Optical probing of laser-induced indirectly driven shock waves in aluminum

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 similar or equal to 22km/s, pressure similar or equal to 8 Mbar) 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 (lambda = 532 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-Unsold opacity.