Transverse shadowgraphy and new recovery techniques to investigate dynamic fragmentation of laser shock-loaded metals

With the development of high energy laser facilities for inertial confinement fusion, such as the Laser M\'egaJoule and the National Ignition Facility, the question of debris ejection from metallic shells subjected to intense laser irradiation has become a key issue. We have developed two diagnostics to investigate this phenomenon. Transverse shadowgraphy is an optical time-resolved diagnostic. It provides successive images of the fragments motion, that allow characterizing different fragmentation processes such as micro-jetting and spallation. A continuous laser probe is divided by beamsplitters and sent to cameras (acquisition time is few tens nanoseconds) with different delays. Quasi-instantaneous pictures of the debris clouds are obtained and ejection velocities can be measured. Complementary data are provided by post-shock analysis of recovered fragments. Such recovery can be achieved in aerogels, but their brittleness and low transparency make the analysis difficult. Instead, we have used a new technique, based on a highly transparent gel of density 0.9\,g/cm$^3$, which allows soft recovery and easy observation of the fragments sizes, shapes and penetration depths, with a spatial resolution of micrometer-order.