Absorption and scattering of high-power laser radiation in low-density porous media

We have experimentally investigated the interaction of high-power neodymium laser pulses in the intensity range 10(13)-10(14) W/cm(2) with flat low-density (0.5-10 mg/cm(3)) agar-agar targets under conditions of interest for problems of inertial nuclear fusion. Optical and x-ray methods with high temporal and spatial resolution were used to examine the dependence of absorption and scattering of the incident beam on the initial mean density and thickness of the irradiated samples. We show that when a porous target is irradiated, a bulk absorption layer of high-temperature plasma is produced inside the target whose dimensions are determined by the initial density of the material. The time dependence and spectral composition of the harmonics 2 omega(0) and 3 omega(0)/2 observed in the plasma-scattered radiation are measured. A theoretical model is developed that describes the interaction of high-power laser pulses with a porous medium. Predictions of the model, based on the hypothesis of two stages of homogenization of the target material-a fast stage (0.1-0.3 ns) and a slow stage (1-3 ns), are in good agreement with the experimental data. (C) 1999 American Institute of Physics. [S1063-7761(99)00303-0].