Anomalies in Ionization Coefficients and in Uniform Field Breakdown in Argon for Low Values ofEp

Abstract

Prebreakdown ionization currents in argon have been measured in uniform fields for low values of the ratio of field strength to pressure $\frac{E}{p}$ [5 to 12 v ${\mathrm{}\left(\mathrm{cm}\mathrm{mm}\mathrm{Hg}\right)\mathrm{}}^{-1\mathrm{}}$]. Currents obtained with varying electrode separation $d$ at constant $\frac{E}{p}$ and constant $p$ could not be analyzed to yield values of the Townsend coefficients $\frac{\alpha }{p}$ and $\gamma$. Currents obtained with varying $p$ at constant $\frac{E}{p}$ and constant $d$ could be analyzed to yield values of $\frac{\alpha }{p}$ and $\gamma$, but such currents yielded coefficients which depend on $d$. The dependence of the values of $\frac{\alpha }{p}$ on $d$ is attributed to the production of highly excited atoms by resonance radiation at some distance from the positions where the electrons lose their energy; these highly excited atoms then produce molecular ions and electrons in collisions with ground-state argon atoms. The secondary mechanism and the dependence of $\gamma$ on $d$ are associated with resonance radiation. Sparking potential measurements in argon made by varying both $p$ and $d$ for values of $\mathrm{pd}$ corresponding to breakdown for the above range of $\frac{E}{p}$ show deviations from Paschen's law.