Electron Density and Temperature Decay in Mercury Afterglows

Abstract

Average electron number density and electron-temperature measurements are presented for the mercury vapor afterglow in a hot-cathode cylindrical discharge tube at a pressure range where ambipolar diffusion predominates. The average electron number density is measured using cylindrical microwave cavities in the T${\mathrm{M}}_{010}$ and dipole resonance modes. The electron temperature is measured by means of a time-resolving 3.0-GHz radiometer. Three pressure regions are identified: (1) $p_0<0.08\mathrm{}$ Torr, where $\frac{p_0}{{\tau }_n}$ increases with $p_0$, (2) $0.08<\mathrm{}{p}_0<0.18\mathrm{}$ Torr, where $\frac{p_0}{{\tau }_n}$ is roughly constant, and (3) $0.18 \mathrm{Torr}<p_0$, where $\frac{p_0}{{\tau }_n}$ increases. Here $p_0$ and ${\tau }_n$ are the pressure reduced to 0 °C and the density decay time constant, respectively. The work presented here is concerned mainly with the first two pressure regions. In the lowest pressure region, electron-temperature measurements in the afterglow under experimental conditions identical to those used for density measurements are used to determine after which point in time the electrons reach the neutral-atom temperature. This information, along with the density decay time constant, is used to predict a mobility of 0.21 ${\mathrm{cm}}^2$ ${\mathrm{V}}^{-1\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ at 760 Torr for ${\mathrm{Hg}}^{+}$ in mercury vapor at 443 °K. This value is in good agreement with previous theoretical and experimental work. In the second pressure range, the electron-temperature measurements indicate that, after an initially rapid drop, a slow decay of electron temperature occurs from roughly 1050 to 600 °K between 0.5 and 5 msec. In spite of this temperature change, the density decay curves were exponential over two decades. This curious observation, which has led previous workers to predict a constant electron temperature around 2000 °K for periods of several milliseconds in the afterglow, has been explained by postulating that the ion mobility could vary over this period. The reasons for this assumption are given in the paper and involve Coulomb collisions which are very important in these experiments. The initially rapid electron-temperature decay occurring during the first 0.5 msec of the afterglow is pressure independent and is well explained by electron-ion elastic collisions. The slow electron temperature decay after 0.5 msec which occurs in the second pressure region is pressure dependent and is explained by a balance between energy gain from collisions of the second kind in the reaction $e+{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}\to {}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}+e$ and losses due to electron-ion elastic collisions. A complete discussion of these processes is given and the neglect of other mechanisms justified.