Ionization, Attachment, and Breakdown Studies in Oxygen

Abstract

Uniform field steady state pre-breakdown currents have been measured in oxygen as a function of pressure corrected to 0°C, $p_0$, at constant $\frac{E}{p_0}$ ($E$ is field strength) and at constant electrode separation $d$ for $32<\mathrm{}\frac{E}{p_0}<100\mathrm{}$ V ${\mathrm{}\left(\mathrm{cm}\mathrm{Torr}\right)\mathrm{}}^{-1\mathrm{}}$. Similar currents have been measured in oxygen-helium mixtures for $16<\mathrm{}\frac{E}{p_0}<36\mathrm{}$ (here $p_0$ is the sum of oxygen and helium pressures). Uniform field breakdown potentials have been measured in oxygen for a $p_{0}d$ range corresponding to $46<\mathrm{}\frac{E}{p_0}<100\mathrm{}$ at breakdown. The currents have been used to evaluate the Townsend ionization coefficients $\frac{\alpha }{p_0}$ (or $F_0$) and $\gamma$, and the electron attachment coefficient $\frac{\eta }{p_0}$ (or $G_0$). From the currents measured in oxygen, values of $F_0$ and $G_0$ were obtained for $32<\mathrm{}\frac{E}{p_0}<60\mathrm{}$. For $60<\mathrm{}\frac{E}{p_0}<100\mathrm{}$, it was not possible to evaluate $F_0$ and $G_0$ separately; however, for this $\frac{E}{p_0}$ range, values were obtained for $F_0-G_0$. It was possible to obtain values of $F_0$ and $G_0$ for oxygen for $16<\mathrm{}\frac{E}{p_0}<30\mathrm{}$ from the currents in the oxygen-helium mixtures. Values of $\gamma$ for oxygen were obtained from pre-breakdown currents for $46<\mathrm{}\frac{E}{p_0}<50\mathrm{}$ (nickel cathode) and for $50<\mathrm{}\frac{E}{p_0}<60\mathrm{}$ (molybdenum cathode). In addition, values of $\gamma$ for oxygen were obtained from sparking potential measurements for $46<\mathrm{}\frac{E}{p_0}<100\mathrm{}$ (nickel cathode) and for $50<\mathrm{}\frac{E}{p_0}<100\mathrm{}$ (molybdenum cathode). The values of $\gamma$ obtained by the two methods are in agreement and are independent of cathode material. $\gamma \cong {10}^{-5\mathrm{}}$ at $\frac{E}{p_0}=100\mathrm{}$ and decreases to about ${10}^{-6\mathrm{}}$ at $\frac{E}{p_0}=46\mathrm{}$. The values of $F_0$ and $G_0$ obtained in the present work were used in conjunction with previous sparking potential measurements of Kachickas in oxygen (brass cathode) to evaluate $\gamma$ for $39<\mathrm{}\frac{E}{p_0}<44\mathrm{}$. The results of this analysis indicate that in this $\frac{E}{p_0}$ range $\gamma$ increases very sharply with decreasing $\frac{E}{p_0}$, reaching a value of about ${10}^{-3\mathrm{}}$ at $\frac{E}{p_0}=39\mathrm{}$.