INTERACTION OF A HIGH-POWER LASER-BEAM WITH LOW-DENSITY POROUS-MEDIA

We have experimentally investigated the processes of laser light absor ption and energy transfer in porous targets made of ''agar-agar'' (C14 H18O7) With an average density of 1-4 mg/cm(3) illuminated by the focu sed beam of a neodymium laser with an intensity of 10(14) W/cm(2) with in a pulse of duration 2.5 ns. Many important scientific and technical problems, e.g., inertial-confinement thermonuclear fusion, the creati on of lasers in the x-ray regime, and the modeling of astrophysical ph enomena under laboratory conditions, can be successfully addressed by using low-density porous media as components of such targets. In our e xperiments with porous targets of variable density and thickness we us ed optical and x-ray diagnostic methods, which ensured that our measur ements were made with high temporal and spatial resolution. We show th at a region forms within the porous target consisting of a dense high- temperature plasma which effectively absorbs the laser radiation. Ener gy 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 g ood agreement with the predictions of our theoretical model, which tak es 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 Institu te of Physics.