Suppression and Excitation of Electron Oscillation in a Beam-Plasma System

Standing waves of longitudinal electron oscillation are excited spontaneously in an electron beam-plasma system. Amplitude pattern of each standing mode whose frequency ω n is nearly equal to n ω 1 , where n is a mode number and ω 1 is the first mode frequency, has been measured. Antinodes of oscillating potential are observed at boundaries. When the signal whose frequency ω m is applied to the gun spontaneous oscillation with ω n is suppressed but the resonant oscillation with ω m =ω n ±1 is excited. The results are quantitatively examined by solving simultaneously two van der Pol equations including a mode-mode coupling term. The excited oscillation is also a standing wave and the amplitude pattern of it is similar to that of the spontaneous one.