Motonao Nakahara

Shock Hugoniot compression curve for water up to 1 GPa by using a compressed gas gun

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 thermodynami...

Output Characteristics Of Field Sensor Used For Optical Current Transformers Applied To The Flat-shape Three-phase Bus-bar

The open-air-type optical current transformer is studied for application to three-phase power distribution equipment. To make this type of OCT, laminations of non-oriented Si-Fe sheet used as shielding are tested for the effect of shielding. The linearity of the magneto-optical field sensor used in the OCT is discussed. It is shown that the proposed OCT will allow 0.5% accurate measurements for three-phase AC currents.

Analysis of magnetic fields due to three-phase bus bar currents for the design of an optical current transformer

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 shield-core, 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.

Magneto-optical properties of CdMnTe films on quartz glass and sapphire substrates in the transparent, visible region

Magneto-optical properties in the visible wavelength region are studied on CdMnTe films grown on quartz glass and sapphire (0001) single crystal substrates by MBE. Good quality films with sharp spectra of X-ray diffraction are obtained on the sapphire substrates polished carefully and pre-heated up to 950/spl deg/C. Weak (111) film-growth is observed for films deposited on the quartz glass substrates. A value of Faraday rotation angle of -0.31 deg/cmG at 700 nm is measured in the Cd/sub 0.59/Mn/sub 0.41/Te film grown on the sapphire substrate. The values for the films on the quartz glass substrates are 1/3 of those on sapphire. It is shown that the specific rotation in CdMnTe depends strongly upon the crystalline quality of films deposited on the substrate. >

Water shock wave emanated from the roughened end surface of an optical fiber by pulse laser input

Abstract We have discovered an interesting effect of surface roughness of optical fiber on the generated shock pressure pulse in the case of an intense laser pulse input. The phenomenon of shock strengthening by roughening the end surface of the optical fiber has been found. Water shock waves emanated from the fiber end surface have been evidenced by the laser shadowgraph technique. Other interesting features are the use of aluminium coating to enhance the shock pressure, repeated but finite lifetime of the effect of the surface roughness, bubble formation, etc. The detailed physics of these phenomena is unknown.

New water shock sensor

A new shock sensor for liquids is described in this article which is suitable for precise shock Hugoniot compression curve measurement of water in the relatively low pressure region of ⩽1 GPa. The present method is based on the fact that the refractive index of water has a very large pressure dependence. A laser beam incident on the liquid–glass boundary from the proper angle of incidence is totally reflected at the interface. The condition of total internal reflection is violated if the refractive index changes instantaneously by shock compression. The shock target assembly was designed for a gas gun experiment based on the above concept. Two examples of the shock Hugoniot measurement data of water were given.

Shock Hugoniot Compression Data for Several Bio‐Related Materials

Shock wave data for several bio‐related materials have been obtained. Plane and steady shock waves have been induced by using a modest compressed gas gun facility. Shock pressure covered in this study ranges at least up to 1 GPa. In order to realize the sensitive detection of shock front in these materials in this relatively low pressure region, an optical method has been developed by our group. Optical prism was placed on the sample such that incident laser beam is totally reflected at the prism sample interface. Samples used in the experiment include gelatin with different initial density, NaCl aqueous solution, and finally chicken breast meat. Shock data obtained in the present study are compared with the shock Hugoniot curve for water. It is found that slope of the shock velocity‐particle velocity Hugoniot compression curve for all the materials tested is almost 2. While value of the intercept of the relationship, corresponding to the sound velocity, is apparently dependent on the material and ambient...

Liquid Atomization Induced by Pulse Laser Reflection underneath Liquid Surface

We observed a novel effect of pulse laser reflection at the interface between transparent materials with different refractive indices. The electric field intensity doubles when a laser beam is completely reflected from a material with a higher refractive index to a material with a lower index. This effect appreciably reduces pulse laser ablation threshold of transparent materials. We performed experiments to observe the entire ablation process for laser incidence on the water–air interface using pulse laser shadowgraphy with high-resolution film; the minimum laser fluence for laser ablation at the water–air interface was approximately 12–16 J/cm2. We confirmed that this laser ablation occurs only when the laser beam is incident on the water–air interface from water. Many slender liquid ligaments extend like a milk crown and seem to be atomized at the tip. Their detailed structures can be resolved only by pulse laser photography using high-resolution film.

Shockwave Dynamics of High Pressure Pulse in Water and Other Biological Materials Based on Hugoniot Data

An equation of state of the form F( p, ρ, e) suitable for the description of flow problems in biological materials is examined using two approaches. First, the effects of irreversible heating of water by shock compression on the elevated pressure and internal energy are discussed using thermodynamic theory. It is revealed that pressure increases due to entropy production contribute almost nothing to the pressure–volume equation of state, although irreversible internal energy itself contribues appreciably to the total internal energy. Shock Hugoniot data for water and biological materials measured at pressures below 1 GPa suggest that the form of the equation of state in terms of pressure and volume can safely be approximated as an isentrope, especially a so-called Tait-type equation of state. To describe shock-induced flows in liquids, parameters in the Tait equation are determined from the available Hugoniot data. Hydrodynamics of shock pressure pulse evolution in water induced by pulse laser energy have been developed. An approximate but adequate description of the process can be obtained with the sound velocity function derived from the Tait-type equation of state for water and biological materials.

High-Speed Photographic Observation of Shock-Pressure Pulses in Water Induced by Laser Energy Absorption at Roughened Surface

Absorption enhancement of laser energy at roughened surface was discovered by our group years ago. Laser ablation threshold apparently decreases by intentional roughening of the surface. Another important condition of threshold reduction is found for transparent material surface, if the pulse laser beam is irradiated from the rear side through the transparent plate. Several examples of the shock pulse induced in water and in air by pulse laser ablation of ground glass, roughened PMMA plates and fiber has been observed in detail by high-speed photography. Simple preliminary experiments to study the physical mechanism of enhanced absorption of laser energy have also been planned and demonstrated.