Electron wave cavity formation in the electron beam-plasma interaction

Nonlinear evolution of the electron-beam instability is examined experimentally in a plasma unstable for ion-acoustic wave excitation. Amplitude modulation of the electron wave is observed, with the threshold for the energy density practically equal to zero. When the beam density is more than a few percents of the plasma density, the electron wave energy grows up to several percents of the thermal energy. The peaks in the amplitude are found to correspond, for the wave, to the trapping in the density depressions executing ion-acoustic oscillation. A striking feature of the present experiment is a coupled relaxation oscillation between the electron wave and the ion-acoustic wave. (auth.)