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

The properties of a nonequilibrium plasma produced by high-power laser pulses with intensities I_L ≈ 10¹⁴—10¹⁵ W cm^-2 irradiating plane targets made of a porous material are investigated. The mean density of matter in targets was substantially higher than the critical plasma density corresponding to a plasma resonance. The density of porous material was ρ_a ≈ 10—20 mg cm^-3, whereas the critical density at the wavelength of incident radiation was ρ_cr ≈ 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 flow through the irradiated target surface in the direction opposite of the direction of the laser beam was noticeably suppressed. Only about 5% of absorbed laser energy was transformed into the energy of particles in this flow during the laser pulse. Absorbed energy was stored as the internal plasma energy at this stage (the greenhouse effect). Then, this energy was transformed, similar to a strong explosion, into the energy of a powerful hydrodynamic flow of matter surrounding the absorption region. The specific features of the formation and evolution of a nonequilibrium laser-produced plasma in 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 material, the expansion of a laser-produced plasma inside the pores, stochastic collisions of plasma flows, and hydrothermal energy dissipation. These processes give rise to long-lived oscillations of plasma density and lead to the formation of an internal region where laser radiation is absorbed.