Computer study of many waves in the small cold beam and plasma interaction

When a small and fast, monoenergetic electron beam enters a relatively cold plasma, the combined system is unstable to the growth of waves. Although the Single Wave Model has been successful in predicting initial linear growths, the nonlinear saturation level and the early stages of amplitude bouncing, it does not predict correctly the subsequent nonlinear wave behaviour observed in experiments. A very broad spectrum of waves is measured at this point, suggesting that many waves determine the later stages of nonlinear development. Fifteen waves are modeled and their linear and nonlinear self-consistent evolution with the beam and a non-resonant plasma are followed. Since nonlinear theory shows that temporally and spatially growing waves satisfy different dispersion relations, the problem for both kinds of boundary conditions is solved. (U.K.)