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-de
nsity (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 te
mporal and spatial resolution were used to examine the dependence of absorp
tion and scattering of the incident beam on the initial mean density and th
ickness of the irradiated samples. We show that when a porous target is irr
adiated, a bulk absorption layer of high-temperature plasma is produced ins
ide the target whose dimensions are determined by the initial density of th
e 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 m
easured. A theoretical model is developed that describes the interaction of
high-power laser pulses with a porous medium. Predictions of the model, ba
sed on the hypothesis of two stages of homogenization of the target materia
l-a fast stage (0.1-0.3 ns) and a slow stage (1-3 ns), are in good agreemen
t with the experimental data. (C) 1999 American Institute of Physics. [S106
3-7761(99)00303-0].