Optical emissivity from a laser-driven shock-heated dense plasma

The aluminium side of two-layer Al-Plastic targets were irradiated with a long pulse (∼2.4 ns FWHM) 1.06-μm laser light at intensities up to 7 x 1013 W/cm2. The time history of the thermal emission of the confined rear surface of the aluminium was measured. Visible emission only occurs for a short time after the arrival of the laser-generated shock waves. Over the range of the measurements, the duration and the intensity of the emission reduce with increasing laser intensity. The experimental results are in good agreement with the results of a simple phenomenological model that assumes a linear temperature and density profile on the shock front. The values of shock velocity and maximum temperature and density that were used in the model were found using the hydrodynamic simulation code MEDUSA. From the model the shock width was measured for different conditions by matching the emission of the experiment and model. It is found that the time history of the emission is strongly sensitive to the ionization potential of the plastic that is assumed to change with density from ∼4eV at zero pressure to zero at high densities. The technique provides a way to measure the pressure metallization of large band gap insulators. (Author)