Observation of free-streaming electrons in plasmas

The excitation and propagation of electron waves in a plasma have been studied experimentally in a low-density and large-volume plasma. For frequencies larger than the electron Langmuir frequency, an electron plasma wave was observed which is predicted by the linear theory for a Maxwellian plasma. For smaller frequencies, free-streaming electron or ballistic signals are observed. For frequencies close to the electron Langmuir frequency, both the electron plasma wave and the free-streaming electron are excited and show interference patterns. These observations are compared with the theoretical predictions. The effect of a sheath formed around a transmitter on the free-streaming electron is also discussed.