Hydrodynamic evolution of picosecond laser plasmas

The density profile evolution of picosecond laser-plasmas has been inferred from the angular dependence of the specular reflectivity of a probe picosecond pulse. Time-resolved density profiles, critical density scale-lengths as short as 3 nm and values for the plasma collisionality are presented for a gold target irradiated by a 580 nm, 0.8 ps FWHM laser pulse with a peak intensity of up to 2x10 13 Wcm -2 . The results are best fitted at early times (<10 ps) by the self-similar isothermal exponential density profile and a critical density collision frequency of 0.3ω, in good agreement with 1-D hydrodynamic simulations.