Experimental Determinations of the First Townsend Ionization Coefficient in Helium

Abstract

The first Townsend ionization coefficient $\frac{\ensuremath{\alpha}}{{p}_{0}}$ ${(\mathrm{c}\mathrm{m}\ifmmode\times\else\texttimes\fi{}\mathrm{T}\mathrm{o}\mathrm{r}\mathrm{r})}^{\ensuremath{-}1}$ has been experimentally determined for an $\frac{E}{{p}_{0}}$ (volts/cm\ifmmode\times\else\texttimes\fi{}Torr) range from 3 to 300 in cataphoretically cleaned helium. The results obtained have been compared with several experimental and theoretical determinations of $\frac{\ensuremath{\alpha}}{{p}_{0}}$. The present values are in agreement within the combined experimental errors with those of Davies, Jones, and Morgan. Comparison is also made with the experimental results of Townsend and MacCallum where good agreement is found except at their lowest $\frac{E}{{p}_{0}}$ values, where the present results lie below their corresponding data. At low $\frac{E}{{p}_{0}}$, the present results agree reasonably well with the theoretical values of Phelps and Dunlop except at very low $\frac{E}{{p}_{0}}$ where Dunlop's calculated values are considerably smaller than the present experimental results. The significance of the experimental method at very high $\frac{E}{{p}_{0}}$ where equilibrium conditions do not prevail is also discussed.