COLLISIONLESS THERMALIZATION OF ELECTRONS IN A BEAM-PLASMA SYSTEM

Collisionless thermalization of a stream of energetic electrons injected into a plasma has been observed by means of an electron energy spectrometer. Accurate measurements of the energy distribution of the injected electrons have shown that a mean energy loss as well as an energy spread of the injected electrons occur when the distance traveled by these electrons through the plasma exceeds 10 to 15 e-folding lengths of the fastest growing plasma wave mode. The effects of electron-neutral collisional damping on collisionless thermalization have also been established. It has been shown that collisionless thermalization can be inhibited, if the electron collision mean free path is made small enough, or if the effective plasma thickness is kept smaller than about 20 plasma wavelengths for the injected electrons. These experimental results have been found to be consistent with the theory of double stream interaction between injected electrons and stationary plasma electrons.