Comparison of amplifying modes and feedback oscillations in a beam-plasma experiment

The beam-plasma instability in a strong magnetic field, (ω/sub c e/>ω/sub p e/), has been studied experimentally. Although the electron plasma waves have positive group velocity, oscillating instabilities are observed when reflections become important. The amplifying (convective) instability has waves which grow in space and saturate due to beam trapping. After saturation, the spectrum broadens and the beam spread increases due to nonlinear generation of sidebands. When reflective feedback is sufficiently strong, an oscillating instability appears. It has large amplitude near the electron gun, and also saturates by trapping for a weak beam. For stronger beams, several oscillating modes are present and quenching occurs, due to interactions between modes. The difference between amplifying and oscillating modes, quiescent broad spectrum versus quenching, is similar to the absolute instability observations, which occur for