Interaction of a high-power laser pulse with supercritical-density porous materials

The properties of a nonequilibrium plasma produced by high-power laser puls es with intensities I-L approximate to 10(14)-10(15) W cm(-2) irradiating p lane targets made of a porous material are investigated. The mean density o f matter in targets was substantially higher than the critical plasma densi ty corresponding to a plasma resonance. The density of porous material was rho(a) approximate to 1-20 mg cm(-3), whereas the critical density at the w avelength of incident radiation was rho(cr) approximate to 3 mg cm(-3). An anomalously high absorption (no less than 80%) of laser radiation inside a target was observed. Within the first 3-4 ns of interaction, the plasma flo w through the irradiated target surface in the direction opposite of the di rection of the laser beam was noticeably suppressed. Only about 5% of absor bed laser energy was transformed into the energy of particles in this flow during the laser pulse. Absorbed energy was stored as the internal plasma e nergy at this stage (the greenhouse effect). Then, this energy was transfor med, similar to a strong explosion, into the energy of a powerful hydrodyna mic flow of matter surrounding the absorption region. The specific features of the formation and evolution of a nonequilibrium laser-produced plasma i n porous media are theoretically analysed. This study allows the results of experiments to be explained. In particular, we investigated absorption of laser radiation in the bulk of a target, volume evaporation of porous mater ial, the expansion of a laser-produced plasma inside the pores, stochastic collisions of plasma flows, and hydrothermal energy dissipation. These proc esses give rise to long-lived oscillations of plasma density and lead to th e formation of an internal region where laser radiation is absorbed.