Laser-supported Combustion Wave Research Articles

Measurement and analysis of species distribution in laser-induced ablation plasma of an aluminum–magnesium alloy

We proposed a theoretical spatio-temporal imaging method, which was based on the thermal model of laser ablation and the two-dimensional axisymmetric multi-species hydrodynamics model. By using the intensity formula, the integral intensity of spectral lines could be calculated and the corresponding images of intensity distribution could be drawn. Through further image processing such as normalization, determination of minimum intensity, combination and color filtering, a relatively clear species distribution image in the plasma could be obtained. Using the above method, we simulated the plasma ablated from Al–Mg alloy by different laser energies under 1 atm argon, and obtained the theoretical spatio-temporal distributions of Mg I, Mg II, Al I, Al II and Ar I species, which are almost consistent with the experimental results by differential imaging. Compared with the experimental decay time constants, the consistency is higher at low laser energy, indicating that our theoretical model is more suitable for the plasma dominated by laser-supported combustion wave.

Species distribution in laser-induced plasma on the surface of binary miscible alloy

The spatiotemporal species distribution and dynamics of plasma play a crucially important role in improving the quality of the deposited film fabricated by pulsed laser deposition (PLD) and optimizing the spectral acquisition to enhance the quantitative performance of spatial-resolved laser-induced breakdown spectroscopy (LIBS). In this work, the monochrome imaging is used to analyze the plasmas induced from binary miscible alloys and explore the dependence of species distribution in plasma on laser-supported combustion (LSC) and laser-supported detonation (LSD) wave propagation regimes. This paper includes three conclusions: 1) the species distribution in LSC-wave dominated plasma induced from immiscible alloy is heavily influenced by the melting points of constituted elements, while in LSC-wave dominated plasma induced from miscible alloy, it is related to the boiling points; 2) whether the alloy is miscible or immiscible, the species distribution in LSD-wave dominated plasma is only related to the atomic masses; 3) to a great extent, the reduction rate of species in LSC-wave dominated plasma is associated with the transition probability, while that in LSD-wave dominated plasma is associated with the number density of species in the upper energy level.

Species distribution in laser-induced plasma on the surface of binary immiscible alloy

Investigation on species distribution of vapor plasma is of great significance for understanding the laser-target interaction, the plasma dynamics and verifying theoretical models. It is also helpful for optimizing the spectral acquisition to improve the quantitative performance of spatial-resolved laser-induced breakdown spectroscopy (LIBS). In this work, the laser-induced plasma on the surface of binary alloy is analyzed by using spectrally, spatially and temporally resolved dual-wavelength differential imaging to explore the dependence of species distribution in plasma on laser-supported absorption (LSA) wave regime and sample composition. The laser-supported combustion (LSC) and laser-supported detonation (LSD) wave dominated plasmas with different elemental mass ratios are induced from a series of binary immiscible alloys by using low and high irradiance laser pulses, respectively. By comparing the spatial-temporal emissivity images of species, the following conclusions are drawn: 1) laser irradiance can change the species distribution of plasma, while the sample composition cannot; 2) for LSC-wave dominated plasma, the species distribution mainly depends on the melting points of constituted elements in the sample, while for LSD-wave dominated plasma, it mainly depends on the atomic masses of elements. The above behaviours on species distribution in plasma produced from binary immiscible alloys are expected to be suitable for other elements or even multi-element plasmas.

Monocrystalline Silicon Plasma Expansion Induced by Millisecond Laser

Ablation of monocrystalline silicon to atmospheric environment induced by a millisecond pulse of an Nd:YAG optical laser with a wavelength of 1064 nm is studied. Shadowgraphy is applied to study the process of monocrystalline silicon plasma expansion for the laser energy density of 955.4–2736.0 J/cm2. It is shown that the outer boundary of the plasma plume diffuses outside with time. Plasma expansion occurs in both axial and radial directions, but the velocity of plasma expansion in the radial direction is smaller than in the axial direction. Two centerlines of the laser action are symmetric. The maximum expansion velocity of 162.1 m/s is reached with the laser energy density of 2376.0 J/cm2, and a laser-supported combustion wave is generated in this case. In contrast to monocrystalline silicon under the action of a short-pulse laser, the millisecond laser action can make the plasma expansion velocity increase for the second time. A material splash can be observed from the expansion images in the case of a high laser energy density.

Gas dynamics and combustion wave expanding velocity of laser induced plasma

Fused silica is an indispensable basic element in a laser system and the weakest link in all components. When the laser interacts with fused silica, the target absorbs the laser energy so that its own temperature rises, and then it melts and vaporizes. The vaporization of the target gasification further absorbs the laser energy and produces a low density ionization reaction, resulting in the laser supported combustion wave (LSCW) phenomenon. In this paper, taking into account the effects of temperature residual, change in target morphology, distribution of splash material, and distribution of target surface airflow condition, we model and simulate the process of LSCW in stages. The laser energy transfer process, including the inverse bremsstrahlung radiation, thermal radiation, heat conduction and convection processes, is simulated by establishing a two-dimensional axisymmetric gas dynamic model. In addition, the LSCW in the visible light band has a strong radiation characteristic, which is significantly different from the laser induced target melting and vaporization phenomenon. The LSCW is easily received and displayed by high-speed camera. Therefore, a shadow system is established to measure the expanding velocity of the combustion wave in the process of fused silica damaged by laser, and the evolution process image of the combustion wave is obtained. The results show that under the action of parallel laser beam, the propagation of the combustion wave is in a steady-state and the gas dynamic behavior is stable. For the pulse widths of 1 ms and 3 ms, the average propagation velocity of the LSCW is calculated to be about 24 m/s, which is consistent with the experimental result in the literature available. This verifies the correctness of our theoretical model. For the pulse width of 3 ms, the average velocity of the flow field near the wavefront is calculated to be about 200 m/s. The numerical relationship between the velocity of the flow field and the propagation velocity of the LSCW is also basically consistent with the theoretical derivation result. Under the action of focused laser beam, the propagation of the combustion wave is unsteady. For the pulse widths of 1 ms, the laser intensity at the front of the plasma decreases gradually and the beam radius becomes larger. For the pulse width of 1.8 ms, both a similar pattern of mushroom cloud in the combustion wave and turbulence are observed, which is basically consistent with the evolution process of the combustion wave appearing in our experiment. The simulation results are in good accordance with the experimental results, and also provide a theoretical and experimental basis for studying the LSCW of fused silica.

Millisecond laser machining of transparent materials assisted by a nanosecond laser with different delays.

A millisecond laser combined with a nanosecond laser was applied to machining transparent materials. The influences of delay between the two laser pulses on processing efficiencies and modified sizes were studied. In addition, a laser-supported combustion wave (LSCW) was captured during laser irradiation. An optimal delay corresponding to the highest processing efficiency was found for cone-shaped cavities. The modified size as well as the lifetime and intensity of the LSCW increased with the delay decreasing. Thermal cooperation effects of defects, overlapping effects of small modified sites, and thermal radiation from LSCW result in all the phenomena.

The Process of a Laser-Supported Combustion Wave Induced by Millisecond Pulsed Laser on Aluminum Alloy

We study the process of a laser-supported combustion wave (LSCW) when an aluminum alloy is irradiated by a millisecond pulse laser based on the method of laser shadowgraphy. Under the condition of different laser parameters, the obtained results include the velocity, ignition threshold of LSCW and the variation law. The speed of LSCW increases with the laser energy under the same irradiation laser pulse width, and the speed of LSCW reduces with the increase of the laser pulse width under the same irradiation laser energy. Moreover, the ignition time of LSCW becomes shorter by increasing the laser number of the pulse and is not effected by changing the frequencies, when keeping the laser pulse width and energy unchanged. The results of the study can be applied in the laser propulsion technology and metal surface laser heat treatment, etc.

Numerical simulation of laser-supported combustion wave induced by millisecond-pulsed laser on aluminum alloy

In this paper, the energy transmission of a laser-supported combustion wave (LSCW) is numerically studied, which includes inverse bremsstrahlung, thermal conduction and convection. A physical model is established to simulate an LSCW induced by millisecond-pulsed laser on aluminum alloy. This physical model is a 2D axis-symmetric numerical model of radiation gas dynamics. Moreover, the simulation focuses on the interaction process in different laser conditions such as various pulse widths and peak energies. As a result, the speed of the LSCW increases by increasing the laser energy while keeping the laser pulse constant, whereas the speed is reduced by increasing the laser pulse width while keeping the laser energy constant. After a comparison of the theoretical, numerical and experimental results, analyses are performed while the experimental results are explained reasonably. Furthermore, the consistency between the numerical and experimental results implies that the numerical calculation model used in this paper can describe the motion of the LSCW of the millisecond-pulsed laser on aluminum alloy very well.

Morphology and characteristics of laser-induced aluminum plasma in argon and in air: A comparative study

In laser-induced breakdown spectroscopy (LIBS), ablation takes place in general in an ambient gas of the atmospheric pressure, often in air but also in noble gas such as argon or helium. The use of noble gas is known to significantly improve the performance of the technique. We investigate in this work the morphology and the characteristics of induced plasma in argon and in air. The purpose is to understand the mechanism of the analytical performance improvement by the use of argon ambient with respective to air ambient and the dependence on the other experimental parameters such as the laser fluence. The observation of plasma morphology in different ambient gases provides also information for better design of the detection system which optimizes the signal collection according to the used ambient gases. More specifically, the expansion of the plasma induced on an aluminum target with nanosecond infrared (1064nm) laser pulse in two ambient gases, argon and the atmospheric air, has been studied with spectroscopic imaging at short delays and with emission spectroscopy at longer delays. With relatively low ablation laser fluence (65J/cm2), similar morphologies have been observed in argon and in air over the early stage of plasma expansion, while diagnostics at longer delay shows stronger emission, higher electron density and temperature for plasma induced in argon. With higher ablation laser fluence (160J/cm2) however, different expansion behaviors have been observed, with a stagnating aluminum vapor near the target surface in air while a propagating plume away from the target in argon. The craters left on the target surface show as well corresponding difference: in air, the crater is very shallow with a target surface chaotically affected by the laser pulse, indicating an effective re-deposition of the ablated material back to the crater; while in Ar a deeper crater is observed, indicating an efficient mass removal by laser ablation. At longer delays, a brighter, denser and hotter plasma is always observed in argon than in air as with lower ablation laser fluences. The observed different influences of the ambient gas on the plasma expansion behavior for different laser fluences are related to the different modes of laser-supported absorption waves, namely laser-supported combustion (LSC) wave and laser-supported detonation (LSD) wave.

Ultraviolet versus infrared: Effects of ablation laser wavelength on the expansion of laser-induced plasma into one-atmosphere argon gas

Laser-induced plasma from an aluminum target in one-atmosphere argon background has been investigated with ablation using nanosecond ultraviolet (UV: 355 nm) or infrared (IR: 1064 nm) laser pulses. Time- and space-resolved emission spectroscopy was used as a diagnostics tool to have access to the plasma parameters during its propagation into the background, such as optical emission intensity, electron density, and temperature. The specific feature of nanosecond laser ablation is that the pulse duration is significantly longer than the initiation time of the plasma. Laser-supported absorption wave due to post-ablation absorption of the laser radiation by the vapor plume and the shocked background gas plays a dominant role in the propagation and subsequently the behavior of the plasma. We demonstrate that the difference in absorption rate between UV and IR radiations leads to different propagation behaviors of the plasma produced with these radiations. The consequence is that higher electron density and temperature are observed for UV ablation. While for IR ablation, the plasma is found with lower electron density and temperature in a larger and more homogenous axial profile. The difference is also that for UV ablation, the background gas is principally evacuated by the expansion of the vapor plume as predicted by the standard piston model. While for IR ablation, the background gas is effectively mixed to the ejected vapor at least hundreds of nanoseconds after the initiation of the plasma. Our observations suggest a description by laser-supported combustion wave for the propagation of the plasma produced by UV laser, while that by laser-supported detonation wave for the propagation of the plasma produced by IR laser. Finally, practical consequences of specific expansion behavior for UV or IR ablation are discussed in terms of analytical performance promised by corresponding plasmas for application with laser-induced breakdown spectroscopy.

Numerical Analysis of Threshold between Laser-Supported Detonation and Combustion Wave Using Thermal Non-Equilibrium and Multi-Charged Ionization Model

Numerical Analysis of Threshold between Laser-Supported Detonation and Combustion Wave Using Thermal Non-Equilibrium and Multi-Charged Ionization Model

基于压电探测判定激光支持燃烧波和爆轰波的点燃阈值

基于压电探测判定激光支持燃烧波和爆轰波的点燃阈值

Near-critical phase explosion promoting breakdown plasma ignition during laser ablation of graphite

Removal rate, air shock, and ablative recoil pressure parameters were measured as a function of laser intensity I(peak) during nanosecond laser ablation of graphite. Surface vaporization of molten graphite at low intensities I(peak)<0.15 GW/cm(2) was observed to transform into its near-critical phase explosion (intense homogeneous boiling) at the threshold intensity I(PE)≈0.15 GW/cm(2) in the form of a drastic, correlated rise of removal rate, air shock, and ablative recoil pressure magnitudes. Just above this threshold (I(peak)≥0.25 GW/cm(2)), the explosive mass removal ended up with saturation of the removal rate, much slower increase of the air and recoil pressure magnitudes, and appearance of a visible surface plasma spark. In this regime, the measured far-field air shock pressure amplitude exhibits a sublinear dependence on laser intensity (∝I(peak)(4/9)), while the source plasma shock pressure demonstrates a sublinear trend (∝I(peak)(3/4)), both indicating the subcritical character of the plasma. Against expectations, in this regime the plasma recoil pressure increases versus I(peak) superlinearly (∝I(peak)(1.1)), rather than sublinearly (∝I(peak)(3/4)), with the mentioned difference related to the intensity-dependent initial spatial plasma dimensions within the laser waist on the graphite surface and to the plasma formation time during the heating laser pulse (overall, the pressure source effect). The strict coincidence of the phase explosion, providing high (kbar) hydrodynamic pressures of ablation products, and the ignition of ablative laser plasma in the carbon plume may indicate the ablative pressure-dependent character of the underlying optical breakdown at the high plume pressures, initiating the plasma formation. The experimental data evidence that the spatiotemporal extension of the plasma in the laser plume and ambient air during the heating laser pulse is supported by fast lateral electron and radiative heat conduction (laser-supported combustion wave regime), rather than by propagation of a strong shock wave (laser-supported detonation wave regime).

Laser-supported combustion wave in the field of gravity

Numerical simulation is used for studying the propagation of slow laser-supported combustion waves (LCW) in atmospheric air. The gasdynamic structure is investigated of the flow field which accompanies the movement of laser-supported combustion wave. A special feature of the problem being solved is the inclusion of gravity; this results in the emergence of convective motions in the thermal wake of LCW. It is demonstrated that different modes of thermal gravitational convection are observed depending on the conditions of LCW movement. A two-dimensional radiation-gasdynamic model is given, which includes the continuity and Navier-Stokes equations, the energy equation, and the equations of laser and thermal radiation transport in a multi-group approximation. The real thermal and spectral optical properties of air in the temperature range from 300 to 20 000 K are taken into account.

Laser–plume dynamics during excimer laser nitriding of iron

On the time scale of tens to hundreds of nanoseconds, high intensity pulsed excimer laser irradiation of iron in nitrogen atmosphere produces thin iron nitride layers with high nitrogen concentration. The laser plasma, or laser plume, which plays a crucial role in the complicated interactions within the laser–plasma–metal system, depends strongly on the ambient nitrogen gas pressure. Its influence was investigated in the nitrogen gas pressure range from 0.05 bar to 10 bar. The nitrogen depth profiles were measured via the nuclear resonance reaction N15(p,αγ)12C, while the phases formed in the surface layer were analyzed by conversion electron Mössbauer spectroscopy and x-ray diffraction. Utilizing sequentially N15-enriched and natural nitrogen atmospheres, the evolution of the nitrogen depth profiles during the laser nitriding process was traced. The experimental results suggest that the one-dimensional laser-supported combustion wave model reasonably describes the laser–plume dynamics and the nitriding effect.

Microscale Laser Shock Processing—Modeling, Testing, and Microstructure Characterization

This paper reports modeling improvements for microscale laser shock processing (LSP), fatigue testing of laser shock processed copper and nickel, and microstructure characterization of the processed materials. Plasma expansion is modeled as laser-supported combustion wave, and radial and axial expansions are considered. Stress/strain analysis is extended to three dimensions and takes into account finite geometry, which again is important for microscale LSP. Tests are designed to demonstrate that microscale LSP can improve fatigue performance of the materials while offering a level of flexibility. The influence of LSP on the microstructures of the materials is studied quantitatively using the orientation imaging microscope (OIM) technique, and grain size, texture, and subgrain structures are analyzed.

Continuous optical discharge stabilized by gas flow in weakly focused laser beam

For the first time, an old experiment is described where a continuous optical discharge is maintained in a large, weakly focused beam of a gasdynamic CO 2 -laser (P = 150 kW), the discharge being stabilized by nitrogen flow at a velocity of 10 m/s opposite to the plasma propagation. This experiment, in other words, produced a large, optical plasmotron. A detailed model, developed to simulate such phenomena numerically (including the laser supported combustion wave), is described. The most important factors, two-dimensional gas flow, heat conductivity, radiation heat exchange, and refraction of the laser radiation in the plasma, are taken into account. The numerical results for the aforementioned experiment are given, the temperature and velocity fields are plotted, and the gas energy balance is analyzed. The computed gas flow velocity (9 m/s) agrees well with the experimental value. For such a velocity, the numerical model shows that the plasma exists stably in a 150-kW beam at that section of the beam (a diameter of 3.2 cm) where the plasma was observed to stop in the experiment. On the basis of the developed theory (confirmed by comparison with the experiment and the numerical model), various other laser beam/plasma phenomena can be simulated ; for example, the operation of a laser-plasma rocket thruster.

A re-examination of the laser-supported combustion wave

Kantrowitz (1972) and Minovitch (1972) have proposed the use of laser sustained plasmas as a means to heat a rocket propellant. Recent studies of laser-powered propulsion have been directed toward the application of high-specific-impulse space propulsion systems for orbital transfer missions. Analyses of rocket performance relied heavily on the concept of the laser-supported combustion (LSC) wave. Raizer (1971) first drew the analogy between laser-sustained plasmas and combustion waves in an analysis. The Raizer model was later applied to hydrogen by Kemp and Root (1979). In connection with certain problems arising with the approach considered by Kemp and Root, the present investigation is concerned with a reexamination of the Raizer model. Attention is given to a numerical approach for the entire LSC wave in hydrogen, taking into account the incorporation of the proper boundary conditions far downstream of the wave.

Pulsed Laser-Generated Impulse on a Surface in Supersonic Flow

I is now well known that pulsed CO2 laser radiation can generate high pressures over a surface through formation of laser-supported detonation (LSD) waves at fluxes greater than 10 W/cm (Refs. 1-3). It is also possible for hot, dense plasmas to be produced in the strong laser-supported combustion (LSC) wave regime at fluxes between 10-10 W/cm (Refs. 4 and 5). The impulse generated at the surface by these phenomena are reasonably well understood. Previous analytical and experimental work has dealt with a static environment. The phenomenology should be different with a supersonic crossflow over the surface. This work examines the behavior of the laser/surface/supersonic flow interaction and presents impulse data from aluminum targets.

Plasma Energy Transfer to Metal Surfaces Irradiated by Pulsed Lasers

Recent experimental data have indicated that when highly reflective metal targets are irradiated by pulsed lasers in an air environment, a larger fraction of the incident laser energy is coupled into the target when a plasma is ignited above the surface than when plasma formation does not occur. Models of the plasmadynamics and subsequent energy transfer to the surface by radiation and conduction are constructed in order to predict the thermal coupling coefficient (coupled energy/laser energy) for pulsed laser irradiation of a metal target. It is found that the largest fraction of the incident laser energy is coupled into the target when a laser-supported combustion wave is ignited in close proximity to the surface. Energy transfer is via reradiation from the absorbing air plasma. Techniques for obtaining maximum thermal coupling are suggested as a result of the theoretical models. Results are compared with both computer simulations of the plasmadynamics and existing experimental data.