E. Schifano

Nonlinear modification of laser–plasma interaction processes under crossed laser beams

Effects of multiple crossed laser beams on stimulated Brillouin and Raman scattering (SBS and SRS) growth rates, as well as on the SBS and SRS competition, have been investigated in a preformed plasma using Thomson scattering of a short-wavelength probe beam. Reduction of the amplitude of ion acoustic waves (IAW) associated with SBS and amplification of the amplitude of electron plasma waves associated with SRS in the case of irradiation by two or three beams compared to a single beam are reported. Mechanisms by which crossed laser beams can reduce SBS by detuning the interaction or by nonlinearly enhancing the damping rate of the IAWs will be discussed. The same mechanisms could explain the enhancement of SRS backscatter through modified secondary decay processes of SRS. In a second experiment, nonlinear enhancement of extreme forward scattering of one laser beam going through a preformed plasma has been observed when a second interaction beam was present. The role of ion wave instabilities on forward sc...

Observation of the Langmuir decay instability driven by stimulated Raman scattering

Thomson scattering was used to measure the Langmuir wave spectrum driven by stimulated Raman scattering. The Thomson scattering signals measured in the experiment showed Langmuir waves with components both parallel and antiparallel to the incident laser’s wave vector, k0. The parallel component was attributed to stimulated Raman scattering. However, the Langmuir waves with components antiparallel to k0, which cannot be explained by stimulated Raman scattering, were attributed to the Langmuir decay instability (LDI). The relative amplitude of the two Langmuir wave features and their angular width supported the conclusion that the Langmuir waves traveling antiparallel to the incident laser’s wave vector were driven by the Langmuir decay instability.

Measurements of the angular and temporal structure of second‐harmonic emission from laser‐produced plasmas

We have measured and analyzed the second harmonic emission, both in the plane of the laser electric field and perpendicular to it, at several angles near 135° from the laser wave vector. The experiments used from 1 to 80 J of 1.053 μm light to irradiate carbon–hydrogen (CH) targets with a 550 ps pulse. A random phase plate was used, producing characteristic intensities in the range of 1013–1014 W/cm2. This was sufficient to drive the Ion Acoustic Decay Instability, producing Stokes emission well‐separated from the emission spike at the second harmonic of the laser frequency. The spectral structure of the Stokes emission was qualitatively similar for all intensities and angles of observation. The duration of the signals showed trends anticipated from linear theory. To explain the scaling of the signal strength and spectral width requires nonlinear theory.