In this paper, precision high speed imaging of the pulse laser ablation of liquid surface has been described. This study is based on our previous findings that appreciable reduction of pulse laser ablation threshold of transparent material in case the pulse laser beam is incident from the water side on the interface of the transparent material and air or water. We have performed a series of experiments to observe the ablation process for laser incidence on the interface of water and air. Whole processes were observed by shadowgraphy optics by using a ns pulse laser and a high-resolution film. Within the tested experimental conditions, minimum laser fluence for laser ablation at water-air interface is shown to be around 12-16 J/cm2. We have confirmed that laser ablation phenomena will take place only when laser beam is incident on the water-air interface from inside the water medium. Many slender liquid ligaments extend like milk crown and seem to be atomized at the tip of them. Jet tip is moving at supersonic velocity but is decelerated very rapidly. By changing the laser energy with keeping laser fluence at the interface, temporal evolution changes appreciably at least in the early stage of the process. These detailed structures can be resolved only by pulse laser photography by using high-resolution film.
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.
A new concept of pulse laser ablation target were proposed. The idea is two-fold; that is, the target material to be ablated is a very thin layer deposited on a surface of some substrate material. As the substrate material, we will choose a transparent material like polymer or glass. Through the transparent material, a pulse laser beam is focused onto the thin film layer from rear side. In this way, we have tried to produce high energy particle beam without slow particles and droplets. Behavior of plume induced by the ablation of thin carbon film of 200 nm deposited on a polymethylmethacrylate (PMMA) substrate has been examined. Thin film target is found to achieve high energy density states by absorbing the pulse laser energy. An Nd:YAG laser of infrared wavelength is used in this study due to the efficient absorption of energy by the ablated plasma plume. Ablation emission was observed by the high-speed streak camera. Plume is shown to be induced at the carbon fllm surface and also at the opposite PMMA surface. Several velocity groups are recognized in the plume. Stress waves in PMMA plate placed close to the thin film target was observed by the pulse laser shadowgraphy.
New method is presented to observe the nanosecond pressure pulse field developed by the pulse laser energy deposition into the water. The present method is based on the pressure dependence of the refractive index of water, and the change in pressure can be visualized by the change in the reflectivity of the optical prism-water interface. Two procedures are tested to record the pulse laser induced high-pressure shock wave front in water, that is, (1) monitoring the time evolution of the laser intensity by a photomultiplier reflected from a point on the interface, and (2) taking an instantaneous photograph of the interface. They will give the pressure-time profile by the procedure, and to give the pressure distribution at that instant. Experimental results shows the feasibility of the method.
A new optical 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 liquids upon compression. This new shock sensor is suitable for the precise measurement of shock Hugoniot states for liquids and bio-related soft materials. Experiments were performed on water, NaCl aqueous solution and gelatine gel in the pressure range of less than 1 GPa. It is found that the method is very effective for the sensitive detection of shock front in these materials. By comparing the data for these materials, precision of the method was discussed.
In this study, unsteady propagation of high-pressure shock waves in PMMA and polyethylene specimens was first observed. Shock velocity decreases with the propagation distance for almost all cases, but the rate of decay process may depend on the material, shock strength and propagation distance. To observe the shock decay process, a high-speed photographic method was developed. The results obtained in this study may be closely related to the shape of the shock Hugoniot compression curve for these materials. Furthermore, stress relaxation occurring behind the wave front and its development with propagation are considered to have a close connection with the above results. Some of the data are presented and discussed compared with the Hugoniot and relaxation structures.
Optical breakdown of air is explored by using high-speed photography for the realization of laser propulsion system in aerospace engineering. The multiple ionization and subsequent pear-shaped emission by laser pulse through a convex lens are recorded and analyzed by image converter camera. The improved pulsed-laser shadowgraphy employed in this system successfully enables us to visualize the transient structures of complicated shock waves more clearly than ever. The ionization just above the surface of an aluminum target is also examined in comparison with the case of no target, which may be the major mechanism of Myrabo's demonstration of a small flyer launching by a pulsed carbon dioxide laser. Not only the high- enthalpy states of the ionized atmosphere are calculated but also the precise history of breakdown initiation in the nanosecond laser pulse is obtained.
Spectroscopic method was used to observe pulse laser induced ablation plume of graphite. Time-integrated emission spectrum of ablation plume of graphite was measured for laser wavelengths of 1064 nm, 532 nm and 308 nm. TOF measurements were also made to determine both the species of plume constituents and their velocity as they move from the target surface. By analyzing the obtained spectrum, several species were identified, i.e., at least a carbon ion, a carbon atom and a C2 molecule. Dynamic vibrational temperature was estimated for the obtained spectra of swan bands based on the statistical mechanics. The ablation mechanisms were discussed considering the non-equilibrium nature of the plume.
Hugoniot curves for several polymeric materials in the stress region of 0.5 GPa are measured by two methods developed for this purpose. High-speed photography or in-material gauge method is used combined with new prism technique based on the principle of total internal reflection. We measured the following materials, i.e., polymethylmethacrylate (PMMA), three kinds of polyethylene (PE specimens), polytetrafluoroethylene (PTFE), nylon 6 (N6), polycarbonate (PC), polypropylene (PP), polyvinylchloride (PVC). All of the Hugoniot curves are found to be nonlinear. In particular, three kinds of PE specimens are measured in detail, which have the different density, crystallinity, molecular weight distribution and manufacturing process. It is found that these PE specimens have different Hugoniot curves, but qualitatively similar.
Precision optical observation method was developed to study impact-generated high-pressure shock waves in condensed materials. The present method makes it possible to sensitively detect the shock waves of the relatively low shock stress around 0.5 GPa. The principle of the present method is based on the use of total internal reflection by triangular prisms placed on the free surface of a target assembly. When a plane shock wave arrives at the free surface, the light reflected from the prisms extinguishes instantaneously. The reason is that the total internal reflection changes to the reflection depending on micron roughness of the free surface after the shock arrival. The shock arrival at the bottom face of the prisms can be detected here by two kinds of methods, i.e., a photographic method and a gauge method. The photographic method is an inclined prism method of using a high-speed streak camera. The shock velocity and the shock tilt angle can be estimated accurately from an obtained streak photograph. While in the gauge method, an in-material PVDF stress gauge is combined with an optical prism-pin. The PVDF gauge records electrically the stress profile behind the shockwave front, and the Hugoniot data can be precisely measured by combining the prism pin with the PVDF gauge.
Pressure enhancement of the generated shock waves in water has been found, when pulse laser energy is transmitted through an optical fiber whose end surface is intentionally roughened. More effective high-pressure shock generation can be possible by the aluminum coating on the roughened fiber surface. In case of the moderate laser energy of about 50 mJ input to the fiber, it is found that the phenomena are dependent on (1) the roughness, (2) the fiber diameter, and (3) the ambient medium. Shock wave generation can be detected successively by the laser input, but found to degrade down. Cavitation bubbles have also been observed after each shot. When the fiber end is in air, an intense and long-stretched flash can be observed. We have observed the phenomena by the pulse laser shadowgraphy.
This paper describes a novel idea of image analysis method of pulse laser in-line hologram of particles to obtain information on particle diameter. Present method is based on the following property of the hologram. Fringe interval of the hologram decreases with radius, and the corresponding spatial frequency increases with radius. Fourier transformed pattern of the hologram, therefore, has the similar properties. We have proposed here the use of this FFT pattern of the hologram for the image analysis. It is shown that the FFT pattern has a white ring zone, which corresponds to the zero-point of the envelope function of original fringe pattern. It is demonstrated that measurement of the diameter of this ring gives a reliable way of obtaining particle diameter. Feasibility of the method has been shown by numerical experiment for Fraunhofer and Fresnel hologram.
For continuous recording of interior acceleration process of high-speed projectile in a gas gun, a new method using inline hologram of a slender wire is proposed and demonstrated. The hologram pattern is recorded by a streak camera via a mirror, which is moving between the wire and the recording plane. The mirror is glued onto the nose of an accelerating projectile. Fringe spacings of the pattern depend only on distance Z between the wire and the recording plane for a fixed wavelength of a laser light source. The acceleration process, therefore, can be obtained precisely by analyzing the fringe spacings which gradually narrow with decreasing the distance Z due to the projectile motion. It was found that (1) the acceleration time of the projectile is about 14.1 ms, (2) the acceleration is almost constant except in the early stage, and (3) two kinds of oscillatory behavior of the projectile during the acceleration are recorded simultaneously.
To visualize the 3D motion history of a particle, an analysis method has been developed for streak holography. The method is based on the fact that inline hologram of a particle has circular fringe pattern that can be calculated either Mie scattering theory, or Fraunhofer or Fresnel diffraction theory. Generalized formulation of the analysis method has been made under the assumption of circular fringe patterns followed by giving various fomulae for the case of Fraunhofer approximation. The image analysis can be further simplified at discrete times when one of the fringes is just tangent to the camera slit. This method is successfully applied to one of particle streak hologram to obtain 3D trajectory of it. The results of these examinations suggest the feasibility of streak holography for the visualization of particle field such as aerosol transport, combustion, multiphase flow, etc.
Air bubbles are observed at the moment of there emergence from a vertical nozzle in water. We can classify them into two types by formation process. The bubbles broken off from a large air bulk always emit sound pulses, but those not being split but keeping their initial volume hardly produce any sound. By comparing their motions and distortions not only by means of the sequential series of photographs but also by the streak photographs of the vertical linear portion of the axisymmetric bubble, we obtain the difference of the subtle distortion of their surface which causes the emission of sound pulse.
A new high speed recording procedure of holographic information is proposed, named `streak holography.' This kind of record is useful particularly for velocity and acceleration measurement, and for the observation of a moving object whose trajectory cannot be predicted in advance. A very high speed camera system has been designed and constructed for streak holography. A ring-shaped 100-mm-diam film is cut out from the high-resolution sheet film, mounted on a thin duralmin disk, which has been driven to rotate directly by an air-turbine spindle. The feasibility of the camera system has been examined by the experiments of relatively slow phenomena, like free fall of small glass spheres and the motion of them to the direction of the optical axis.
For the precise observation of high-speed impact phenomena, a compact high-speed streak camera recording system has been developed. The system consists of a high-pressure gas gun, a streak camera, and a long-pulse dye laser. The gas gun installed in our laboratory has a muzzle of 40 mm in diameter, and a launch tube of 2 m long. Projectile velocity is measured by the laser beam cut method. The gun is capable of accelerating a 27 g projectile up to 500 m/s, if helium gas is used as a driver. The system has been designed on the principal idea that the precise optical measurement methods developed in other areas of research can be applied to the gun study. The streak camera is 300 mm in diameter, with a rectangular rotating mirror which is driven by an air turbine spindle. The attainable streak velocity is 3 mm/microsecond(s) . The size of the camera is rather small aiming at the portability and economy. Therefore, the streak velocity is relatively slower than the fast cameras, but it is possible to use low-sensitivity but high-resolution film as a recording medium. We have also constructed a pulsed dye laser of 25 - 30 microsecond(s) in duration. The laser can be used as a light source of observation. The advantage for the use of the laser will be multi-fold, i.e., good directivity, almost single frequency, and so on. The feasibility of the system has been demonstrated by performing several experiments.
A new procedure is proposed of generating converging or of colliding shock waves in solids. The method is based on the refraction phenomena of a plane shock front at a shaped material interface, due to the difference in shock velocity of the materials of each side. A high-pressure gas gun is used to produce plane shock waves in a composite target assembly. The assembly is composed of aluminum and of polyethylene. The conically converging shock wave is generated in a lower-impedance material, i.e., polyethylene. In this case, polyethylene material is machined to the shape of a cone, and is inserted and glued to the aluminum plate having just the same inner surface. The processes of shock convergence are observed by a compact high-speed streak camera together with a pulsed dye laser as a light source. We have performed a series of experiments by varying several parameters. The realized converging angle of shock waves is found to be about 55 - 60 degrees in polyethylene medium. It is shown that the converging wave front looks almost continuously curved, and it is not easy to discriminate the boundary of the Mach stem. In other words, the growth of the Mach region seems faster than expected. This result is attributed to the combined effects of wave convergence and Mach reflection.
A new method for continuous recording of holographic information, "streak holography," is proposed. This kind of record can be useful for velocity and acceleration measurement as well as for observing a moving object whose trajectory cannot be predicted in advance. A very high speed camera system has been designed and constructed for streak holography. A ring-shaped 100-mm-diam film has been cut out from the high-resolution sheet film and mounted on a thin duralmin disk, which has been driven to rotate directly by an air-turbine spindle. Attainable streak velocity is 0.3 mm/μs. A direct film drive mechanism makes it possible to use a relay lens system of extremely small F number. The feasibility of the camera system has been demonstrated by observing several transient events, such as the forced oscillation of a wire and the free fall of small glass particles, using an argon-ion laser as a light source.
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