A shock Hugoniot compression curve for water has been measured up to less than 1 GPa. A plane and steady shock wave is produced in water by the flat plate impact of a projectile accelerated by a compressed gas gun. A new experimental procedure was proposed to detect the shock wave front sensitively, which makes it possible to measure shock Hugoniot in higher precision than the previous method. The present method is based on the very large pressure dependence of the refractive index of water upon compression. By using this method, the shock compression curve was determined within the precision of 2% to 3% of the estimated shock pressure. The precision is better than that of the previous data. It was confirmed that within the pressure range covered in this experiment, the shock-particle velocity Hugoniot can be described by a linear relation with a large slope. Shock Hugoniot states on the pressure-temperature plane were calculated by using the obtained Hugoniot data combined with the values of thermodynamical variables. Thermodynamic analysis of the shock compression process was developed to estimate the contributions of irreversible heating by shock compression.
Burst of small fragments of glass has been evidenced in the present study, when ground glass surface is laser ablated. Production of macro particles by laser ablation is an inherent characteristic of ground glass, and no similar phenomena have been observed in case of metal or polymer ablation. In this case, no additional metal coating has been made to further enhance absorption of laser pulse. Pulse laser shadowgraph has been taken to study the details of the phenomena in air and in vacuum. At least in vacuum, particle burst is found almost normal to the surface. By using ns-duration Nd:YAG laser of 100 mJ/pulse, observed particle velocity ranges 0.5 km/s to 1.5 km/s in case of in air and the maximum velocity is extended up to 1.5-2 km/s in vacuum. SEM observation of the ground surface reveals that glass surface is covered with micro cracks with several microns deep, which might attribute to macro particle production. In this sense, not surface roughness but also surface structure will be important in the ablation phenomena of glass. It is plausible that absorption of laser beam at the glass surface causes spallation like phenomena as well as production of an amount of plasma, and the plasma production might be responsible for the acceleration of broken fragments of glass. We applied the phenomena to ignite PETN powder explosive, and succeeded in igniting PETN powder only by laser ablation of ground glass.
The dc and ac magnetic fields due to transmission currents flowing through bus bars arranged side-by-side are analyzed to design the three-phase optical current transformer. The structure of optical current transformer with a double core for magnetic shielding is proposed based on the analysis of the interference effect of magnetic fields originated in the three-phase transmission currents and those of the eddy current fields induced by the component of three-phase ac fluxes perpendicular to the wide flat face of plate-form bus bar. It is shown that when a value of 100 is assumed as a relative permeability for the inner shieldcore, the eddy current fields which hinder the exact measurement of transmission currents can be decreased by less than 1/10 in the space between the inner core and the outer one, where the magneto-optical field sensor is arranged.Index Terms-Flat-shape three-phase bus bar, magnetic shielding, magneto-optical field sensor, optical current transformer.
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