Electron Scattering and Plasma Oscillations

The dependence of electron scattering upon plasma oscillations was studied in a mercury arc discharge tube containing a movable probe. The best conditions for studying these relations were found when the vapor pressures and arc currents were small. Probe volt-ampere characteristics showed the presence of ultimate electrons with a Maxwell-Boltzmann distribution corresponding to about 30, 000^ and a superposed stream of fast electrons emitted from the cathode. These fast electrons were scattered without appreciable change in direction in well-defined, approximately plane regions only a few tenths of a millimeter wide. The plasma oscillations were studied with a crystal detector in the probe circuit supplemented by a Lecher wire system. In addition to "turbulent" disturbances with no measurable frequencies found throughout the tube, narrow regions were found in which stable periodic oscillations of considerable magnitude were detected. These regions and the scattering regions were, in general, equal in number and coincided except for a small shift of the former in the direction of the anode. The observed frequencies agreed well with the formula derived by Tonks and Langmuir. The scattering regions and the regions in which periodic oscillations were observed became less marked and moved toward the cathode as either the vapor pressure or arc current were increased. The periodic oscillations were found only in regions traversed by the fast electrons. The results are interpreted as showing that scattering is due to plasma oscillations which receive their energy from the fast electrons. The process by which the oscillations were detected by the probe is discussed to account for the shift of the regions in which oscillations were observed with respect to the scattering regions.