The Rate of Change of Electron Temperature in the Mercury Afterglow

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Abstract
A study has been made of the behavior of a plasma in which no new ions are found, as regards electron temperature and concentration, to determine the sources of the electron energy. Measurements were made with a movable probe in a stream of ionized mercury vapor passing out from an arc discharge. The velocity and vapor concentration were determined by stroboscopic measurements on moving striations produced by an impressed alternating voltage. The vapor concentration, expressed in pressure reduced to 0°C, varied between 0.05 and 1.5 mm of mercury. The rate of cooling of the electrons was very rapid at first, but below 2500°K this rate decreased rapidly. Finally the electron temperature reached a constant value which was several hundred degrees above that of the vapor, the difference being proportional to the vapor concentration. The most reasonable source of energy necessary to keep up the electron temperature seems to be either the local recombination process, directly or through the metastable atoms formed, or the metastable atoms brought down the tube by the vapor stream.