32×{}10^-15 V cm², 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 E/N=6.32×10^-15 V cm². The low-energy behavior of the experimental energy distribution indicates an increase of the most probable energy with increasing E/N.