Characteristics Of Laser-induced Plasma Research Articles

Characteristics of laser-induced plasma generated in water and in air with different nanosecond laser pulse durations

The characteristics of laser-induced plasma generated in water and in air were compared with two laser pulse durations of 6 ns and 17 ns.

Acoustic characteristics of laser-induced plasmas from the forming dynamics perspective

The acoustic signal has demonstrated its capabilities in assisting laser-induced breakdown spectroscopy (LIBS) measurements. In this study, the acoustic characteristics of laser-induced plasmas (LIPs) under different levels of energy deposition were analyzed, and their correlation with LIP forming dynamics was investigated. In the deposited energy space, two zones in the acoustic pressure and duration were observed, featuring a clear transition point in 100 mJ. The analysis based on self-emission spectra and images suggested that this transition is a result of the change in plasma forming dynamics. Above 100mJ, the plasma temperature and electron density were saturated; thus, any further increase in deposited energy only contributes to the plasma size. In this regime, the acoustic wave from the significantly elongated plasma no longer satisfied the ideal spherical assumption. The observation was also strengthened by the analysis in the frequency domain. Moreover, the correlation between acoustic and radiation signals was also changed significantly with plasma forming dynamics. This study offers a systematic analysis of LIP acoustic signals on the deposited energy space. The potential of using acoustic measurement to interpret the plasma forming dynamics was demonstrated, which could be beneficial for the successful implementations of acoustic-aided LIBS.

Characterization of Magnetized-Plasma System Induced by Laser

This study investigated the effect of applying an external magnetic field on the characteristics of laser-induced plasma, such as its parameters plasma, magnetization properties, emission line intensities, and plasma coefficients, for plasma induced from zinc oxide: aluminum composite (ZO:AL) at an atomic ratio of 0.3 %. Plasma properties include magnetization and emission line intensities. The excitation was done by a pulsed laser of Nd:YAG with 400 mJ energy at atmospheric pressure. Both the electron temperature and number density were determined with the help of the Stark effect principle and the Boltzmann-Plot method. There was a rise in the amount of (ne) and (Te) that was produced by applying a magnetic field and, on the other hand, using the 532 nm wavelength rather than the fundamental wavelength of a laser. The emission lines in the atmosphere's plasma have an appearance of Lorentzian shape. The 532 nm laser exhibited a decrease in both the Larmor radius and the confinement factor compared with the 1064 nm laser. By applying the magnetic field, the Laser Induced Breakdown Spectroscopy (LIBS) intensities increased by 1.44 times when compared to the emissions before applying the field. In addition, the spectral line intensities improved with the fundamental wavelength compared to the second harmonic frequency as a result of the increase in the extracted materials. This is due to the increase in the absorbance of the laser by the target, as some of these materials are excited, so they act as emission sources, which makes them more detectable.

Dynamics of laser-induced plasma and cavitation bubble at high pressures and the impacts on underwater LIBS signals

Characteristics of laser-induced plasma depends strongly on its ambient pressure, and a deeper understanding of the high pressure impact on the underwater plasma emission is essential for the application of LIBS in the deep-sea. In this work, we investigated the temporal evolution of underwater plasma and cavitation bubble at the high pressures up to 50 MPa, by using fast imaging and shadowgraph techniques. It showed that as the ambient pressure increases, the plasma expansion is much suppressed with a stronger emission intensity and a slower emission decay, but it quenches much earlier and ends with a sharp drop at the higher pressures. From the shadowgraph imaging results, it showed that the increase in pressure has a great impact on the bubble evolution. Both the bubble maximum radius and the bubble lifetime decrease dramatically with the pressure as a consequence of faster dynamics. By comparing between the plasma and bubble results, we demonstrated the critical role of bubble dynamics on the characterization of plasma emission at high pressures. At relatively low pressures, the bubble expansion time is long enough to finish the plasma radiation. While at high pressures, strong bubble-plasma interaction occurs that will guide the plasma evolution behavior at the late stage. The plasma can gain energy from the confinement effect caused by the bubble shrinking process which leads to an increase in the emission intensity, but it will quench immediately after the bubble collapse. Such effects caused by bubble dynamics at high pressures explain well the evolution behaviors of underwater LIBS signals of OH, Ca II, Ca I, and CaOH, considering the different lifetimes of these emitting species in the plasma. This work reveals the complex impacts of high pressure on underwater plasma emissions and underwater LIBS signals.

Characteristics of laser-induced steel plasmas generated with different focusing conditions.

Laser focusing is an important parameter that affects the characteristics of laser-induced plasma. Focusing lenses with different F-numbers form different energy density distributions near the surface of a sample, thus affecting the characteristics of plasma. In this study, the plasma generated by a nanosecond laser ablation of a micro-alloy steel certified sample at 1atm of air was investigated. We compare the spectrally integrated plasma images obtained at different defocusing distances for short- and long-focus lenses and investigate the optical emission spectra of laser-induced plasma on steel alloy by using focusing lenses with different F-numbers. With an increase in the defocusing distance, the plasma plume changes from flat to hemispherical and then splitting occurs. The spectral line intensity increases first and then decreases, then increases slightly, and finally decreases gradually. For the long-focus lens, when the focal point is above the sample surface, the laser beam strongly interacts with air over a longer distance, leading to longer air plasma and weaker sample plasma compared with the short-focus lens. Thus, the relative intensity of the second peak in the spectral line intensity, according to the defocusing distance, gradually decreases with increasing F-number. We also obtain two-dimensional spatial distributions of the spectral line intensity according to the F-number and defocusing distance. The optimal defocusing distances for all focusing lenses occur when the focal point is below the sample surface. The relation between the optimal defocusing distance and F-number follows a single- exponential decay function.

Radial characteristics of laser-induced plasma under the influence of air pressure

Air pressure is one of the key factors affecting laser-induced breakdown spectroscopy (LIBS), and the mechanism of its influence on the spatial–temporal evolution of laser-induced plasma (LIP) is still not fully understood due to complex physical processes. In this study, the spatially and temporally resolved LIP’s spectra at different pressures were collected from the direction of laser incidence, and the radial distribution characteristics of LIP along the target surface under the influence of air pressure were studied. Furthermore, the spatial–temporal evolution of the radial distribution of the electron density n e and electron temperature T e was studied using Stark broadening and a Boltzmann plot. Finally, the radial distribution of LIP satisfying the McWhirter criterion and the influence of air pressure on its spatial–temporal evolution were studied. It was found that air pressure has a significant effect on the radial distribution of LIP. The spectral intensity, electron density n e, and the electron temperature T e of the LIP decrease faster against distance r from the LIP core and slower with the delay time T d in a higher air-pressure environment. Furthermore, the LIP will gradually fail to satisfy the McWhirter criterion with the increase in the radius r and delay time T d; in addition, the lifetime of LIP, which satisfies the McWhirter criterion, is longer at higher pressure. This study is helpful in clarifying the influence of air pressure on the spatial–temporal evolution of LIP, optimizing the experimental parameters of LIBS, and providing a reference for application of LIBS.

Plasma characteristics of a novel coaxial laser-plasma hybrid welding of Ti alloy

A novel coaxial laser-plasma hybrid welding method defined as “laser-soft plasma coaxial hybrid welding” was developed and studied. The plasma characteristics evolution, including plasma morphology, static electrical characteristic, and physical information of this welding method, was studied with the based material of TC4 alloy. Laser welding and plasma welding were carried out and used as comparative experiments. The plasma morphology of hybrid welding combines the characteristics of laser-induced plasma and arc plasma. The static electrical characteristic indicates that laser and plasma interact and influence each other but with different mechanisms under different current conditions. Physical information such as plasma electron temperature and ion density show that composite plasma can rapidly increase the heat source energy and ionization rate. To sum up, the coaxial plasma arc can increase the laser effect and redistribute the heat of the compound heat source, leading to a widened bead weld and better welding gap adaptability. This work provides an effective method and reference for hybrid welding process design and control in the practical industry.

Characteristics of Temporal and Spatial evolution of laser-induced plasma under different laser repetition rate

To explore the influence of the laser repetition rate on the characteristics of laser-induced plasma, the spatial and temporal evolution characteristics of the plasma generated from a copper alloy sample were compared when the laser repetition rate was varied from 1 to 20 Hz. The intensity and signal-to-back ratio (SBR) of atomic lines gradually increased with increasing laser repetition rate, reached the maximum at 10 Hz, and then decreased, whereas the intensity and SBR of ionic lines continually increased as the laser repetition rate increased. The morphology of the two-dimensional spatial distribution of the spectral line intensity changed from flat to elongate as the laser repetition rate increased. The plasma emission extended over a longer distance. The changes in the temporal and spatial evolution of the plasma temperature with the laser repetition rate were consistent with those of the ionic line intensity. The results indicate that a greater ablation amount of the sample material and a larger high-temperature region in the plasma were formed when the sample was ablated at a higher laser repetition rate. At this time, the heat accumulation in the sample and the confinement effect of the ablation crater on the plasma intensified the collision of particles inside the plasma, forming plasma with a higher degree of ionisation.

Time- and Position-Dependent Breakdown Volume Calculations to Explain Experimentally Observed Femtosecond Laser-Induced Plasma Properties

Focusing of femtosecond laser pulses in gases can produce different gas breakdown phenomena depending on the focusing conditions: from simple optical breakdown like laser “sparks” to a nonlinear optical breakdown like filamentation. The dynamics of such plasmas after the pulse exposure is dependent on the energy deposited by the laser in the breakdown volume. Using a time- and position-dependent breakdown model, we estimate the breakdown volume and show that the energy deposited by the laser in this breakdown volume determines the characteristics of laser-induced plasma in the post-pulse exposure regime. Experimentally we find that for different focal lengths there exists a threshold value of the energy density beyond which a transition from an ellipsoidal shape to a spherical shape can be observed, followed by a toroidal expansion of the produced plasma. When electron density and electron temperature are expressed as a function of the energy density, deviations from the parabolic dependence on irradiance are observed. They imply additional ionization by multiphoton ionization in the plasma volume that occurs when the peak power of the laser pulse is above the critical power for self-focusing in air. The relevance of this experimental and theoretical study is to prevent undesired self-focusing conditions during material processing, a step toward well-controlled laser-plasma etching without laser ablation.

Modeling temporal and spatial evolutions of laser-induced plasma characteristics by using machine learning algorithms

It is well-demonstrated that laser-induced plasma (LIP) is a transient phenomenon in which plasma characteristics vary with space and time. The study of the spatial and temporal distribution of the temperature and electron number density of LIP can elucidate the process of plasma expansion and plasma interaction. Therefore, establishing an experimental-based LIP model would open a new era in the quantitative explanation of the LIP and its applications. To this end, two popular machine learning algorithms, i.e., ANN, and ANFIS, are proposed in this study for modeling the cadmium plasma characteristics under the local thermodynamic equilibrium (LTE) conditions. The main aim of these models is to predict the plasma temperature and electron number density (representing the LIP characteristics) in terms of the influencing parameters. The accuracy and generalization capability of the developed models are evaluated by measuring performance functions for the training and testing data. The results show that both machine learning algorithms can successfully cope with the complexity of LIP variations over time and space. Moreover, the well-trained models are used to investigate the temporal and spatial evolution of LIP by calculating the temperature and electron number density in ranges of the influencing parameters.

Spectral characteristics of laser-induced plasma generated on porous silicon produced by metal-assisted etching

Porous silicon (Si) produced by metal-assisted etching using nanoparticles (Si nanohole array with straight pores) has potential as a sample loading substrate of laser-induced breakdown spectroscopy (LIBS). In this work, we investigated the characteristics of laser-induced plasma generated on the porous Si with different pore depths (pore depth: approximately 180, 320, and 990 nm) from spectroscopic aspects. The intensity of Si I line was enhanced by using the porous Si instead of a flat Si, it increased with increasing the pore depth of porous Si, and its enhancement factor reached 45 (pore depth: approximately 990 nm). The atomic excitation temperature and atomic number density increased with increasing the pore depth, whereas the intensity ratio of Si II line to Si I line and the intensity of nitrogen I line decreased. These results provide an insight into the plasma formation on nanomaterials.

Contrasting time-resolved characteristics of laser-induced plasma spatially confined by conical cavities with different bottom diameters

Contrasting time-resolved characteristics of laser-induced plasma spatially confined by conical cavities with different bottom diameters

Morphology of Meteorite Surfaces Ablated by High-Power Lasers: Review and Applications

Under controlled laboratory conditions, lasers represent a source of energy with well-defined parameters suitable for mimicking phenomena such as ablation, disintegration, and plasma formation processes that take place during the hypervelocity atmospheric entry of meteoroids. Furthermore, lasers have also been proposed for employment in future space exploration and planetary defense in a wide range of potential applications. This highlights the importance of an experimental investigation of lasers’ interaction with real samples of interplanetary matter: meteorite specimens. We summarize the results of numerous meteorite laser ablation experiments performed by several laser sources—a femtosecond Ti:Sapphire laser, the multislab ceramic Yb:YAG Bivoj laser, and the iodine laser known as PALS (Prague Asterix Laser System). The differences in the ablation spots’ morphology and their dependence on the laser parameters are examined via optical microscopy, scanning electron microscopy, and profilometry in the context of the meteorite properties and the physical characteristics of laser-induced plasma.

Determining the spatial distribution of laser-induced plasma by laser-induced voltaic measurement

Distribution characteristics of laser-induced plasma (LIP) plays a significant role in the mechanism studies and applications. Owing to various technologies and limitations, the quantitative analysis of plasma channel distribution has not been fully expounded. Here in this work, laser-induced voltage (LIV) measurement was employed to assess the distribution of LIP channels. An exponential relationship was observed between the energy and the voltage, indicating that the laser energy has a controllable influence on the plasma channel. The distribution of plasma channels is captured by moving the electrode position in two directions. LIV is provided as an innovative method to describe the distribution of plasma channels, which is a promising means for characterizing the formation, diffusion and expansion of plasma.

Spatiotemporal and emission characteristics of laser-induced plasmas from aluminum-zirconium composite powders

Laser-induced plasmas and reactions therein depend on the ablated target's chemical composition and morphology. Recently, we have characterized plasma properties such as spatiotemporal morphology, electron density, species temperature within the plasma, ionization degree, etc. for pure Al under variations in morphology. Here, we utilize a bi-metal powder system to study similar characteristics as a function of powder chemistry. Micron-sized ball milled Al/Zr composites of three chemistries (3Al:2Zr, Al:Zr, and Al:3Zr) are compared to similarly-sized pure Al, pure Zr, and a mixture of Al + Zr powder. We find that the introduction of increasing concentrations of Zr has several interesting effects on the plasma compared to pure Al, including a dramatic increase in the electron density (to ~3–5 × 1020 cm−3), and the earlier onset of ZrO emission bands which have a higher disassociation energy and are thermodynamically preferred over AlO at high temperatures. Differences in plume morphology, the spatial distribution of molecular species, and the emission intensities of ZrO, Zr I and Al I were observed for Al + Zr and ball-milled Al:Zr samples, indicating the influence of microstructure on the chemical reactions. We measured plasma temperatures using the Saha-Boltzmann plot method; the composites demonstrate higher temperatures than pure Zr, Al + Zr mixtures, and pure Al. We observe only minor differences in temperature as a function of increasing Zr-content within the composites. This work continues to build on our understanding of the relationships between plasma properties and microsecond-timescale chemical reactions of reactive materials.

Spatiotemporal evolution of laser-induced plasmas in air: Influence of pressure

In this paper, we focused on the influence of gas pressure on the dynamic characteristics of laser-induced plasma (LIP) in air. The energy-dependent transmittances of a 1064 nm laser to LIPs generated at the pressure of 0.2–5 atm were measured. Laser Rayleigh scattering and Thomson scattering methods were applied to investigate the evolution of laser-produced shock wave and radial distribution of electron number density (ne) and electron temperature (Te) of LIP in air. The fraction of the shock wave energy to the total laser energy absorbed is about 40%–52% and increases upon increasing pressure. The higher the gas pressure, the greater ne and Te within the central of LIP and a faster decaying rate of Te and ne. The electron number density decays exponentially with time, and the decay index is between −0.891 and −1.177. A torus structure in LIPs is generated during the later stage of plasma decay and significantly affects the distribution of Te, ne and the intensity of plasma emission.

Temperature measurement with compositional correction of gas mixture based on laser-induced plasma

Laser-induced breakdown plasma was used for gas temperature measurement based on the residual energy of a laser pulse defined as the laser energy detected in the beam path as well as plasma emissions. Gas mixtures with different compositions (O2/N2, O2/CO2, and O2/N2/CO2) were used to simulate the main components of combustion products at different temperatures. First, the correlation of residual energy and gas temperature was investigated, which showed that the residual energy increased with an increase in the gas temperature. The results showed that it also relates to the gas composition, which would affect the characteristics of laser-induced plasma. Then the spectral emission ratio of the plasma (O/N, O/C, and C/N) was obtained simultaneously to correct the compositional effect on the temperature measurement. Finally, the gas temperature with different components can be obtained by the equation coupled with the gas temperature, residual energy, and gas composition. The corrected temperature is consistent with that obtained by thermocouple and the gas temperature measurement error is less than 3.5% in the range from 309 K to 548 K.

Two-lines method for estimation of plasma temperature and characterization of plasma parameters in optically thick plasma conditions.

In this paper, the characteristics of laser-induced plasmas are studied by investigation of the spectral line features in laser-induced breakdown spectroscopy (LIBS) experiments. The plasma is produced by focusing of a Nd:YAG laser on standard Al-alloy samples at 30 mJ energy. Here, with the assumption of having a homogenous plasma and by using a semiemperical technical method, the plasma temperature is calculated by the proposal of a new two-lines method. Moreover, by utilizing some theoretical equations, the plasma parameters and the self-absorption magnitude are evaluated according to the radiative transfer equations in local thermodynamic equilibrium (LTE) conditions. The main advantages of this method are that without discrimination between thin plasmas and thick ones, and as well as without straight quantification of the degree of self-absorption, the plasma temperature can be calculated. The results showed that determination of the intensities of the spectral lines, transition parameters, and Stark broadening parameter is adequate for plasma characterization in a typical LIBS experiment.

Gas dynamics and vorticity generation in laser-induced breakdown of air.

Research has shown that the ignition characteristics of laser-induced plasmas in fuel-air mixtures are influenced by the gas dynamics effects induced during the gas breakdown stage. Here, we present the numerical modeling of the fluid mechanics induced by breakdown (plasma formation) from a nanosecond near-infrared (NIR) laser pulse in air. The simulations focus on the post-discharge kernel dynamics with the goal of developing a better understanding of how vorticity is generated during the kernel cooling phase. Initial conditions (ICs) of kernel shape, temperature, and pressure (corresponding to the end of the laser pulse) are found from experimental Rayleigh scattering data. It is shown that this method for determining ICs is preferred versus the use of the Taylor-Sedov blast wave theory as it provides a more accurate description of the starting field. Past experimental observations have revealed that the gas dynamics of nanosecond laser sparks typically lead to the formation of an asymmetric torus with a frontal lobe propagating towards the laser source. We show that the development of the asymmetric torus is governed by strong vorticity generated through baroclinic torque arising from the blast wave that forms at the kernel boundary. Initially, the blast takes the shape of the teardrop kernel but then evolves into a spherical front during the first ∼10 µs because the blast wave strength varies along its circumference. This spatial variation leads to a misalignment between the pressure and density gradients and generation of vorticity by baroclinic torque. Ultimately, the observed flow-field is dictated by how the energy was initially deposited around the beam waist during breakdown. As such, one can tailor the aerodynamics induced during the cooling and recombination phase by controlling the energy deposition profile.

Effect of sample morphology on the spectral and spatiotemporal characteristics of laser-induced plasmas from aluminum

Species stratification and local plasma composition can affect microsecond-timescale oxidation reaction rates of metals such as Al in an oxidizing atmosphere. Here, we utilize fast, gated emission spectroscopy and a high-speed framing camera to determine the intensity and spatiotemporal evolution of various Al ablation products within a laser-induced plasma. Using a high-purity Al plate, micron- and nano-sized Al powders, and inert micron Al2O3 powder, we studied the effect of Al morphology and reactivity on the oxidation characteristics and plasma hydrodynamics in air at 1 atm over two temporal regimes (2–10 μs and 20–100 μs). We observed an increase in the spatial distribution and intensity of emission from vaporized Al within the plasma for powder-based samples compared to plate Al due to enhanced material dispersion. In nano-Al, AlO emission forms at, and propagates along, the surface of the powder bed from 2–10 μs, whereas for micron powder, there is a delay in AlO formation within the bulk of the plasma until tens of microseconds. We measured electron densities from a variety of spectral lines, which can range from ~ 2 × 1015 to 2 × 1018 cm−3, and which scale inversely with the rate of plasma expansion across morphologies. The Al I and Al II species temperatures from 2 to 10 μs calculated via Boltzmann plots are similar (from ~ 10,000 to 14,000 K), and we performed a suite of local thermodynamic equilibrium (LTE) validity calculations to establish that these two species are in LTE, while H is not. Using image co-registration, we calculated the thickness of the AlO layer surrounding the expanding Al cloud at times > 20 μs, which can range from ~ 50 to 300 μm. These results allow us to begin to understand the complexities of laser ablated metal powder reactions at microsecond timescales.