Optoacoustic analysis of the laser-cleaning process

Laser cleaning is an optodynamic process in which the optically induced removal of a liquid or a solid contaminant from a substrate is accompanied by a optoacoustic wave in the surrounding air. In our experiments we used both dry and steam laser-cleaning techniques for various samples. Optoacoustic wave, produced by the abrupt heating and detachment of the contaminants, was observed with a probe-beam deflection technique. We determined two characteristic parameters of the optoacoustic wave: the amplitude and the time-of-flight of the acoustic signal. With an analysis of these waves we also determined possible generating mechanisms. The decrease of the amplitude and the velocity of propagation, which approaches sonic speed, of the consecutive waves indicate that the dynamics during the laser-cleaning process is progressively weakened. The cleaning process is over when both the parameters reach a constant value, so with measuring optoacoustic waves the progress of the cleaning process could be observed on-line.     

Real-time optodynamic monitoring of pulsed laser decoating rate

This paper deals with a laser-cleaning process for removing a layer of paint. The effects of each individual laser pulse were monitored by a three-probe system. Ultrasonic signals from the rear surface of the substrate were obtained using an arm compensated Michelson interferometer, the optoacoustic waves in the surrounding air were detected using a laser beam deflection probe and the shape of the growing crater was measured using a laser anamorph profilometer. Two different cleaning lasers were used: an Excimer and a Nd:YAG laser. Typical parameters of the optodynamic signals versus the laser-pulse number are presented. A linear correlation between the quantity of ablated material and the integral of the ultrasonic signals was found together with an exponential relationship between the time-of-flight of the optoacoustic signals and the decoating rate.     

Generation of X-rays at bubble cavitation in a fast liquid jet in dielectric channels

The paper presents the results of studying the combined shock-wave radiation-emission processes associated with cavitation phenomena that occur at fast directional motion of a liquid jet into a closed working chamber through narrow dielectric channels. These processes induce high-power tunable X-ray radiation outside the chamber. At a relatively small liquid pressure, cavitation has been shown to generate shock waves in the chamber walls, which excites surface atoms and leads to the emission of X-rays from the outer surface of the chamber. At a high liquid pressure, the liquid jet does not touch the chamber walls and the cavitational shock waves lead to the excitation of the surface atoms of the jet itself accompanied by the generation of optical and X-ray radiation in the jet, which has been also observed in experiments.     

Generation and detection of shear acoustic waves in metal submicrometric films with ultrashort laser pulses

We present experimental and calculational results demonstrating the thermoelastic generation of shear acoustic waves using femtosecond laser pulses in submicrometric isotropic aluminum films. We show that the generation of the shear waves is correlated to the reduction of the width of the optoacoustic source on the surface. The presence of shear waves is related to acoustic diffraction and acoustic mode conversion at the thin film interfaces.     

Acoustic emission monitoring during laser shock cleaning of silicon wafers

A laser shock cleaning is a new dry cleaning methodology for the effective removal of submicron sized particles from solid surfaces. This technique uses a plasma shock wave produced by laser-induced air breakdown, which has applied to remove nano-scale silica particles from silicon wafer surfaces in this work. In order to characterize the laser shock cleaning process, acoustic waves generated during the shock process are measured in real time by a wide-band microphone and analyzed in the change of process parameters such as laser power density and gas species. It was found that the acoustic intensity is closely correlated with the shock wave intensity. From acoustic analysis, it is seen that acoustic intensity became stronger as incident laser power density increased. In addition, Ar gas has been found to be more effective to enhance the acoustic intensity, which allows higher cleaning performance compared with air or N₂ gas.     

Initiation and propagation of detonation waves in combustible high speed flows

The initiation and propagation of detonation waves in combustible high speed flows were studied experimentally. A planar detonation wave traveling in an initiation tube was transmitted into a test section where a combustible high speed flow was induced by an incident shock wave generated in a shock tube. In this study, the flow Mach numbers were obtained as 0.9 and 1.2. The experimental results show that depending on the flow velocity, the apparent propagation velocity of a detonation wave is higher in the upstream and lower in the downstream direction than the CJ velocity. Smoked plate records reveal cellular patterns deformed in the flow direction, and the calculated aspect ratios of the cell were found to agree well with the experimental ones on the basis of the assumption that the velocity of the transverse wave is not affected by the flowing mixture. By analyzing the shock-wave diffraction at the position where there is an abrupt change in the area, on the basis of Whitham’s theory, it was deduced that in the present experimental set-up, the detonation was initiated by the reflection of the diffracted shock waves on the sidewalls of the test section. The agreement between the experimental and calculated results regarding the position of the cellular patterns on the smoked plate record indicated that the position of detonation initiation in high speed flows is shifted downstream due to the flow velocity.     

Ion-acoustic shock waves with negative ions in presence of dust particulates

Dust acoustics shock waves have been investigated experimentally in a homogeneous unmagnetized dusty plasma device containing negative ions. When the negative ion density larger than a critical concentration ‘rc’ negative shock waves were observed instead of positive shock waves. Again when it is nearly equal to ‘rc’ both positive and negative shock waves propagate. The experimental findings are compared with modified KdV-Burgers equation. The velocity of the shock waves are also measured and compared with the numerical integration of modified KdV-Burgers equation.     

Shock-cavitational mechanism of X-ray generation during fast water stream cavitation

The process of generation of X-ray radiation formed during the free exit of a fast stream of water in a state of cavitation from a narrow channel was investigated both experimentally and theoretically. It was observed for the first time that the initial part of the stream and the outlet of the channel are the sources of intensive X-ray radiation (with an activity of 0.1 Cu), which is connected with the cavitation phenomena and the following excitation of shock waves. The frequency of the X-ray radiation depends on the composition of the radiating surface (water or the surface of the channel walls) and increases with the increase of the atomic number of the surface. The mechanism of radiation generation is connected with the influence of cavitation shock waves formed on the nearest interface (water-air, water-metal, metal-air, air-metal, etc.).     

激波聚焦反射的实验和数值研究

 采用高速摄影技术和数值模拟方法,对入射激波在两种不同形状的抛物形反射器表面聚焦和反射的过程进行了研究,得到了激波聚焦反射过程的波系结构的实验阴影照片和数值计算结果,两者符合得很好。对激波聚焦形成的气体动力学焦点的特性进行了分析,结果表明,入射激波在两种反射器反射后聚焦所形成的气体动力学焦点均是由三波点在轴心处的会聚所导致的,气体动力学焦点位于相应的反射器壁面的几何焦点附近。不同的反射器中,激波聚焦前后的波系结构也不同,对较浅的反射器,入射激波反射前在反射器壁面形成了弓形激波,反射之后需要相对较长的时间完成聚焦,形成聚焦反射激波之后弓形激波仍未相交;对较深的反射器,入射激波反射后在更短的时间内聚焦,聚焦时弓形激波已经相交,聚焦反射激波之后的流场波系结构更加复杂。     

Molecular dynamics simulation of plastic effects upon spalling

The molecular dynamics method is applied to simulate spalling during the plane shock interaction between plates. The effect of lattice defects in a material on the propagation of a shock wave and the process of spalling is studied. The plastic effects are described using a model of imperfect particle packing with defects (vacancies). The model proposed can describe the separation of the shock-wave front into an elastic precursor and a plastic front and give velocity profiles for the free target surface close to the experimental profiles.     

Dynamic compaction of copper powder: 2D numerical simulation and experimental results

Abstract

A method for plate-impact dynamic compaction of copper powder has been developped. The optimization of the experimental set-up (impedance adjustments, tensile wave traps, relative thickness of impactor and target,…) is presented.

2D axisymetrical numerical simulations have been performed with a Lagrangian finite element code. Geometrical characteristics of the experimental set-up as well as the dynamic response of the powder (Reaugh equation of state) and of the material of the set-up have been taken into account. These simulations show that, due to the difference in shock velocities in the container and in the powder, the powder is submitted to 2D loading waves. As a matter of fact the powder may be loaded by a non-planar shock wave propagating in the as-expected direction, as well as by a sweeping wave initiated at the bottom of the powder container, and propagating obliquely from the bottom-up. This second wave loads the bottom of the powder first. The influence of the impactor thickness as well as its material on the shock front shape and on the shock density-pressure history of the material has been studied. 1D simulations are shown not to evaluate properly the stress history and the energy deposition in the powder sample.

Metallographic observations as well as X-ray tomography experiments have been performed on consolidated samples. A very good agreement has been found between results of 2D numerical simulations and the observed final shape and density maps of the samples. The shape of deformed powder particles are also in agreement with the expected shock history.     

Intermediate regime of light diffraction by surface ultrasonic waves in planar gyrotropic optical waveguides

The intermediate regime of optoacoustic interaction, similar to the Bragg one, with Rayleigh surface ultrasonic waves in gyrotropic planar waveguides is studied. A system of equations of bound waves is presented that describes planar optoacoustic interaction in the intermediate, Raman-Natoh, and Bragg regimes of light diffraction by surface ultrasonic waves. It is shown that the optical activity of a wavequide film favors the incident light energy pumping to diffracted light for a TE-polarization wave and decelerates this process for an incident TM-polarization wave. Mozyr' State Pedagogical Institute, 28, Studencheskaya St., Mozyr', 247760, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 3, pp. 412–416, May–June, 1997.     

Observation of locked optical kink-antikink spatial shock waves

We report the first experimental observation of optical spatial shock-wave pairs. The shock waves consist of two coupled kink and antikink beams that remain locked to each other throughout propagation in a nonlinear diffusion-driven photorefractive crystal. These coupled shock-wave pairs move undistorted at angles that fall outside their original angular sector of propagation.     

Investigating shock-wave transient processes in explosives by means of synchrotron radiation

The results from studying the transient processes induced by a shock in porous TATB, obtained using an original and tested method based on employing the soft X-ray component of synchrotron radiation, are presented. The method enables us to determine the parameters of a shock-wave striker, the distribution of velocity and density behind the front of the shock and detonation wave, and the characteristics of flow after a shock wave is reflected from a rigid wall, all in one experiment. Trials with charges 1.8 and 1.9 g/cm³ in density show that modes such as the absence of detonation and initiation in direct and reflected shock waves, are possible depending on the loading conditions.     

Densification of Crystalline Boron Carbide during Shock-Wave Loading

Crystalline powder boron carbide samples are subjected to explosive loading by 2-μs shock waves with an amplitude of 38 GPa and shock heating to 700 K and to subsequent conservation. The samples recovered after shock-wave loading are studied by X-ray diffraction, and new effects of shock-wave loading on boron carbide are revealed. The explosive treatment is shown to shift the X-ray diffraction reflections of initial boron carbide toward high angles, which is attributed to an increase in the boron carbide density at the level of atomic volume in the unit cell of boron carbide. The X-ray diffraction reflections are found to broaden, which is interpreted as an increase in the coherent scattering region in the crystalline boron carbide subjected to the explosive treatment.     

Hugoniot curves for mixtures of aluminum and magnesium with oxides

The Hugoniot curves for mixtures of aluminum and magnesium with metal oxides and calcite were calculated in the absence and presence of chemical transformations behind the shock wave front. The calculation results were used to interpret the measured parameters of shock-wave loading and of the attenuation of short-duration shock waves.     

Formation of dispersive shock waves by merging and splitting Bose-Einstein condensates

The processes of merging and splitting dilute-gas Bose-Einstein condensates are studied in the nonadiabatic, high-density regime. Rich dynamics are found. Depending on the experimental parameters, uniform soliton trains containing more than ten solitons or the formation of a high-density bulge as well as dispersive shock waves are observed experimentally within merged BECs. Our numerical simulations indicate the formation of many vortex rings. In the case of splitting a BEC, the transition from sound-wave formation to dispersive shock-wave formation is studied by use of increasingly stronger splitting barriers. These experiments realize prototypical dispersive shock situations.     

Ultra-short pulse laser interaction with transparent dielectrics

The interaction of intense, ultra-short laser pulses (USLP) with a surface of transparent dielectrics is considered. The combination of multi-photon absorption and impact ionization generates a plasma layer at the dielectric boundary. Interaction with the plasma self-consistently determines the amount of reflected, transmitted and absorbed light, and the spatial distribution of electron density. In the present paper, we model the interaction of USLP with transparent dielectrics. We calculate the evolution of electron density profiles and the variation of reflection, transmission and absorption of laser radiation during the pulse. We show that the laser-created surface plasma acts as a filter transmitting only the leading edge of the laser pulse. The transmitted energy is approximately fixed, nearly independent of input pulse energy. The transmitted energy increases with pulse duration. This increased energy is manifested in the formation of cylindrical shock waves directly applicable to recent experiments investigating absorption and shock generation in water. PACS 79.20.Ds; 81.15.Fg; 05.45.Pg     

An optodynamic method for the real-time determination of the depth of a laser-drilled hole

An experimental method for determining the real-time depth of laser-drilled holes is presented. The proposed method involves detecting the laser-induced optoacoustic waves generated during the interaction of the laser beam with the material. Our optodynamic study involved measuring the propagation times of these waves as they traveled through the material and analyzing their temporal behavior during the drilling process. The experimental observations revealed an exponential relationship between the propagation time of the longitudinal stress wave and the number of consecutive laser pulses. Received: 25 October 2001 / Accepted: 27 October 2001 / Published online: 20 December 2001