Destruction of trapping oscillations

A somewhat unconventional traveling wave tube was built to investigate the nonlinear behavior of the beam-plasma instability beyond the first trapping oscillation of the wave amplitude. In the small cold beam limit, the equations governing the evolution of the beam-plasma instability are mathematically identical to those describing the traveling wave tube. The traveling wave tube has the advantage that the slow wave structure will remain linear for the wave amplitudes reached in the experiments; furthermore, it does not introduce noise. Five trapped particle oscillations are observed following the saturation of a single launched wave. Two mechanisms for destroying these oscillations have been found. The first involves wave damping and can occur for decrements smaller than 0.01 k0. The second is a result of the modulation of the main wave by unstable sidebands. In addition to the experiments, the equations which describe the interaction are solved numerically. The experimental observations are in excellent agreement with the results of the numerical calculations.