Recent advancement of underwater laser-induced breakdown spectroscopy is reviewed, especially emphasizing on our recent studies on laser irradiation procedure, mechanism of the plasma and cavitation bubble formation, the effects of hydrostatic pressure, and the application to deep sea.

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Laser spectroscopy has wide potential to realize online, on-site, rapid, in-situ, remote, and direct analyses in various industrial fields. The demand for such advanced analytical technologies has been fueled year by year due to growing interest in environmental protection and quality control in manufacturing lines. The great progress of such optical equipment as lasers, spectrographs, and photo-detectors as well as their price-reduction is also promoting the research and the development of laser diagnostics. Compared with other laser spectroscopic technologies, laser induced breakdown spectroscopy (LIBS) is simple, inexpensive, and flexible. LIBS is a promising application to monitor various major and trace elements in vapor and/or aerosol particles, although its accuracy and sensitivity are slightly lower than its applications to solid samples. This report reviews recent works on LIBS for combustion diagnostics and the analyses of related substances.

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The measurement of such trace substances as chlorine in concrete is important in evaluateing the durability of reinforced concrete structures because reinforcing bars in concrete are eroded by chloride ions. Laser-induced breakdown spectroscopy (LIBS) is attractive for the fast on-site measurement of trace elements. We measured a chlorine concentration of 0.18 kg/m3 in concrete. Thus, a chlorine concentration of 0.6 kg/m3, at which the reinforcing bars in concrete structures start to corrode, can be detected. We also measured the two-dimensional distribution of the emission intensity of multi-elements on the surface of a concrete specimen's cross-section. These results show that LIBS is an attractive tool for quick, high spatial resolution, and high sensitivity on-site measurement of chlorine concentration in concrete structures. Here we review the LIBS applications for the diagnostics of concrete structures.

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Laser-induced breakdown spectroscopy (LIBS) is an attractive technique for determining elemental composition in real time, in-situ, and remotely without any sample preparation. The LIBS analysis of metal ions in an aqueous solution is available in process control and environmental monitoring. In the present paper, we reviewed LIBS methods for a liquid phase to improve detection sensitivity. We performed LIBS measurements with the sheet flow for the simultaneous determination of elements in simulated high-level radioactive waste liquid and discussed the application potential as a tool for online process monitoring.

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This report explained the local equivalence ratio (air fuel ratio) measurements of engine exhaust gas and direct fuel injection in engine cylinder using laser-induced breakdown spectroscopy (LIBS) and sparkinduced breakdown spectroscopy (SIBS). When we use LIBS/SIBS in combustion system, we can discuss simultaneous laser/spark ignition and equivalence ratio measurement of premixed mixture. The intensity ratio of H/O or H/N showed a good agreement with preset equivalence ratio in engine exhaust gas and premixed mixture in engine cylinder. The influence of spark discharge characteristics on emission spectra was investigated by simultaneous measurement of spark discharge visualization and spectroscopy. The relationship between air-fuel ratio and the emission intensity ratio was obtained in premixed gas combustion in a spark ignition engine. There was a tendency that local air-fuel ratio decreases with advance of the injection timing from the fixed ignition timing. It was observed that spark discharge was affected by the flow due to injection. LIBS/SIBS is very good tool to discuss the combustion characteristics in real engine researches.

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The measurement of coal and fly ash contents is the essential foundation for the adjustment of boiler combustion in thermal power plant. Rapid detection of coal and fly ash is significant to improve the efficiency of thermal power plant and to reduce the environmental pollutions. The pollutions of trace heavy metals in exhaust result in the serious influence on environment and human. First step of the pollution control is the fast and precise measurement of pollutants. Laser-induced breakdown spectroscopy (LIBS) has a great potential application of on-line measurement in thermal power plant. The direct measurement of particles using LIBS was studied. The LIBS method was also improved to measure the gas phase trace species, as well as the combined method of LIBS and time-of-flight mass spectrometry (TOFMS). These results will be useful for the applications of LIBS and TOFMS.

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For the rapid and precise sorting of steel scrap, laser-induced breakdown spectroscopy (LIBS) is a promising method. It has several advantages such that it can work under ambient air atmospheres, and specimens can be tested without any pretreatment, such as acid digestion, polishing of the surface of the specimens, etc. For the application of LIBS for actual steel scrap, we obtained emission spectra by an LIBS system, which was mainly comprised of an Nd:YAG laser, an Echelle-type spectrometer, and an ICCD detector. Considering spectral interferences from the emission lines of the iron matrix in the alloys, Cu I lines having wavelengths of 324.754 and 327.396 nm could be chosen. In five replicate measurements of each SRM, shorter delay times after laser irradiation and longer gate widths for detecting the transient emission signal are suggested to be the optimal experiment parameters.

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We applied laser-induced breakdown spectroscopy (LIBS) analysis to the molten alloy production process, in which simulated metals (Zr, Cu, Sm, Ce) are used instead of nuclear metallic fuels containing minor actinide (MA). Our aim is in-situ monitoring of the elementary composition of a molten alloy’s surface in a chamber and the vapor particles generated from that surface. We successfully observed the variation in the ratio of the elementary composition of the molten alloy’s surface in the crucible by depending on the crucible’s temperature. The elementary composition of the vapor particles in the molten alloy chamber was also measured. Practical experimental results show that the LIBS technique is very useful for investigating elementary composition in the molten alloy production process and understanding molten alloy behavior in crucibles.

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For the remote analysis of a next generation nuclear fuel material containing minor actinide (MA), we applied laser-induced breakdown spectroscopy (LIBS) to uranium oxide (U3O8), which included a small amount of neodymium oxide (Nd2O3), as a simulated sample of MA. By using a deconvolution technique for the Nd spectra in U, we separated the complex, overlapped, and confused spectra and determined their actual intensities. As a result, a calibration curve with good linearity and a detection limit of less than 700 ppm were demonstrated.

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Material development is becoming critical to enhance the thermal efficiency of steam turbines. However, such conventional methods as tension testing machines and strain gauge usage are inappropriate to evaluate material for steam turbine blades because they cannot simultaneously estimate the stress state under the operating condition of turbine blades. To visualize the surface of turbine blades in real time, we developed a high speed shooting system that is composed of a rotary encoder, a digital delay pulse generator, a personal computer, and a timing controller, which was a specially designed unit. With our developed system, we successfully obtained still pictures under conditions where the rotational speed and the circumference temperature were 3,000 rpm and 650 degrees, respectively.

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Ultraviolet absorption spectroscopy using a gas cell based on hollow optical fiber is proposed for breath analysis. Compared to conventional infrared gas spectroscopy systems, ultraviolet-based systems are simpler and lower in the cost maintaining high sensitivity. In addition, the hollow-fiber based gas cell enables measurement with a long optical path and extremely small sample volume. Firstly the length of the hollow-fiber gas cell is optimized for analysis of low concentration gas components. Secondly a laser-driven ultraviolet light source having much higher brightness than conventional deuterium light sources is newly utilized in the system. By using the optimized hollow fiber gas cell and the new light source, it is shown that nitric oxide gas with a concentration of a few hundred ppb is successfully measured.

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