Backscattered supercontinuum emission from high-intensity laser-plasma interactions.
We performed high-intensity subpicosecond laser-plasma interaction experiments to examine nonlinear scattering mechanisms in underdense plasmas. At incident laser intensities of 2 x 10(18) W/cm(2) the stimulated-Raman-backscattered spectrum exhibited an extremely broad, supercontinuumlike structure (Deltaomega/omega(0) > 1) extending from ~500 to >1200 nm (limited only by detector sensitivity). Large-amplitude modulations in the spectrum of the backscattered light were measured and are attributed to an interaction of the stimulated-Raman-scattered radiation with ion plasma waves.
Analysis and simulation of Raman backscatter in underdense plasmas
A new formalism to describe the spatiotemporal evolution of relativistic Raman backscatter (RBS) of ultrashort laser pulses in underdense plasma has been developed. This theory is based on an eikonal representation for the RBS field and averaging over the oscillation frequency. Equations are derived for the evolution of the RBS radiation field amplitude and phase and for particle motion in the ponderomotive potential of the pump and RBS fields. A technique similar to that used in Raman free electron lasers is used to include the plasma density oscillation and its electric field in the particle equations. The equations have been solved numerically in a code that provides an accurate description of the nonlinear electron motion on a short spatial scale of half the radiation wavelength, while at the same time evolving the laser field on a much coarser scale, characteristic of the growth rate of the instability. Two-dimensional nonlinear effects, such as the return current, are analytically estimated and phen...
Nonlinear decay and instability of a nonuniform finite-amplitude plasma wave
The effects of spatial nonuniformities in the amplitude of a decaying finite‐amplitude plasma wave are examined. It is found that the usually neglected, but experimentally important, initial damping (or growth) of the finite‐amplitude wave may have important consequences upon the decay process. The mode‐coupling equations are solved for both forward‐scattering and back‐scattering type decay processes. The latter case is particularly interesting, since both amplification in space, and instability in time, of the perturbed waves may occur in the presence of damping mechanisms. Calculations are presented which show that longitudinal waves with frequencies in the vicinity of cyclotron harmonics may be subject to such instabilities.