High-energy GARPUN KrF laser interaction with solid and thin-foil targets in ambient air

摘要

Hydrodynamic regimes of KrF laser interaction with solid and thin-film targets in atmospheric and reduced pressure air were investigated at high-energy GARPUN installation. These experiments were performed with 100-J, 100-ns laser pulses in planar focusing geometry and compared with numerical simulations with ATLANT code to verify the concept of laser-driven shock tube (LST), which could accelerate a gas to hypersonic velocity and produce strong shock waves (SW). Laser beam was focused by a prism raster optical system that provided very uniform intensity distribution at moderate laser intensities q ≤ 1 GW/cm2 over a square spot of ～ 1-cm size. Dynamics of laser-produced plasma and SW in a surrounding gas were investigated by means of high-speed photo-chronograph and streak camera in combination with shadow or schlieren techniques, time and space resolved spectroscopy in a 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 a surrounding gas. Planar shock waves with velocities up to 7 km/s towards the laser beam were observed in a normal density air and up to 30 km/s in a rarefied air. Acceleration of thin CH films of 1 to 50-μm thickness was investigated both in a free-expansion and plasma-confined regimes with the highest achieved velocities up to 4 km/s. The SW damping law in a free space independently on laser intensity and air pressure could be approximated by a power law x ～ t~n with a power indexes n_1 = 0.85 - 0.95 at the initial stage and n_2 = 0.5 - 0.6 later, when a distance of the SW front from a target became comparable with a 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. They were introduced by a grid, which was set in front of the film target.