Laser-driven shock-wave studies using optical shadowgraphy

Abstract

The propagation of laser-induced shock waves in a transparent solid (Plexiglass) is investigated by optical shadowgraphy. The shock waves are generated by concentrating 100-J, 300-ps laser pulses from the Asterix-III iodine laser (\ensuremath{\lambda}=1.3 \ensuremath{\mu}m) onto areas 85 or 450 \ensuremath{\mu}m in diameter on the target surface---this corresponding to laser irradiances of 3\ifmmode\times\else\texttimes\fi{}${10}^{15}$ and 2\ifmmode\times\else\texttimes\fi{}${10}^{14}$ W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, respectively. It is observed that the initial phase of shock-wave propagation is obscured by an electrical breakdown phenomenon, being particularly strong at the smaller laser spot size. The possible cause of this phenomenon is discussed, and it is pointed out that the field strengths of ${10}^{8}$--${10}^{9}$ V/m required for breakdown are generated as a consequence of charge separation during two-dimensional plasma expansion. The shock-wave trajectories observed at subsequent times are compared with one- and two-dimensional numerical simulations. Good agreement is found when the same input data as for the simulations of previous thin-foil acceleration experiments are used.