COLLISIONLESS DAMPING OF LOCALIZED PLASMA-WAVES IN LASER-PRODUCED PLASMAS AND APPLICATION TO STIMULATED RAMAN-SCATTERING IN FILAMENTS

Observations of stimulated Raman scattering (SRS) in laser-produced pl asmas often yield results at odds with theoretical predictions. For ex ample, SRS is commonly seen at incident laser intensities below the th eoretical threshold, and the spectrum of SRS light often extends to mu ch shorter wavelengths than models predict. To account for these anoma lies it is often proposed that SRS is occurring in high-intensity, sel f-focused light filaments. A serious problem with this model is that p lasma wave damping rates estimated on the basis of the usual Landau th eory for homogeneous plasmas would seem to rule out this explanation f or many cases of interest. Damping rates for plasma waves confined to small-radius filaments, however, could be significantly different than damping rates for plane waves. Using a novel method for calculating t ransit-time damping, this paper analyzes the collisionless damping of plasma waveguide modes in a cylinder. It is found that the actual damp ing rates for waveguide modes in a suitable filament model are much le ss than for the plane waves in a homogeneous plasma producing the same wavelength of SRS emission. Consequently, the filament model remains viable as an explanation of the anomalous SRS observations. (C) 1998 A merican Institute of Physics. [S1070-664X(98)01012-X].