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•Study of cavitation bubble and shock wave dynamics near a concave surface.•Shock wave interaction with the concave reflector and its scattering on a bubble.•Secondary cavitation ...induced by the refocused shock wave.•Visualization by shadow or schlieren photography with adaptive illumination system.•Theoretical treatment of shock wave propagation with geometrical acoustics.
The interplay among the cavitation structures and the shock waves following a nanosecond laser breakdown in water in the vicinity of a concave surface was visualized with high-speed shadowgraphy and schlieren cinematography. Unlike the generation of the main cavitation bubble near a flat or a convex surface, the concave surface refocuses the emitted shock waves and causes secondary cavitation near the acoustic focus which is most pronounced when triggered by the shock wave released during the first main bubble collapse. The shock wave propagation, reflection from the concave surface and its scattering on the dominant cavity is clearly resolvable on the shadowgraphs. The schlieren approach revealed the pressure build up in the last stage of the collapse and the first stage of the rebound. A persistent low-density watermark is left behind the first collapse. The observed effects are important wherever cavities collapse near indented surfaces, such as in cavitation peening, cavitation erosion and ophthalmology.
We found a fast hydrodynamic process in air double pulse Laser Induced Plasma (LIP) during the first 50 ns of plasma life. This process leads to strong inhomogeneity in the refractive index of air ...LIP, disturbance in the Gouy phase shift, and an increase in Third Harmonic Generation (THG). The efficiency of THG strongly depends on the confocal parameter of the focusing beam. The decrease in the focus of the pumping lens, despite the increase in the pump intensity, leads to a decrease in TH intensity. This can be attributed to the change from weak to tight focusing conditions in THG.
A microwave-assisted laser-induced breakdown spectroscopy (MA-LIBS) was developed to enhance the signal-to-noise ratio (SNR) measurements of alumina oxide plates. A semiconductor-based 2.45 GHz ...microwave device was coupled to a semiconductor 1064 nm laser source using a helical antenna, which replaced the larger-size capacitor-like antenna. This work was motivated by the applications of laser-induced breakdown spectroscopy (LIBS) in decommissioning of Fukushima Daiichi Nuclear Power Station (FDNPS). When using a 50-meter optical fiber from the main source into the nuclear vessel, the LIBS laser energy was expected to decrease significantly. Using the low laser energy of equal to or less than 2.0 mJ, the MA-LIBS significantly enhanced the excitation of the neutrals of Aluminum (Al) and the background signals contained no continuum emissions. Significant increase of the emission intensities of Al I with the addition of microwave energy was observed which also increased the full-width-half-maximum (FWHM). The correlation of the laser and microwave parameters and the SNR were then analyzed. Results show the logarithmic correlation of the microwave pulse width and microwave energy to the intensity enhancement and SNR which is attributed to the absorption of the microwave energy. This correlation is consistent with variations of laser energy density, laser pulse number, and laser spot diameter. Using a delayed acquisition time, the emission intensities of Al I were only observed with the addition of microwaves. At the experimental threshold of laser breakdown energy using 0.3 mJ, the emission intensity of Al I was only observed with the addition of microwaves. Attempts to duplicate the same SNR measurements using the stainless steel and lead targets were also done. The microwave successfully enhanced the intensity emission and SNR of the Al I, Cr I, and Pb I excited species.
•Microwave-assisted laser-induced breakdown spectroscopy was developed.•Microwave improves the resolution and signal-to-noise (SNR) ratio of Al I emission.•The laser energy density influenced the electron seed used in the microwave enhancements.•The microwave energy absorption by the laser-induced plasma influenced the sustained intensity enhancement of the enlarged plasma.
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Flame kernel formations of close dual-point laser induced sparks were investigated experimentally, focusing on the hydrodynamic effects induced by an interaction of shock waves produced by the laser ...induced sparks. Dual sparks were produced near the center of the combustion chamber by splitting of a ray emitted by a 532 nm Nd:YAG laser. Methane/air mixtures were ignited under a quiescent condition in a constant volume chamber with detailed measurements of the ignition energy and the pressure history. The minimum ignition energy was derived as an ignition energy having an ignitability of 50% using the logistic regression method. The flame kernel initiation process was also observed by Schlieren photography using a high-speed video camera. The offset of laser induced sparks were adjusted by tuning angles of mirrors and lenses. The ignition performance of single- and close dual-point laser breakdown induced sparks was investigated in detail in terms of the minimum ignition energy and the combustion induction time. Time resolved Schlieren photographs indicated that two hump shaped kernels grew rapidly during the initial stage in the vicinity of the plane of symmetry defined by the laser sparks under certain conditions. Their formation was due to the hydrodynamic effects induced by Mach shock waves, which resulted from interactions of the dual shock waves. The minimum ignition energy of the close dual-point laser induced sparks near the lean limit at 1.0 MPa was much lower than that of single-point laser induced sparks, although it was greater than that of the single ones at 0.1 MPa. The combustion induction time, which was defined as the time corresponding to the maximum pressure increase rate, was shortened for close dual-point laser induced sparks, especially for lean mixtures at high pressure. Robust flame kernels were formed by close dual-point laser induced sparks with Mach shock wave formation, and improved ignition performance for lean mixtures at high pressure was observed.
•Laser induced absorption phenomena in transparent materials are simulated.•A simulation tool to be used for accurate prediction of laser breakdown is proposed.•Time shapes of total electron density ...and photoionization generated electrons density are calculated.
The laser-induced damage in transparent optical materials represents an important active field of research as part of laser/material interactions studies. Most of research activities within this field are aiming to laser micro-processing of transparent optical materials, glasses and ceramics. An example of such laser micro-processing techniques is drilling micro channels through a glass plate and drilling transverse holes through single mode optical fibre cladding and core. The latter example of research activity has an important purpose consisting of designing and manufacturing micro-nanoscale optical fibre sensors with improved capabilities. Regarding these applicative research activities, there are two important correlated issues here to be underlined. The first one consists in the fact that high intensity laser field induced electron density variation into a transparent material is the main mechanism of breakdown and damage, that is, basis of micro-processing. The second one refers to the necessity of developing simulation procedures based on accurate theoretical models of these physical processes in order to use an accurate computer control of micro-processing technology. The main purpose of this work is to present the results of simulations in electron density variation induced by high intensity laser pulses in various transparent materials.
Ignition behaviors of close dual-point laser breakdown spark ignition were investigated experimentally for methane/air mixtures at 0.1 and 1.0MPa in a constant volume combustion vessel. Absorbed ...energy was measured from the difference between incident and transmitted laser rays using two joule meters as the ignition energy, and the behaviors of the initial flame kernel were observed with Schlieren photography using a high-speed video camera. First, the ignition behaviors of a single-point laser breakdown spark ignition were investigated. The results indicated that the effect of the focusing lens on the minimum ignition energy (MIE) was limited in terms of the absorbed energy. Although the MIE at 1.0MPa was lower than that at 0.1MPa near the stoichiometric equivalence relationship was reversed near the lean limit. For lean mixtures, local quenching of the initial flame kernel was clearly observed including third lobe region especially at 1.0MPa. In the case of the close dual-point sparks for an equivalence ratio of 0.6, formation of a third lobe was suppressed. When the dual spark gap was large, two flame kernels were formed as observed in the case of the single spark. An optimal gap in which the absorbed energy was minimal for successful ignition existed depending on the pressure, although the magnitude of the associated energy was not so different from that in the case of the single spark. However, the growth rate of the initial flame kernel formed by the close dual sparks was considerably higher than that formed by the single spark, especially at 1.0MPa. Enhancement of the flame kernel development due to the close dual spark was clearly observed.
A direct spectrum matching method for laser-induced breakdown spectroscopy is proposed to simultaneously measure gas density and concentration in turbulent reacting environments with improved ...measurement accuracy. The breakdown spectrum recorded in the target flow is directly matched with a spectrum out of a database consisting of various emission spectra recorded under well-defined conditions in a range of gas density and composition. It is shown that the wavelength, intensity and line width of the atom/ion emission lines in the spectrum indicate atom composition and gas density that are independent of parent molecular species in the target flow. Once a matching spectrum (within 550–830 nm containing O, H, N, and C lines) in the database of a known gas condition is found, the concentration and gas density at the location of the breakdown can be accurately derived. A 532-nm Nd:YAG laser with 10-Hz pulse repetition rate is used to induce breakdown in fuel/air mixtures in a variable pressure combustion chamber to build the spectrum database.In addition, it is used in a cavity flameholder of a model supersonic combustor to measure the gas density and concentration fields in a turbulent reacting environment. All the measurements are completed within 100 ns after laser firing, before breakdown affects the flow and the fast evolving environment alters the breakdown spectrum.
A simple laser-assisted method to transform metallic Zn to a ZnO-based random lasing medium is reported. The method uses Nd+3 laser-induced air breakdown to treat the surface of a Zn target and thus ...produce ZnO nanoparticles randomly scattered over the broken area. Photoluminescence spectra of some places of the area at room temperature under resonant powerful photoexcitation exhibit the emission band at 395nm with regularly spacing narrow spikes (<1nm), which is typical of multi-mode lasing. The band is suggested to be originated from electron-hole plasma arising at high excitation in ZnO-based structures. The micro lasers revealed are possibly composed of ZnO nanoparticles scattered in chains inside self-made random resonators, with Zn flinders being mirrors.
We have developed a new volumetric ignition method called Laser Breakdown-Assisted Long-distance Discharge Ignition (LBALDI). In LBALDI, laser breakdown plasma is utilized to assist spark discharge ...by facilitating formation of ionization channel. It is expected to improve lean ignitability limit by long distance discharge for extended initial flame kernel, which was proved by our previous research. The authors investigated the influence of laser incident angle on the probability of discharge by LBALDI. It was found that discharge was the most successful when the incident laser was directed perpendicular to the discharge path.
•Gas dynamic regimes of a dual-pulse (UV+NIR) laser ignition scheme are studied.•Four different flow patterns are observed during the plasma cooling phase.•Flow field in the post-discharge depends ...strongly on initial energy deposition profile.•Comparison with experiments shows the key role of dual-pulse plasma in controlling early flame kernel development.
A numerical study of the gas dynamics induced by a dual-pulse laser pre-ionization plasma used in laser ignition applications is presented herein. Past experimental observations have revealed important differences in the gas dynamics of the dual-pulse plasma, generated by overlapping a pair of ultraviolet (UV) and near-infrared (NIR) pulses, as compared to those of single-pulse (typically near-infrared) laser breakdown that has been more commonly studied. The simulation results reported here show that the dual-pulse pre-ionization scheme can lead to various gas dynamic regimes depending on the axial offset of the focal points of the two beams along the optical axis. If the UV and NIR pulses are perfectly overlapped (no offset) the energy deposition is uniform along the optical axis, leading to the formation of a toroidal structure in the post-discharge cooling phase. Alternatively, if the NIR energy addition pulse is focused with a small axial offset, (i.e., upstream or downstream of the UV pre-ionization pulse by ~0.5 mm) then an asymmetric torus forms that exhibits a third lobe which propagates away from the main kernel (to the side where the NIR was focused). Finally, if the two beams are focused with a larger offset of ~2.5 mm (weaker coupling), then another regime with a fourth lobe can arise. The four main flow regimes revealed from the model are in agreement with OH* experimental chemiluminescence images. These flow regimes influence the development of the early flame kernels, therefore playing an important role in practical laser ignition applications.
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