Statistical ray tracing in plasmas with random density fluctuations.

Abstract

The propagation of light rays in a plasma is considered for small density fluctuations superimposed upon a given background density profile. The lowest-order angular spreading, focusing, and drift effects are calculated in the geometrical-optics limit using a statistical random-walk approach. This method is an economical semianalytic alternative to a purely numerical ray-trace approach and is of particular but not exclusive interest in describing the propagation of laser light in laser-produced plasmas. The model is applied to a simple fluctuation spectrum characterized by an rms amplitude and by a correlation length. Results are presented for the evolution of the intensity profiles of beams incident on a plane-parallel linear-profile plasma slab as a function of the angle of incidence. Comparisons of these results with numerical Monte Carlo ray-trace solutions show good agreement. Density fluctuations as small as a few percent of the critical density can, for example, produce significant angular broadening in the specularly reflected beam and reduce the sensitivity of the absorption fraction to the incidence angle, particularly near normal incidence.