ELECTROSTATIC MODE COUPLING OF BEAT EXCITED ELECTRON PLASMA WAVES (RAMAN, BRILLOUIN SCATTERING)

The nature of the spectrum of electrostatic waves which is excited when two laser beams beat in a plasma containing a density ripple is investigated theoretically, experimentally and computationally.;CO(,2) laser experiments designed to study and characterize beat wave excitation and mode coupling were conducted. The results of collective ruby laser Thomson scattering and other measurements of the electrostatic and scattered electromagnetic spectra are presented. Despite the richness in physics of the experiment, the simple model is able to predict the observed behavior and characteristics of the beat wave and coupled modes. Direct comparisons between the experimental results and theoretical predictions are made regarding mode saturation amplitudes. Evidence for electrostatic modes driven by SRS driven plasma waves and by the lasers when their frequency difference equals twice the plasma frequency is also presented.;Computer simulations were employed to both model the experiment and test the theory. The results of the simulations are in good qualitative agreement with both theory and experiment. In addition a mechanism for self stabilization of the mode coupling saturation mechanism is demonstrated.;A theoretical model for the beat wave excitation process in a rippled-density plasma is developed. The model predicts the generation of a rich spectrum of plasma modes with frequencies equal to the beat frequency of the two laser beams and wavenumbers equal to integer multiples of the ripple wavenumber. For commonly encountered experimental parameters a new beat wave saturation mechanism has been found which can limit the amplitude of the beat wave to a value below that expected for the widely quoted relativistic detuning process. The effects of stimulated Raman scattering, ion wave harmonics, and other coupling mechanisms are also discussed.