Masatake Yoshida

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 ...

Planar shock wave generated by uniform irradiation from two overlapped partially coherent laser beams

Two partially coherent light laser beams, coupled with a random phase plate were focused at an angle of 31.7° with the centers of the beams offset by 250 μm. This produced a relatively uniform (7% root-mean-square) irradiated spot of 400 μm. When this technique was used to produce a shock wave in a copper wedge, a relatively uniform shock of 2 ns duration was produced. In addition, a multilayered flyer plate was accelerated using this method to an average speed of 21 km/s and produced a planar impact on a glass window. It is anticipated that this technique can be used to produce important multi-Mbar equation of state information in future experiments.

Flyer acceleration by a high-power KrF laser with a long pulse duration

The experiments of flyer acceleration by the irradiation of a high power laser are carried out using the ASHURA system at the Electrotechnical Laboratory, in which the laser has a short wavelength (∼249 nm) and a long pulse duration (∼30 ns). Three-layered targets (aluminum–polyimide–tantalum) are irradiated. The laser ablates the aluminum and polyimide layers and the rear layer (tantalum) is accelerated as a flyer. It is suggested that the tantalum flyer is in a condensed state for the duration of flight. The flyer velocity estimated from the acceleration profile is at least 8 km/s. One-dimensional numerical simulation indicates that the terminal flyer velocity becomes higher than 15 km/s. The energy conversion from laser energy to flyer kinetic energy is more efficient than that in the previous experiments using the three-layered targets with a longer wavelength and a shorter pulse duration (1–2 ns). Thus, it is found that a facility with a long pulse duration and a short wavelength is suitable for the ...

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.

Dynamic failure of steel under hypervelocity impact of polycarbonate up to 9 km/s

We performed hypervelocity impact experiments on SS400 steel with a polycarbonate projectile at velocities up to 9 km/s. Spall fracture damages were observed near a rear surface of the impacted target. The microstructure and microdamages were examined using optical microscopy and scanning electron microscopy. The α−e phase transition region was observed near the crater. Cracks parallel to the impact direction were observed below the crater, and radial cracks grew from the α−e phase interface at high velocity impact tests, especially above 6 km/s. Cleavage was the dominant mechanism for a spall fracture surface, and ductile fracture structures were also observed at the edge of spall plane. Geometric spall behaviors were well reproduced by numerical simulations using a hydrocode. These simulation results also showed that the cracks below the crater would be due to dynamic tensile stresses. The calculated results using the value of 13 GPa as the transition pressure showed that the duration necessary for the ...

Development of railgun accelerator at ISAS

A square-bore railgun accelerator system is described in which energy stored in capacitor banks (300 kJ maximum) is supplied to a railgun through a pulse transformer. The use of the transformer reduces the electric current and joule dissipation in the primary circuit while keeping the magnitude of the current in the secondary circuit high. It also makes it possible to use electric components having a low maximum current rating and keeps the current waveform from oscillating without a crowbar. However, if the electric contact between the armature and rail is not complete at ignition the core of the transformer saturates magnetically. This causes very low transformer impedance and anomalously large current in the primary circuit. The performance of the square-bore gun is poor, probably due to the leakage of plasma armature in front of the projectile. The projectile velocity with a hybrid armature is higher than with a plasma armature. The performance is greatly improved by changing to a round bore cross and refining the bore wall precisely with a reamer machine. >

Observation of Shock Initiation Process in Gap Test

We have conducted the experiments for shock sensitivity of high energetic materials by gap test. The set up of gap test have been improved to observe the shock initiation phenomena in acceptor charge by high‐speed video. The length of gap material was varied to observe the reaction process under various situations. The luminescence at the surface of acceptor holder was used Go/Nogo decision. The distance from the gap end to the luminescence area increases with increasing in gap length. In the critical gap length in which the sympathetic detonation does not occur, the remarkable decomposition of acceptor charge was observed as gas expansion.

Electromagnetic processes and launch efficiency of railgun systems

Detailed analysis of electromagnetic processes in railgun armatures shows that an electromagnetic drag mechanism caused by the generation of eddy currents is an ubiquitous effect in any railgun system. The results of an energy balance analysis allowed us to construct the equivalent electric circuits of railgun armatures. The acceleration in any railgun is equivalent, from an energetic point of view, to charging of a leaking capacitor. Simple numerical evaluations of the value of the electromagnetic drag force show that its role in the acceleration process is unavoidable for typical railgun launcher experimental conditions, although a quantitative evaluation of the drag force is difficult to make. Even discounting any known performance loss mechanism of a non-electromagnetic nature, our study gives a consistent qualitative explanation for all irreconcilable phenomena known from experiments; the saturation of attainable velocities, the degradation of plasma armatures, and the problem of high velocity sliding contact for metal armatures. Possible ways of optimizing railgun acceleration and achieving a higher velocity are discussed.

Hugoniot measurements for polyimide with laser and explosives

Equation-of-state measurements for a polyimide are presented. High-power KrF laser and chemical explosive-driven experiments provided Hugoniot data on the polyimide up to about 65 GPa. Conventional reflected-light measurements in the explosive experiments and velocity interferometry measurements in the laser experiments were performed. From both laser and explosive results the change of Hugoniot was indicated at near 30 GPa.