The interaction between the intense laser pulse and the CH film allows a strong shock wave to propagate towards the statically compressed water sample. The density of water is inferred according to the well-determined equation of state up to 25 GPa 20.įor the dynamic compression, a square laser pulse (about 3 ns in duration, 351 nm in wavelength, 800–1500 J in energy) generated from the Shenguang-II laser platform, is focused on the CH film with a spot size of about 0.65 mm in diameter. Under static compression, the fluorescence signal from the ruby particle is used to determine the pressure of water 17, 18, 19. The quartz plate (with a 200 nm aluminum coating on the side contacting the diamond) serves a standard material for performing impedance matching calculation to determine the Hugoniot state 16. In the chamber between the two anvils, there are a quartz plate, a ruby particle, and the water sample. The rear anvil (1200 μm in thickness) has an anti-reflection coating (with respect to the VISAR probe light at 660 nm) to enhance the signal-to-noise ratio of the VISAR signals. The front anvil (150 μm in thickness) was coated by a gold film (1.5 μm in thickness) to eliminate preheating effects and a polypropylene (CH) film (25 μm in thickness) to serve as the laser ablator. Doubly distilled pure water was statically compressed to about 0.57 GPa (about 1.16 g/cm 3 in density) between two diamond anvils. The experimental details and the results will be presented in the Experimental and Result and discussion sections.įigure 1 shows a schematic of the target assembly for water as well as the diagnostics. This technique makes it possible to obtain the sound speed of water in the off-Hugoniot region with reference to the Hugoniot measurement of liquid water. In this work, we report the measurement of sound speed of water in the case of a combination of pre-compression using diamond anvil cell (DAC) and laser shock compression. The establishment of these methods makes it possible to determine the sound speed of water under laser shock compression. In addition to that, there are other indirect methods proposed to determine the sound speed of materials under laser shock compression, such as the detection of acoustic perturbations in the target with reference to that in a standard material 13, 14, 15. Recently, a new method is demonstrated to determine the sound speed of quartz under laser shock compression, which uses the bending boundary formed by the propagation of the lateral rarefaction wave into the shock compressed region, as probed by a line-imaging Velocity Interferometer System for Any Reflector (VISAR) 12. The difficulty is related to the fact that the position of the ablation front is changing with respect to the laser intensity, the target material and the amount of ablated material 11. ![]() The technique becomes difficult to apply in the case of laser shock compression, which aims at reaching higher pressure along the Hugoniot line. ![]() ![]() However, the method of measuring the sound speed of a material has been well established, especially in the case of gas gun experiment 10, where the thicknesses of the flyer and target can be measured precisely for determining the speed of the overtaking rarefaction waves. The report on the sound speed of water at high pressure is scarce. Apart from this, the sound speed data is also valuable to test equation-of-state (EOS) models since as a second derivative of the Gibbs free energy they are generally more sensitive to the minor differences between various EOS models. Therefore, sound speed information is important if a planetary interior model needs to be built purely based on experimental equation of state data. Since the sound speed is usually measured on a Hugoniot line, the information of the intersection of the Hugoniot line with the local isentropic line is intrinsically carried by the sound speed. The thermodynamic states of water inside the icy giant planets can be approximated by isentropic lines 8, along which the derivative of pressure with respect to density is the sound speed 9. Water at extreme conditions is one of the most concerned topics due to its significant abundance in icy giant planets like Uranus and Neptune 1, 2, 3, 4, 5, as well as extrasolar ‘hot Neptunes’ and ‘mini-Neptunes’ 6, 7.
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