Search

A concise general dispersion relation is found to describe electron parametric instabilities driven by relativistically intense circularly polarized laser light in plasma. It is valid for densities from rarefied plasma up to overdense regions accessible by self-induced transparency due to the relativistic electron mass increase. In the form of a fourth order scalar difference equation, it describes stimulated harmonic generation as well as stimulated Raman scattering, the relativistic modulational and filamentation instabilities, and the two-plasmon decay. It is also related to analytic results for stimulated harmonic generation from rarefied plasma and to synchrotron radiation from single electrons. [ABSTRACT FROM AUTHOR]

The occurrence of stimulated Raman scattering from within localized density cavities that may be present in many laser-produced plasmas is studied. Then scattering in both the backward and forward directions from a single cavity becomes absolutely unstable, with a common growth rate and zero inhomogeneous plasma thresholds whenever electron plasma waves are totally trapped within the cavity. Both back- and forward scattering mechanisms contribute to this common growth rate. A cavity therefore equalizes the growth of emission in backward and forward directions. Forward scattering in its own right gives rise to absolute growth. The threshold for absolute growth is then set by damping, and by Landau damping of the plasma wave in particular. Only very modest density depressions are needed to trap the long-wavelength weakly damped plasma waves. Absolutely unstable back- and forward scattering from within shallow cavities in the underdense coronal plasmas of laser fusion targets may prove to be commonplace rather than the exception, although growth rates are generally slow and sensitive to the cavity density and scale. [ABSTRACT FROM AUTHOR]

The effect of a plasma filament on the characteristics of stimulated Raman scattering is examined. The model representation of the filament allows an assessment of the way in which Raman growth rates are influenced not only by the density inhomogeneity transverse to the beam direction, but by the nonuniform intensities of the incident laser and scattered light, as well as by Landau damping in the plasma. The model determines global eigenvalues without recourse to local (WKB) approximations, or to phenomenological damping factors. In shallow filaments the concentration of the light gives rise to modest gains in growth, while for deeper filaments the effects of the inhomogeneity and of Landau damping are dominant and suppress the instability. In both cases the strongest growth occurs at the bottom of the filament. Waveguide effects in deep filaments can result in emission being constrained to wavelengths significantly shorter than twice that of the incident laser light. [ABSTRACT FROM AUTHOR]

Thresholds and growth rates for stimulated Raman scattering from a plasma density maximum, corresponding to laser–target burnthrough experiments, are obtained by numerical solution of the full-wave Raman scattering equations. This supplements and extends to new regimes the WKB analysis of Rosenbluth [Phys. Rev. Lett. 29, 565 (1972)] and the quasiglobal analysis of Williams and Johnston [Phys. Fluids B 1, 188 (1989)]. The results show unexpected scalings with laser intensity well above threshold, and permit absolutely unstable forward scattering with thresholds comparable to those for backward scattering. [ABSTRACT FROM AUTHOR]

The linear mode conversion of a plasma wave to a light wave in a magnetized plasma has been examined theoretically and by computer simulation. This conversion is the inverse of resonance absorption exhibiting an identical dependence on magnetic field and density scale length with an optimum conversion efficiency of approximately 60%. Radiation from this source may contribute to the harmonic spectra observed from laser-irradiated plasmas. [ABSTRACT FROM AUTHOR]

A five-wave Raman scattering process is examined in inhomogeneous plasma in the presence of a finite level of ion acoustic waves. This five-wave interaction is a composite of conventional three-wave stimulated Raman scattering (SRS) followed by the decay of the Raman Langmuir wave into either a second Langmuir wave (LD) or a second scattered light wave (ED) and an ion acoustic wave. The presence of a suprathermal level of ion acoustic waves detunes SRS and inhibits the temporal growth. In inhomogeneous plasma these decays take place locally so that phase matching for both decays is assumed to be coincident. The net Raman convective gain is shown to be sensitive to the source levels of the amplifying waves. For LD the gain may be suppressed or enhanced depending on the relative intensities of the two Langmuir waves. For ED the gain is invariably suppressed. In either case, this occurs for very modest levels of ion acoustic waves. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A unified treatment of electron parametric instabilities driven by ultraintense laser light in plasma is described. The method is valid for laser light of any polarization and intensity, for any plasma density both rarefied and classically overdense, and for any scattering geometry. A fully relativistic self-consistent three-dimensional (3D) analysis, based on the cold electron fluid and Maxwell's equations without further assumption, is carried out in the laser pulse proper frame allowing a concise description in terms of a system of six ordinary differential equations. The approach is applied to linearly polarized laser light in underdense plasma, and overdense plasma accessible by self-induced transparency. Electron charge displacement and relativistic mass fluctuations give rise to hybrids of stimulated Raman scattering, the two plasmon decay, the relativistic modulational and filamentation instabilities, and to stimulated harmonic generation. There is vigorous growth over a wide range of wave numbers and harmonics. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]