Interaction of a high-power laser beam with low-density porous media

We have experimentally investigated the processes of laser light absorption and energy transfer in porous targets made of ''agar-agar'' (C14H18O7) With an average density of 1-4 mg/cm(3) illuminated by the focused beam of a neodymium laser with an intensity of 10(14) W/cm(2) within a pulse of duration 2.5 ns. Many important scientific and technical problems, e.g., inertial-confinement thermonuclear fusion, the creation of lasers in the x-ray regime, and the modeling of astrophysical phenomena under laboratory conditions, can be successfully addressed by using low-density porous media as components of such targets. In our experiments with porous targets of variable density and thickness we used optical and x-ray diagnostic methods, which ensured that our measurements were made with high temporal and spatial resolution. We show that a region forms within the porous target consisting of a dense high-temperature plasma which effectively absorbs the laser radiation. Energy is transferred from the absorption region to the surrounding layer of porous material at up to 2 x 10(7) cm/s. Experimental data are in good agreement with the predictions of our theoretical model, which takes into account the specific features of absorption of laser radiation in a porous material and is based on representing the energy transfer within the material as a hydrothermal wave. (C) 1997 American Institute of Physics.