Townsend Ionization Coefficients and Uniform Field Breakdown in Hydrogen and Nitrogen at High Pressures

Abstract

Values of $\frac{\alpha }{p}$, the first Townsend coefficient divided by the pressure, reproducible to a few percent were measured in hydrogen and nitrogen by the standard Townsend procedure for the range of pressure $p$ times electrode separation $d$ of 100 to 800 mm Hg×cm. Values of $\frac{\alpha }{p}$ were obtained for the range of field strength to pressure ratio $\frac{E}{p}$ of 14 to 22 and 30 to 45 volt ${\mathrm{cm}}^{-1\mathrm{}}$ ${\mathrm{}\left(\mathrm{mm}\mathrm{Hg}\right)\mathrm{}}^{-1\mathrm{}}$ in hydrogen and nitrogen, respectively. Current-voltage measurements at constant values of $p$ and $d$ taken to within 0.05% of the breakdown potential check the consistency of the $\frac{\alpha }{p}$ measurements and demonstrate the existence of the second Townsend coefficient $\gamma$ for $\mathrm{pd}$ values as high as 750 in nitrogen and 800 in hydrogen. The primary multiplication at breakdown is only about ${10}^3$ for both gases, and the existence of $\gamma$ is necessary to account for total multiplications of ${10}^5$ measured just below breakdown. The values of $\gamma$ are reproducible to within about 20% for fixed values of $p$, $d$, and $\frac{E}{p}$, provided the nickel cathode used is not unduly abused by heavy currents. The values of $\gamma$ are about ${10}^{-3\mathrm{}}$ for values of $\frac{E}{p}$ of about 20 in hydrogen and 40 in nitrogen, and were found to be unfalsified by space charge. Measured values of the breakdown potential in both gases are given within experimental error by the Townsend condition $\gamma (e^{\alpha d}-1)=1\mathrm{}$. Variations of the state of the nickel cathode for both gases at constant $p$ and $d$ give rise to changes in both $\gamma$ and the breakdown potential in accord with the Townsend condition. Alternately, values of the breakdown potential may be used with accurate values of $\alpha$ to obtain precise values of $\gamma$ from the Townsend condition.