Experiments on the Interaction of a Modulated Electron Beam with a Plasma

New experiments are performed concerning the high frequency interactions of electron beams and plasmas. A modulated (500 Mc and 3000 Mc) electron beam is passed through a uniform plasma region of a mercury arc discharge, after which it is demodulated. Exponentially growing wave amplification along the electron beam is observed for the first time at a modulation frequency equal to the plasma frequency. No constant magnetic fields are used in these experiments. Calculations based on the one dimensional analysis of Bohm and Gross of an electron beam passing through a plasma are made to predict the effects of the random energy of the plasma electrons and collisions. By studying the interaction of a finite diameter beam and a plasma with no thermals or collisions, it is shown that the effect of the finite geometry is to reduce the growth constant. Recent work by Trivelpiece and Gould has pointed out that a plasma column in free space may propagate forward and backward waves at a velocity small compared to the velocity of light. The experimental techniques of passing a modulated electron beam through the plasma, as described above, are applied to observe traveling wave type of interaction with the slow wave mode of propagation in the absence of any magnetic fields. Theory predicting experimental rates of growth is presented. Experimental results in good agreement with theory are presented. In the course of verifying plasma density measurements, the excitation of the dipole resonance of a plasma column is considered. Multiple resonances are observed and discussed