Electrostatic Mode Coupling of Beat-Excited Electron Plasma Waves

Abstract

The process of beat excitation of electron plasma waves in a plasma containing a density ripple is studied theoretically, experimentally, and computationally. A simple theoretical model has been developed which, for modest experimental parameters, predicts a new beat wave saturation mechanism. This mechanism involves the excitation of a spectrum of secondary electrostatic modes which divert pump energy from the beat wave and can lead to saturation of the beat wave at an amplitude well below that expected for relativistic detuning. Experiments designed to study the coupled mode spectrum were performed. The measured properties of the electrostatic spectrum are in reasonable agreement with the theory. To bridge the gap between the idealized model and the experiment, computer simulations were performed for a variety of parameters. The results of the simulations are in excellent agreement with the theory at early times prior to the onset of purely kinetic effects. For later times the simulations exhibit qualitative behavior which is consistent with the experimental measurements. Under certain conditions the model predicts the thermal quenching of the mode coupling saturation mechanism. These predictions are also verified in the simulations.