INTERACTION EFFECTS BETWEEN A PLASMA AND A VELOCITY-MODULATED ELECTRON BEAM

The interaction between a grid modulated electron beam of finite diameter and a stationary plasma column, in the absence of a magnetic field, was investigated theoretically and experimentally. Experiments performed with a beam-plasma tube, with modulation of the electron beam by grids, revealed a maximum interaction when the modulation frequency of the beam was equal to or near the plasma frequency of the plasma. An electronic gain of 7.5 db due to the plasma was observed. The output powers were low with no net gain in any condition, due to the low impedances of the cavities which were used, and to the low beam current available. Nevertheless, it was possible, by driving the beam to saturation power levels, to obtain fundamental components of the beam current. Under conditions for existence of growing waves in the system, strong harmonic generation becomes possible as a result of modifications of space- charge forces on the beam, due to the presence of the plasma. Expressions for the second-harmonic components of the relevant rf quantities in the infinite beam-plasma system were derived, indicating the existence of growing second- harmonics.