Fluid and kinetic nonlinearities of near-acoustic plasma waves
Large amplitude, near-acoustic plasma waves have a temperature-dependent harmonic content and nonlinear frequency shifts. On cold plasmas, experiments show that fluid nonlinearities dominate, and the observed harmonic content is well predicted by a recent fluid analysis. In contrast, the measured nonlinear frequency shift is smaller than this theory prediction, suggesting the presence of other nonlinear effects neglected in this analysis. When the plasma is heated, kinetic effects become important, and we observe an increase in both the harmonic content and nonlinear frequency shift. Quantitative experiments on the stability of these large amplitude, near-acoustic plasma waves strongly support the idea that trapped particles reduce the amplitude threshold for instability below fluid models. Furthermore, at low temperatures, the broad characteristics of the parametric instability are determined by the frequency detuning of the pump and daughter wave and the wave-wave coupling strength, surprisingly consistent with simple cold fluid, three-wave theories. However, at higher temperatures, the pump wave becomes unstable at half the threshold pump wave amplitude, with similar exponential growth rates as for a cold plasma.