Kunihiko Wakabayashi

Hugoniot measurement of diamond under laser shock compression up to 2 Tpa

Hugoniot data of diamond was obtained using laser-driven shock waves in the terapascal range of 0.5–2TPa. Strong shock waves were generated by direct irradiation of a 2.5ns laser pulse on an Al driver plate. The shock wave velocities in diamond and Al were determined from optical measurements. Particle velocities and pressures were obtained using an impedance matching method and known Al Hugoniot. The obtained Hugoniot data of diamond does not show a marked difference from the extrapolations of the Pavlovskii Hugoniot data in the TPa range within experimental errors.

GEKKO/HIPER-driven shock waves and equation-of-state measurements at ultrahigh pressures

The GEKKO/HIPER-laser [N. Miyanaga et al., in Proceedings of the 18th International Conference on Fusion Energy (IAEA, Sorrento, Italy, 2001), IAEA-CN-77] driven shock experiments were characterized in detail for studies on equation-of-state (EOS) at ultrahigh pressures. High-quality shock waves were produced with the bundled 9 laser beams optically smoothed by spectral dispersion technique and Kinoform phase plates. The laser beams were directly focused on targets at up to an intensity of 1014 W/cm2 or higher with a wavelength of 351 nm and a duration of 2.5 ns. Key issues on dynamic EOS research; the spatial uniformity and temporal steadiness of shock wave were estimated, and the preheating problem was also investigated by measurements of the self-emission and reflectivity from target rear surface. The experimental and analytical methods were validated by using double-step targets consisting of two Hugoniot standard metals. Extreme pressures only accessed in nuclear explosion experiments were generated ...

Equation-of-state measurements for polystyrene at multi-TPa pressures in laser direct-drive experiments

Equation-of-state (EOS) measurements for polystyrene in TPa (10Mbar) pressure regions are presented. Polystyrene Hugoniot data were obtained up to 2.7TPa using impedance matching techniques with laser direct drive at the GEKKO/HIPER laser facility [N. Miyanaga et al., in Proceedings of the 18th International Conference on Fusion Energy (IAEA, Sorrento, Italy, 2001), IAEA-CN-77] The results were compared with theoretical models and previous experimental data and found to be in good agreement with the previous data obtained by different drive and diagnostic techniques.

Transient Bond Scission of Polytetrafluoroethylene under Laser‐Induced Shock Compression Studied by Nanosecond Time‐Resolved Raman Spectroscopy

Nanosecond time‐resolved Raman spectroscopy has been performed to study polymer films, polytetrafluoroethylene (PTFE), under laser driven shock compression at laser power density of 4.0 GW/cm2. The CF2 stretching mode line of PTFE showed a higher shift (18 cm−1) at delay time of 9.3 ns due to the shock compression and corresponding pressure was estimated to be approximately 2.3 GPa. A new vibrational line at 1900 cm−1 appeared only under shock compression and was assigned to the C=C stretching in transient species such as a monomer (C2F4) produced by the shock‐induced bond scission. Intensity of the new line increased with increasing delay time along propagation of the shock compression.

Quantitative Visualization of Open-Air Explosions by Using Background-Oriented Schlieren with Natural Background

This paper describes application of a background oriented schlieren (BOS) technique in order to obtain quantitative measurements of shock waves from explosions by processing high speed digital video recordings. To illustrate the technique we present results from analysis of two explosions, one by C-4, the other by emulsion explosives (EMX). The experimentswere performed at theMinistry of Defense Test field, and carried out by the Ministry of Economy, Trade and Industry (METI). The objective of this paper is to show that the shock wave overpressures in a field explosion test can be predicted quantitatively by means of this technique.

REDUCTION OF EXPLOSION DAMAGE USING SAND OR WATER LAYER

The attenuation of blast waves and fragment velocity caused by an explosion was examined. The blast wave was attenuated by covering an explosive with water based material or sand. The relation between density of the materials for covering the explosive and the attenuation of peak pressure and impulse of the blast wave was studied to reduce the volume and weight of the material. The attenuation effect of the blast wave depended not only on the weight of the barrier materials, but also on the porosity. Reduction of fragment velocity accelerated by a high explosive using a sand or water layer was also evaluated.

TEMPORAL CHANGE OF RAMAN SPECTRA OF CARBON TETRACHLORIDE UNDER LASER‐DRIVEN SHOCK COMPRESSION

Nanosecond time‐resolved Raman spectroscopy has been performed to study a molecular response of carbon tetrachloride under laser‐driven shock compression at laser power density of about 5 GW/cm2. Intense Raman peaks of CCl4 at 213, 314, and 459 cm−1 in the Stokes and anti‐Stokes region were clearly observed simultaneously at single‐shot experiment. These peaks showed a blue shift (high frequency shift) and became broad under compression. The intensity of these peaks increased along with the propagation of shock wave. The temporal change of frequency shift and full width at half‐maximum (FWHM) of peaks showed the different behavior depending on each vibrational mode. The anti‐Stokes and Stokes ratio for each peak increased during shock compression due to the shock induced temperature rise. The temporal change of temperature estimated by Raman spectroscopy showed agreement with that of calculated temperature within present experimental resolution.Nanosecond time‐resolved Raman spectroscopy has been performed to study a molecular response of carbon tetrachloride under laser‐driven shock compression at laser power density of about 5 GW/cm2. Intense Raman peaks of CCl4 at 213, 314, and 459 cm−1 in the Stokes and anti‐Stokes region were clearly observed simultaneously at single‐shot experiment. These peaks showed a blue shift (high frequency shift) and became broad under compression. The intensity of these peaks increased along with the propagation of shock wave. The temporal change of frequency shift and full width at half‐maximum (FWHM) of peaks showed the different behavior depending on each vibrational mode. The anti‐Stokes and Stokes ratio for each peak increased during shock compression due to the shock induced temperature rise. The temporal change of temperature estimated by Raman spectroscopy showed agreement with that of calculated temperature within present experimental resolution.

Laser-Induced Shock Compression of Tantalum to 1.7 TPa

An extremely high pressure (1.74±0.22 TPa) was generated in Ta by using strong shock waves driven by direct irradiation of laser beams from the GEKKO XII glass laser system. The shock velocity was measured directly and the particle velocity and pressure were obtained using an impedance-matching technique by considering the intensity profile of the laser beams.

Chemical Reactions and Other Behaviors of High Energetic Materials under Static Ultrahigh Pressures

We have studied behaviors of one of high sensitive explosives, RDX (hexahydro-1,3,5trinitro-1,3,5-triazine), under static ultrahigh pressures (up to 65 GPa) generated by using diamond anvil cells (DACs) using FT-IR spectroscopy and UV-VIS absorption spectroscopy. RDX changed its color into dark red when compressed up to 20 GPa with cesium iodide (CsI), filled as a pressure medium, but not when compressed RDX alone nor with potassium bromide (KBr). When RDX was compressed with CsI, intensities of characteristic IR absorption peaks of RDX decreased as the pressure increased, and did not returned to the intensities measured at ambient pressure, after the pressure was unloaded. On the other hand, when RDX was compressed alone, its color changed into yellow at pressures above 60 GPa. UV-VIS absorption spectra of RDX were also measured. The absorption peak shifted to 410 nm at 65.5 GPa from 243.5 nm at ambient pressure. It is assumed that the HOMO-LUMO band gap of RDX decreases with increasing the pressure. Introduction Chemical reactions can be triggered by mechanical forces in solids because solids support shear strains. When covalent bonds are bent, the energies of their highest occupied molecular orbitals (HOMOs) are raised, whereas the energies of their lowest unoccupied molecular orbitals (LUMOs) are lowered [1]. Thus, the gap between these levels, which determines a bond’s stability, is decreased [2]. When mechanical forces are applied to materials, the electronic structures of the materials will change, so that structural transformations or chemical reactions can proceed Materials Science Forum Online: 2004-09-15 ISSN: 1662-9752, Vols. 465-466, pp 189-194 doi:10.4028/www.scientific.net/MSF.465-466.189

Dependence of Blast Attenuation on Weight of Barrier Materials

The attenuation effect of barrier materials, which covers an explosive completely, on blast waves was studied. The density of the barrier materials was examined to make the barrier materials light and low in volume. Water gel, small spheres of foam polystyrene, and mixtures of these two materials were used as the barrier materials, and the density of the mixture was varied from 0.12 g•cm-3 to 1.0 g•cm-3 by changing the mixed volume ratio. Natural silica sand was also tested for comparison. A spherical PMMA container was filled with the barrier materials and a spherical pentolite (100 g) was ignited at the center of container. The blast pressure around the container was measured. The mixture of the density of approximately 0.55 g•cm-3 maximized the attenuation of the blast wave for the same volume. The attenuation effect depends not only on the weight of the barrier materials but also on the porosity. A mixture of a density of approximately 0.13 g•cm-3 maximized the attenuation of the blast wave for the same weight. Using porous materials, relatively light barrier materials can attenuate the blast wave effectively, if the volume is not restricted. The attenuation effect of sand was greater than that of water gel and a mixture for the same volume.