''Self-Smoothing of Laser Light in Plasmas''.

Abstract

The modification of the optical characteristics of a laser beam by a plasma is a key issue in laser-plasma coupling. it is critical to understand how this takes place, if we are ever to understand the interaction processes in the plasma corona as well as the coupling at super-high intensities--as when laser pulses approach Petawatt intensities. Interpreting and understanding parametric instabilities in laser-produced plasmas has been a problem of increasing complexity. Improvements in diagnostic capabilities in experimental studies, as well as refinements in the modeling (using different numerical techniques), are showing a complex scenario: strong interplay among instabilities, modification of the plasma conditions caused by the instabilities, and modification to the initial distribution of laser intensity inside the plasma. Of particular interest are stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS), instabilities which have been studied extensively during the past 20 years, both theoretically and experimentally. Until now, most studies--mainly driven by requirements associated with inertial confinement fusion (ICF)--have concentrated on backscattering instabilities. The role of forward instabilities has not received much attention, despite having the potentials for strongly modifying the overall laser-plasma interaction region. The objective of this project is to study numerically the nonlinear enhancement of large-angle,more » forward scattering of two identical laser beams propagating in a preformed plasma. it is known that filamentation instability and self-focusing are capable of modifying laser-beam geometry, altering the electromagnetic-field distribution and spectral properties. These instabilities, combined with forward SBS, apparently cause a plasma-induced smoothing (self-smoothing) of the laser light as it propagates through the plasma. The final effect may have consequences similar to the temporal smoothing introduced intentionally in many laser systems. They do not propose this phenomenon as a smoothing technique; however, they claim that the understanding of this effect is crucial to the interpretation of experimental results on parametric instabilities.« less