Optical signals from shock waves emerging at a free surface of metals are expected to yield information about the equation of state and the transport and relaxation properties of hot dense plasmas. We present the results of optical measurements on planar shock waves ͑velocity Ӎ22 km/s, pressure Ӎ8 Mbar͒ in solid aluminum which were generated by exposing a miniature sample to intense thermal x rays from a laser-heated cavity. The reflectivity of the free surface of the sample for the light from a probe laser (ϭ532 nm) and the absolute value of its optical emission were simultaneously registered with a 7-ps temporal resolution. For interpretation we used a two-temperature hydrodynamic code which includes the electron heat conduction and electron-ion relaxation and accounts for the nonequilibrium shock structure. The underlying self-consistent model for the equation of state and the transport coefficients of a metal over the relevant range of thermodynamic parameters are described in some detail. The reflectivity decay signal, which yields direct information on the effective collision frequency in the unloading material, and the emission peak, which is sensitive to the heat conductivity and dielectric permittivity of the hot and dense plasma behind the shock front, are well reproduced by the simulations. The emission signals are, however, longer than predicted, possibly due to the residual surface roughness in the experiment. On a longer time scale of 1-2 ns, the emission signal is well described by a simple radiation transport model with the Kramers-Unsöld opacity. ͓S1063-651X͑97͒09007-7͔PACS number͑s͒: 52.25. Fi, 52.25.Mq, 52.25.Rv, 52.35.Tc

The experience of target fabrication with low-density and cluster heterogeneity is presented. Cluster plasma research is strongly dependent on target fabrication development and target structure characterization. Ten more target parameters should be measured for experiment interpreting in case of micro-heterogeneous plasma. Foam and foil targets, high-Z doped also, are produced and irradiated on the existing laser facilities. The density of 4.5 mg/cc cellulose triacetate in the form of regular three-dimensional polymer networks are achieved which is as low as plasma critical density for the third harmonic of iodine laser light. The possibilities of varying important target parameters, methods of their monitoring are discussed. Experiments with underdense foam targets with or without clusters irradiated on Prague Asterix Laser System (PALS) laser facility are analyzed preliminary for target optimization. Under-critical foams of varying structure (closed-cell foam or three-dimensional networks) and densities are reported for plasma experiments. Thermal and radiation transport in such targets are considered.

We report the first simultaneous measurement of the reflectivity and optical emission of a strong (4-8 Mbar) shock front emerging at a free surface of a solid. Planar shock waves were driven by thermal x rays from a laser-heated cavity. The inferred model-independent brightness temperature of the shock front in silicon turns out to be significantly below the expected Hugoniot temperature. We find that our data cannot be explained within the two-temperature model which assumes instantaneous metallization of silicon in the density jump. [S0031-9007(98)06017-7]

The interaction of powerful laser and X-ray pulses with planar low average density (0.5–10 mg/cm3) porous agar-agar targets was experimentally studied. At a laser power density of ∼5 × 1013 W/cm2 (λ = 1.054 μm) the laser light absorption and following energy transfer processes, as well as dynamics of produced plasma were investigated in detail with a variety of optical and X-ray diagnostic methods. Volume absorption is shown to occur in experiments with laser-irradiated agar targets. An extended laser energy deposition region filled with hot (0.8–1 keV) plasma is formed inside a porous target. The laser light absorption efficiency is as high as ∼80%. The emission of 2ω0 and 3ω0/2 harmonics from laser-produced plasma is observed over the time of the laser pulse even with agar targets of 0.5 mg/cm3 average density. Characteristics of energy transfer in low-density porous media are measured in experiments on illumination of agar targets by laser pulses or X rays emitted by a thin Cu converter. The hydrodynamic mechanism is responsible for the energy transfer in laser-illuminated porous targets and the radiative energy transfer seems to be dominant in the case of X-ray irradiation. The experimental data are in reasonable agreement with predictions of a developed theoretical model describing the hot plasma layer formation and the two-stage homogenization process within the illuminated porous targets.

Interactions of the laser beam of iodine laser 'PALS' with low-density foam targets and acceleration of Al foils by the pressure of heated foam matter are investigated here, both experimentally and theoretically. An x-ray streak camera was used for the evaluation of the speed of energy transfer through the porous foam material. X-ray streak records show no noticeable emission near the Al foil at the rear side of a 400 µm thick foam and, thus, Al foil is most likely accelerated without significant heating and expansion. Accelerated foil velocities, measured by three-frame optical shadowgraphy, reach up to 10 7 cm s −1 . Experimental results compare well with our two-dimensional hydrodynamics simulations and with an approximate analytical model.