Evaluation of plasma-supported ablation processes in pulsed laser-target effects

Transient processes, capable of improving the coupling efficiency of specific targets exposed to laser radiation, are subjects of numerous investigations, both concerning fundamental research and technological applications. The present paper refers to recent studies, related to laser- target effects on optronic materials, carried out at the German-French Research Institute of Saint-Louis. The availability of a unique repetitively pulsed CO 2 -laser with pulse energies up to 150 J and repetition rates up to 100 pps, allowed experiments to be performed in a large range of fluences up to hundreds of J/cm 2 with pulse durations of several microseconds, corresponding to peak power densities up to more than one hundred MW/cm 2 . Ablation processes were studied under various irradiation conditions (ratios of peak power densities to average power densities of more than 10 4 :1. The flexibility of the laser allowed to optimize energy transfer rates according to specific material properties, by varying pulse energies, pulse shapes, repetition rates and lengths of pulse trains. Experiments investigations were aimed at improving the knowledge both on surface effects in case of optically opaque materials and on volume processes for materials which are transparent or translucent for the incident laser wavelength. Special high-speed diagnostic techniques have been used and are discussed. Particular interest was to analyze nonlinear processes at peak power densities close to and above surface plasma ignition thresholds.