Detection of Cerenkov radiation excited by a low-intensity electron beam in magnetoplasma

Abstract

r.f.-Emission has been detected in low-intensity electron-beam experiments in magnetoplasma and identified with Cerenkov radiation since its behaviour is in good agreement with the theory of the Cerenkov mode. In order to facilitate the experiments and to avoid undesirable perturbations, a stable low pressure ECRH-type plasma source, a tenuous electron beam of energies from 5 to 30 keV, and lock-in instrumentation of high sensitivity of r.f.-field measurement were used. The spectrum and polarization of the radiation, as well as the dependence of its intensity upon the plasma and beam parameters, have been investigated. From the observed relation between the radiation intensity and the beam current, it is concluded that the radiation tends to become coherent above a certain threshold value of the beam current. The threshold value and its dependence on beam energy were in good agreement with the stability criterion for a thin beam-plasma interaction.