Temporal Growth of Ionization in Gases

The temporal rate of growth of ionization in uniform electric fields $E$ in hydrogen at pressures $p$ up to some cm Hg was measured over a wide range of $\frac{E}{p}$ between 50 and 400 v/cm mm Hg when the values of the electric fields exceeded that corresponding to the static sparking potential by a few percent. Comparison of the experimental data with a mathematical analysis of the growth based upon the action of primary and secondary ionization processes enabled the relative significance of possible secondary processes to be assessed, and their absolute values to be determined. These results showed that there was a change in the relative importance of the secondary processes as $\frac{E}{p}$ changed. Thus, for low values of $\frac{E}{p}\left(\sim 50\right)$ the predominant secondary process was found to be photoelectric emission from the cathode, but as $\frac{E}{p}$ was increased the relative importance of electron emission from the cathode due to the incidence of positive ions also increased until at high values (∼300) 50% of the emission was due to this cause. At all values of $\frac{E}{p}$ investigated, no single secondary process was exclusively responsible for the cathode emission.