LANDAU-FLUID SIMULATION OF LASER FILAMENTATION

The Landau-fluid model is a recently introduced fluid-moment closure s cheme [G. W. Hammett and F. W. Perkins, Phys. Rev. Lett. 64, 3019 (199 0)] that was designed to include kinetic dissipative effects like Land au damping in fluid calculations. The fluid-moment hierarchy is termin ated by assuming linear relationships among the retained moments in Fo urier-transform space, with coefficients determined by matching the pl asma response to that obtained from a kinetic analysis. This paper gen eralizes the technique to the full range of ion and electron collision ality and applies it to a new fluid simulation code constructed to stu dy laser filamentation in underdense plasmas [Berger et al., Phys. Flu ids B 5, 2243 (1993)]. By matching the ion-acoustic complex frequency derived from the fluid model with that predicted by collisional, Fokke r-Planck, and kinetic analyses, the specific heat ratio, thermal condu ctivity coefficient, and viscosity coefficient for ions and the therma l conductivity coefficient for electrons are determined as functions o f the wave number k. For frequencies much less than the pump frequency this leads to a fourth-order polynomial dispersion relation whose spe ctrum includes damped ion-acoustic waves as well as filamentation mode s whose stability depends on the pump strength. An analytic instabilit y threshold condition on the laser intensity is derived from which the relative importance of ponderomotive and thermal drives can be assess ed. Expressions for the linear susceptibilities in the presence of a f inite-amplitude pump are also given, which might prove useful for unde rstanding spectral linewidths for Thomson scattering.