K. Kondo

Dependence on laser intensity and pulse duration in proton acceleration by irradiation of ultrashort laser pulses on a Cu foil target

The dependence on laser intensity and pulse duration in energetic proton acceleration by irradiation of ultrashort laser pulses on a 5μm thick copper tape target was measured. The laser intensity was varied from 8.5×1017W∕cm2 to 1.1×1019W∕cm2, and the pulse duration from 55 fs to 400 fs. The maximum proton energy increased as the pulse duration was increased while the laser intensity was kept constant. The dependence of the maximum proton energy on laser intensity and pulse duration was in good agreement with an analytical plasma-expanding model.

Dynamic response of fused quartz in the permanent densification region

A shock wave investigation on fused quartz was performed in the permanent densification region, using a double stage light‐gas gun with dc x‐ray velocity meter and the electromagnetic particle velocity measurement technique. The elastic limit obtained in this work [(8.81±0.11) GPa] was somewhat lower than the published value. The compression curve obtained was quite different from the published curve and indicated that a rapid volume contraction occured just above the elastic limit. The tetrahedral coordinations seemed to remain at least up to 22u2009GPa, but above 16u2009GPa, they would be distorted or in some parts destroyed. The pressure of 16u2009GPa was considered as the limit of the permanent densification process, and the specific volume at atmospheric pressure interpolated along a hypothetical release adiabat was about 0.32u2009cm3/g.

Femtosecond-laser-induced spallation in aluminum

Ultrashort laser pulses having 50 fs duration were used to cause planer fracture, i.e., spallation, in pure aluminum foils at intensities of (0.07–1)×1015u200aW/cm2. A linear relationship was obtained between spall depth and foil thickness, approximately corresponding to the proportions that were obtained in other relations for 300 ps and longer laser pulse irradiation. Spall thicknesses less than 5 μm were obtained in foils thinner than 50 μm, and revealed small-scale spallation at a strain rate exceeding 108u200as−1. This ultrashort laser pulse evidently produced a shock wave followed by a steep unloading stress profile.

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.

Nanosecond time-resolved Raman spectroscopy on phase transition of polytetrafluoroethylene under laser-driven shock compression

Nanosecond time-resolved Raman spectroscopy is performed on polytetrafluoroethylene (PTFE) under laser shock compression at approximately 1 GPa. Blueshifts (8u2009cm−1) of a C–C stretching and redshifts (−8u2009cm−1) of a CF2 twisting in Raman spectrum due to a high-pressure phase (phase III) are observed. High-pressure phase transition to phase III of PTFE is suggested to occur within 10 ns.

Free‐surface velocity measurement of shock‐compressed alumina powder compact using a Fabry–Perot interferometer

A Fabry–Perot interferometer system was used to measure the free‐surface velocity of α‐alumina powder compact. In situ pressure profile measurements using a pair of manganin gauges were also carried out to clarify the compaction process during compression and stress release. The Fabry–Perot system indicates that the reflectivity of the free surface of the powder material does not significantly change during the stress release process. An observed feature of the powder’s stress release curve as determined by the manganin gauge suggests the existence of the residual voids in the shock compressed powder matrix even in the high shock pressure regime.

Raman Scattering Studies of CuInS2 Films Grown by RF Ion Plating

CuInS2 films were grown by rf ion plating and their crystallinity was characterized by Raman spectroscopy. Six Raman peaks were observed, most of which were assigned to the phonon modes of CuInS2, except for a peak at 307 cm-1 that was clearly observed in the films with a poor crystalline quality. The peak at 307 cm-1 was assigned to the totally symmetric mode by polarized Raman measurement. This peak was related to some kind of a localized mode with a smaller mean atomic weight of cations.

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 1014u2009W/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 ...

High-pressure phase transition of mullite under shock compression

Hugoniot measurements have been performed on mullite polycrystals in the pressure range up to 80GPa by using a two-stage light-gas gun and a Doppler laser interferometer. The Hugoniot elastic limit is determined to be 16.1±2.2GPa. The Hugoniot data show that the phase transition occurs at 30.4±3.6GPa. Comparison of the Hugoniot data with calculated theoretical Hugoniots suggests that the high-pressure phase transition of mullite is its disproportionation to corundum and stishovite. The mixed phase region is between 30 and 70GPa. It is indicated that this phase transition is very sluggish.

Spectroscopy of Hard X-Rays (2-15 keV) Generated by Focusing Femtosecond Laser on Metal Targets

Spectroscopy of hard X-rays generated by focusing a femtosecond laser (42 fs at 780 nm) onto metal targets consisting elements of various atomic number (Z) is carried out in the energy range between 2 and 15 keV using a direct-detection charge-coupled-device camera.Sharp K-shell line emissions are observed for X-rays generated from medium-Z targets (Ti, Fe, Ni, Cu and Zn), which can be used for X-ray diffraction without further monochromization. A broad continuum and a weak trace of L-shell line emissions are observed for X-rays from high-Z targets (Mo, Nb and W). The energies of the characteristic X-rays agree well with that of X-rays from neutral atoms or slightly ionized ions.