Hydrodynamics of high-energy GARPUN KrF laser interaction with solid and thin-film targets in ambient air

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Hydrodynamic regimes of KrF laser interaction with solid and thin-film targets in air at atmospheric and reduced pressures were investigated at the high-energy GARPUN facility. These experiments were performed with 100 J, 100 ns laser pulses in planar focusing geometry and compared with numerical simulations with the ATLANT code to verify the concept of the laser-driven shock tube (LST). Strong shock waves (SWs) are produced in the LST and the gas is accelerated to hypersonic velocity due to the deposition of laser energy. The laser beam is focused by a prism raster optical system that provides a very uniform intensity distribution at moderate laser intensities q ⩽ 1 GW cm−2 over a square spot of ∼1 cm size. Dynamics of laser-produced plasma and SWs in a surrounding gas were investigated by means of a high-speed photo-chronograph and streak camera in combination with shadow or schlieren techniques, and time and space resolved spectroscopy in the visible spectral range. Both experiments and simulations confirmed that target evaporation and blow-up of expanding plasma are the main mechanisms of UV laser–target interaction in the surrounding gas. Planar SWs with velocities up to 7 km s−1 towards the laser beam were observed in normal-density air and up to 30 km s−1 in rarefied air. Acceleration of thin CH films of 1–50 µm thickness was investigated both in free-expansion and plasma-confined regimes with the highest achieved velocities being up to 4 km s−1. The SW damping law in free space, independent of laser intensity and air pressure, could be approximated by a power law x ∼ tn with power indices n1 = 0.85–0.95 at the initial stage and n2 = 0.5–0.6 later, when the distance of the SW front from the target became comparable with the size of the irradiated spot. Instability growth at contact interfaces between ablative plasma and accelerated film, as well as between plasma and compressed air were observed, and compared for various initial irradiation non-uniformities. These non-uniformities were introduced by a grid, which was placed in front of the film target.