The electron-beam-pumped KrF laser installation GARPUN with a 100-J output energy and long 100-ns pulse duration has been used to investigate laser-target interactions in a broad range of laser intensities for small~150 mm! and largẽ ;1 cm! irradiated spots. For higher intensities~up to 5 ϫ 10 12 W0cm 2 !, a conical shock wave was generated in condensed matter by megabar pressure at the ablation front. It propagated with a supersonic velocity in a quasisteady manner together with a conical shock wave inside a target. Evaporated target material moving with a velocity of ;50 km0s formed an extended plasma corona of ;5 mm length with an electron temperature of ;100 eV. Emission spectra of plasma have been investigated in the extreme UV range 120-250 Å. For lower intensities~10 8 -10 9 W0cm 2 !, planar shock waves in normal density air were produced with initial velocities up to 4 km0s in the forward direction and 7 km0s in the opposite direction toward incident radiation. In rarefied air, the forward shock wave kept velocities constant whereas the backward ones were accelerated up to 30 km0s. Planar compression waves in transparent condensed matter were also demonstrated propagating with sonic velocity.

Two of the key issues of a krypton fluoride (KrF) laser driver for inertial fusion energy are the development of long life, high transparency pressure foils (to isolate vacuum in the electron beam diode from a working gas in the laser chamber), and the development of durable, stable, optical windows. Both of these problems have been studied on the single-pulse e-beam-pumped KrF laser installation GARPUN. We have measured the transport of electron beams (300 keV, 50 kA, 100 ns, 10 × 100 cm) through aluminum-beryllium and titanium foils and compared them with Monte Carlo numerical calculations. It was shown that 50-μm thickness Al-Be and 20-μm Ti foils had equal transmittance. However, in contrast to Ti foil, whose surface was strongly etched by fluorine, no surface modification nor fatal damages were observed for Al-Be foils after ∼1000 laser shots and protracted fluorine exposure. We also measured the 8% reduction in the transmission of CaF2 windows under irradiation by scattered electrons when they were set at 8.5 cm apart from the e-beam-pumped region. However an applied magnetic field of ∼0.1 T significantly reduced electron scattering both across and along the laser cell at typical pumping conditions with 1.5 atm pressure working gas. Thus the e-beam-induced absorption of laser radiation in optical windows might be fully eliminated in an e-beam-pumping scheme with magnetic field guiding.

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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.

This paper describes a performance of 100 J-class KrF laser system "GARPUN" intended for target irradiation experiments by 100 ns pulses. A controllable space-time intensity distribution in a focal spot reaching 5* 1O2 W/cm2 produced megabar ablation pressure, which initiated conical shock wave in solids. It propagated in a quasi-steady manner together with an ablation front that resulted in anomalous high penetration rate of laser radiation throughout the matter. Long-time sample loading together with strong tangential shear flow of compressed layers produced favorable conditions for pressure-induced transformation of the pyrolytic graphite into a diamond-like phase by martensitic mechanism.