Three-dimensional filamentation of light in laser plasmas

The first calculations of time-dependent laser-plasma fllamentation in three dimensions are reported. These calculations are done with a three-dimensional laser propagation code based on a previous two-dimensional code [Phys. Fluids 31, 3079 (1988)]. The effect of incident beam structure, and in particular optical smoothing techniques, on the behavior of filamentation is studied. Both ponderomotive and thermal conduction dominated nonlinearities are considered, and calculations are done simulating both homogeneous non-absorbing plasmas and inhomogeneous laboratory plasmas. Random phase screen (RPS) and induced spatial incoherence (ISI) optical smoothing techniques are investigated and compared to generic unsmoothed laser beams. Qualitative examples are presented and scaling studies are done and compared to a simple theoretical analysis, In typical laser-plasma interactions without optical smoothing, three-dimensional effects lead to greatly increased filament intensities, as expected. Peak filament intensities of order 100-500 times the average intensity are routinely observed (without optical smoothing), as compared to earlier twodimensional calculations where peak intensities were of order 10-50 times average. In spite of this tendency to create stronger filaments, three-dimensional filamentation (when measured on a time-averaged basis) can b