Phase-locked particle motion in a large-amplitude plasma wave

A plasma wave with an oscillating amplitude and phase occurs in two commonly studied situations, the beam-plasma interaction and the launching of a large-amplitude wave in a Maxwellian plasma. Electron motion in such a wave is either regular or stochastic. Theoretical study shows that regular motion can exhibit a phase-locking effect, which explains the persistence of amplitude oscillations observed in simulations and experiments. An additional (''test'') wave of moderate amplitude can prevent phase-locking, causing stochastic motion instead, and thereby destroy the amplitude oscillations. The effects studied are also relevant to the theory of sideband instability