Effects of anomalous high penetration rate of high-power KrF laser radiation throughout the solid matter and shock-induced graphite-diamond phase transformation

Abstract

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.

Deep Penetration of UV Radiation into PMMA and Electron Acceleration in Long Plasma Channels Produced by 100 ns KrF Laser Pulses

“…It should be noted that in these experiments, during an exposure to nanosecond laser pulses, the ablation front passed a distance in the condensed matter less than the size of the irradiated spot, i.e., the one-dimensional (1D) plane geometry of the ablation front propagation and the SW generated by it was realized. In our previous experiments with Al, Pb, graphite and polyethylene (C 2 H 4 ) n targets at GARPUN KrF laser [20][21][22], pressures consistent with the above scaling were measured in the intensity range of 5 × 10 11 -5 × 10 12 W/cm 2 for 100 ns pulses and a focal spot of ~150 µm. The largest pressure value at an intensity of 5 × 10 12 W/cm 2 reached 4 Mbar.…”

“…A deep penetration of the UV radiation in PMMA and high ablation front velocities were confirmed by a spatial-temporal evolution of the visible plasma glow. The modeling experiments with a preliminary drilled capillary channel and Monte Carlo simulations evidence that the crown origin might be caused by an electron beam acceleration of up to a few hundred keV energies, which is much higher than the electron temperature of the plasma corona ~100 eV [21,22]. This indicates the presence of an unusual direct electron acceleration regime.…”

“…Distinct features of the high-energy UV light interaction with PMMA observed in our experiments are: (i) a conical SW evidencing a 2D hydrodynamic regime; and (ii) a thin capillary-like channel with a diameter of 30-40 µm extending deep into the target up to ~1 mm from the top of the ablative crater (Figure 1). The microscope images were obtained with sample illumination by an unpolarized incoherent light source and in a projection scheme in the polarized light of copper vapor laser [20][21][22]. The formation of the capillary channel is explained by the pre-focusing of incident radiation nearby the crater top due to specular reflection by the plasma density gradient near the conical surface and the self-focusing of radiation transmitted through plasma in the PMMA [27].…”

A New Insight into High-Aspect-Ratio Channel Drilling in Translucent Dielectrics with a KrF Laser for Waveguide Applications

“…Indeed, for fused silica glass at KrF laser wavelength λ = 248 nm linear refraction index is n 0 = 1.5 while the nonlinear one due to the Kerr effect is n 2 = (3.4–5.6) × 10 −16 cm 2 /W [ 31 , 32 ]. Those give the critical power for radiation self-focusing = (1.1–1.5) × 10 5 W, which is much lower than the peak power P ~ 10 9 W in high-energy single-shot experiments [ 25 , 26 , 27 , 28 ]. Moreover, it was comparable with P ≥ 10 5 W in small-energy multi-pulse experiments.…”

“…Therefore, it was a goal of the present experiments to locate the true mechanisms of high-aspect-ratio capillary channel formation in typical materials, such as PMMA and translucent fused silica glass K8 (which is an analog of Schott Glass BK7 glass) for a KrF laser in comparison with the drilling of highly transparent KU-1 glass (analog of Corning 7980 glass). Earlier, we observed extended channels with a length of up to 1 mm and a diameter of ~30 μm under entirely different conditions, namely in the interaction of a single high-power KrF laser pulse of 100 J energy and 100 ns duration with PMMA targets [ 25 , 26 , 27 , 28 ]. At a much higher laser intensity, I ~ 5 × 10 12 W/cm 2 , a channel was produced in the PMMA due to pre-focusing of radiation that passed through a hot ( T ~ 50 eV) dense ( ρ ~ 1 g/cm 3 ) ablation plasma and was reflected at a grazing angle by the wall of a deep conical crater with an inlet diameter D ~ 0.5 mm and depth L ~ 1 mm.…”