Experimental Electron Energy Distributions for Townsend Discharges in Argon Gas

In this work the electron energy distribution functions and the anisotropic drift term of the velocity distribution functions in non-self-sustaining (Townsend) discharges in argon were determined by direct measurement for a range of $\frac{E}{N}$ (electric field strength per gas-atom concentration) from 70 to 407 townsends (Td) (1 Td=${10}^{-17\mathrm{}}$ V ${\mathrm{cm}}^2$). Some structure in the form of the distribution functions is observed, but the prediction of Heylen and Lewis for argon is not fully supported. The experimental method employed is to energy analyze electrons effusing from apertures in the anode of a discharge cell with a spherical retarding electric field. The experimental energy distributions were used along with cross-section data from the literature to compute the electron mobilities, diffusion constants, mean energies, and Townsend's first-ionization coefficients. Combination of the data with results from kinetic theory permitted evaluation of the anisotropic part of the velocity-distribution function.