Frequency Shift of an Electron Plasma Oscillation in a Modulated Beam-Plasma System

A spontaneous electron plasma oscillation in a beam-plasma system can be suppressed by modulating the beam density. In this case small shift of frequency of the oscillation is observed. At the same time oscillations with frequencies 2ω-ω m and 2ω m -ω arise, where ω is the frequency of the spontaneous oscillation and ω m is the modulation frequency. However, oscillations having frequencies ω m +ω and |ω m -ω| are not detected. These phenomena are interpreted on the basis of van der Pol equation with a driving term, and a good agreement is found between the theory and the experiment.