Saturation of Langmuir waves in laser-produced plasmas.

摘要

This dissertation deals with the interaction of an intense laser with a plasma (a quasineutral collection of electrons and ions). During this interaction, the laser drives large-amplitude waves through a class of processes known as parametric instabilities. Several such instabilities drive one type of wave, the Langmuir wave, which involves oscillations of the electrons relative to the nearly-stationary ions. There are a number of mechanisms which limit the amplitude to which Langmuir waves grow. In this dissertation, these mechanisms are examined to identify qualitative features which might be observed in experiments and/or simulations. In addition, a number of experiments are proposed to specifically look for particular saturation mechanisms.;In the experiments performed for this dissertation, a probe laser was scattered from the electrons present in the plasma to measure the spectrum of Langmuir waves driven in the plasma. This technique, known as Thomson scattering, was used to identify the mechanisms producing the Langmuir waves, as well as to identify mechanisms responsible for the saturation of these Langmuir waves. The Thomson-scattering measurements indicated that the Langmuir-wave spectrum resulted from stimulated Raman scattering, which is the decay of the laser into an electromagnetic wave and a Langmuir wave. In a related experiment, these measurements detected Langmuir waves which were driven by the Langmuir decay instability, which is the decay of a Langmuir wave into a second Langmuir wave and an ion acoustic wave. This measurement represents the first observation of the three-wave parametric instability known as the Langmuir decay instability in laser-produced plasmas.;In a plasma, a Langmuir wave can decay into an electromagnetic wave and an ion wave. This parametric instability is proposed as a source for electromagnetic emission near half of the incident laser frequency observed from laser-produced plasmas. This interpretation is shown to be consistent with existing experimental data and it is found that one of the previous mechanisms used to explain such emission is not. The scattering version of the electromagnetic decay instability is shown to provide an enhanced noise source of electromagnetic waves near the frequency of the incident laser.