Direct Experimental Verification of Electron-Diffusion Supercooling in the Neon Discharge Afterglow

Abstract

Average-electron-density measurements in a neon discharge afterglow have been made using a T${\mathrm{M}}_{010}$ microwave cavity. The quartz and Pyrex tubes have a 2.18-cm inner diameter and are 90 cm long (${\Lambda }^2=0.206\mathrm{}$ ${\mathrm{cm}}^2$) with hollow nickel and molybdenum cylindrical electrodes. The gas pressure is 0.5 Torr at 300°K for the majority of our measurements, although some results are given at 1.5 and 5.0 Torr. Radiation-temperature measurements are made under identical experimental conditions using a 3.0-GHz microwave radiometer. Three ambient gas temperatures are used: 300, 500, and 673°K. The density decay measurements give an indirect indication of the supercooling of the electrons. The radiation measurements clearly show that the electron temperature falls below the ambient temperature of the gas and the discharge tube. A simple theory for the density and temperature decay compares reasonably well with the measured results.