Non-Linear Phenomena in Beam-Plasma Interaction

A modulated weak electron beam is injected into a low-density diffused plasma and growth rates of convective instabilities are observed. The growth rates obtained experimentally are in pretty good agreement with those calculated from the dispersion relation including only the collisional damping effect if the beam density is relatively low. However, if the beam density is high, the experimental results conspicuously deviate from the calculated values. These results suggest that it becomes difficult for the amplified wave to maintain its coherence as the amplitude increases, and that such non-linear effect would cause breakdown of the amplified waves. The ratio I 1 / I 0 is adopted as a measure of the threshold over which the non-linear effect becomes remarkable, where I 0 and I 1 are the zero-order and first order electron currents in the beam-plasma system respectively. As I 1 / I 0 approaches unity breaking of the wave becomes observable.