Laser-induced Plasma Plume Research Articles

NO PLIF flow visualization and time-resolved temperature distributions in laser induced breakdown plumes

NO planar laser induced fluorescence (PLIF) is used to obtain images of laser-induced breakdown plasma plumes in NO-seeded nitrogen and dry air at near atmospheric pressure. Single-shot PLIF-images show that the plume development 5–50 μs after the breakdown pulse is fairly reproducible shot-to-shot, although the plume becomes increasingly stochastic on longer timescales, 100–500 μs. The stochastic behavior of the plume is quantified using probability distributions of the loci of the plume boundary. Analysis of the single-shot images indicates that the mixing of the plume with ambient gas on sub-ms time scale is insignificant. The induced flow velocity in the plume is fairly low, up to 30 m s−1, suggesting that laser breakdowns are ineffective for mixing enhancement in high speed flows. The ensemble-averaged PLIF images indicate the evolution of the plume from an initially elongated shape to near-spherical to toroidal shape, with a subsequent radial expansion and formation of an axial jet in the center. Temperature distributions in the plume in air are obtained from the NO PLIF images, using two rotational transitions in the NO(X, v′ = 0 → A, v″ = 0) band, J″ = 6.5 and 12.5 of the QR12 + Q2 branch. The results indicate that the temperature in the plume remains high, above 1000 K, for approximately 100 μs, after which it decays gradually, to below 500 K at 500 μs. The residual NO fraction in the plume is ∼0.1%, indicating that repetitive laser-assisted ignition may result in significant NO-generation. These measured temperature and velocity distributions can be used for detailed validation of kinetic models of laser-induced breakdown and assessment of their predictive capability.

Enhancement characteristics of laser-induced plasma confined by hemispherical cavities in different materials

A hemispherical cavity can be used to improve the sensitivity of laser-induced breakdown spectroscopy by increasing the emission spectra of the plasma. In this work, laser-induced plasma plume images were obtained with and without confinement. The results have confirmed that the reason for the signal enhancement caused by the hemispherical cavity was the reflected shockwave “compressing” effect. The dependence of spectral signal enhancement on the material property of the cavities was investigated. Four cavities in different materials, which include K-resin, aluminum alloy, photosensitive resin and nylon, were used to confine the plasma. It has shown that the material physical properties of the cavities influence the enhancement effect. We found that the main influencing factor was surface roughness of the material. Smoother cavity surfaces induced stronger enhancement effects due to the larger specular reflection component of the reflected shockwave and the more intense subsequent interaction between the plasma and shockwave. The spectral intensities of the characteristic lines approximately doubled with surface roughness values changing from 0.2 to 16.1.

Dynamics of laser-induced plasma splitting

The dynamics of splitting of the laser-induced plasma plume is investigated by spatiotemporal plasma imaging and spectroscopy. Plasma is induced on a copper target in a broad pressure range (10−2–105 Pa) and at varying laser pulse energies (5–40 mJ). Splitting the plume into fast and slow components is observed. The analysis of spatially resolved plasma images and spectra shows that the brightest emission region moves from the slow component to the fast component as time evolves. Copper atoms and ions, which radiate between 315 nm and 335 nm and between 485 nm and 504 nm, belong to the fast component whereas copper atoms, which radiate between 315 nm and 335 nm and between 505 nm and 525 nm, belong to the slow component. A charge separation field is proposed to explain the separation of atoms and ions into the slow and fast components, correspondingly. The results can be of importance for laser induced breakdown spectroscopy, pulsed laser deposition, and nanoscale syntheses.

Characteristics of laser-induced aluminum plasma plumes after increasing sample temperature and spatial confinement

This paper exhibits the characteristics of laser-induced aluminum plasma plumes after increasing sample temperature and spatial confinement.

A study on magnetic field assisted laser percussion drilling and its effect on surface integrity

Purpose: of this paper is to reduce the taper angle and surface roughness of the laser drilled hole on Aluminium alloy with the assistance of magnetic field. At lower laser powers, able to achieve higher material removal rate in drilling with reduced taper angle and roughness. Design/methodology/approach: Aluminium alloy is a highly reflective material, while laser drilling it ejects plumes, which makes the drilling unreliable. The plume generated due to this action causes deteriorating effects over the work piece as such affecting surface textures. Removal of plume is the major consideration in laser machining process, especially in laser assisted drilling. The plume is a form of cluster of ions having charges in it. Due to the magnetic field input, the ions line the path along the lines of force of magnets. Thus, the ion cloud can be cleared at the localized plane, where the subsequent laser drilling going to be happens, leads to reduced plume thereby reduces the taper angle and surface roughness. Findings: The defect of percussion laser drilling that is barrelling effect in the drilled hole was reduced with the assistance of magnetic field setup. For the laser energy of 90 mJ, the magnetic assisted laser drilling shows better improvement in the material removal rate of 64.5%, the profile error (spatter height) was reduced to 45% and the taper angle of the drilled hole also reduced by 16.3%. The results confirmed the fact that, the Lorentz force confined the plume particle to be raised upwards and circulated outwards to the sidewall from the centre of the laser beam. This expansion of laser induced plasma plume, improved the material removal rate of the hole. Research limitations/implications: Laser drilling was carried out by a constant magnetic field and the parameters like material removal rate, taper angle, profile error, surface roughness were studied. In the future work, these parameters were studied with the application of varying magnetic field. Practical implications: As a result of the work, laser drilling was carried out on turbine blades or complex shapes for retention properties, with reduced taper hole and surface roughness, thereby improving the efficiency of the systems. Originality/value: The novelty of the work is providing magnetic flux for the laser drilling process, which improves the process parameters. The incorporation of magnetic field to the laser drill needs a cost less setup, which can ensure reliable improvement in the material removal rate, reduction in taper angle and profile error.

Electrical Characteristics of Plasma Plume During Pulse Wave Laser Welding

To detect the real-time plasma plume during pulse wave Nd:YAG laser welding, experimental studies were conducted based on asynchronous signal acquisition system. The electrical signals of the laser-induced plasma plume were obtained by a passive acquisition system. The plume was directly observed and recorded using synchronous high-speed camera. The results showed that the waveform of the signals was in accordance with the periodical laser power. The signals decreased after the laser was turned on and fluctuated relatively steadily on the stable stage and then increased to 0 V after the laser beam was turned off. The decreasing time of the electrical signals was roughly 1.0 ms, and it decreased with the increasing peak power. However, the average power had insignificant effect on the signal decreasing time.

Influence of the distance between target surface and focal point on the expansion dynamics of a laser-induced silicon plasma with spatial confinement

Expansion dynamics of a laser-induced plasma plume, with spatial confinement, for various distances between the target surface and focal point were studied by the fast photography technique. A silicon wafer was ablated to induce the plasma with a Nd:YAG laser in an atmospheric environment. The expansion dynamics of the plasma plume depended on the distance between the target surface and focal point. In addition, spatially confined time-resolved images showed the different structures of the plasma plumes at different distances between the target surface and focal point. By analyzing the plume images, the optimal distance for emission enhancement was found to be approximately 6mm away from the geometrical focus using a 10cm focal length lens. This optimized distance resulted in the strongest compression ratio of the plasma plume by the reflected shock wave. Furthermore, the duration of the interaction between the reflected shock wave and the plasma plume was also prolonged.

Vision and spectroscopic sensing for joint tracing in narrow gap laser butt welding

The automated laser beam butt welding process is sensitive to positioning the laser beam with respect to the joint because a small offset may result in detrimental lack of sidewall fusion. This problem is even more pronounced in case of narrow gap butt welding, where most of the commercial automatic joint tracing systems fail to detect the exact position and size of the gap. In this work, a dual vision and spectroscopic sensing approach is proposed to trace narrow gap butt joints during laser welding. The system consists of a camera with suitable illumination and matched optical filters and a fast miniature spectrometer. An image processing algorithm of the camera recordings has been developed in order to estimate the laser spot position relative to the joint position. The spectral emissions from the laser induced plasma plume have been acquired by the spectrometer, and based on the measurements of the intensities of selected lines of the spectrum, the electron temperature signal has been calculated and correlated to variations of process conditions. The individual performances of these two systems have been experimentally investigated and evaluated offline by data from several welding experiments, where artificial abrupt as well as gradual deviations of the laser beam out of the joint were produced. Results indicate that a combination of the information provided by the vision and spectroscopic systems is beneficial for development of a hybrid sensing system for joint tracing.

Experimental investigation of atomic and ionic titanium lines, diatomic TiO γ transition and continuum background radiation via magnetically confined LIBS

Abstract This paper is focused on the clarification of the behaviour of various atomic, ionic, and TiOγ (A3Φ – X3Δ, Δν =0) diatomic transition lines as well as the continuum radiations in laser-induced titanium plasma plume by a pair of NdFeB. Depending on their different amount of impressionability from applied fields and electron-impact excitation rate, the responses of each component in optical emission spectrum and background continuum radiation to different applied magnetic field strengths (~ 0.42–1.12 T) are significantly different. At magnetic fields of 0.98 and 1.12 T, the amount of impressionability for Ti i: 388.2 nm is higher than others, while for TiOγ molecular transitions, the amount of impressionability factor in a 0.98 T field strength is considerable, because in the first moments after the laser pulse the continuum radiation is dominant, by applying an external magnetic field, this amount of radiation be reduced. The temporal evolution of the optical emission lines shows that choosing a suitable time delay and gate width has a considerable role in further impressionability of externally applied field in case of different elements. The electron density Ne varied from 0.99×1017 to 11.53×1017 cm−3, and the plasma temperature Te, was obtained in a range of 4835–16818 K by employing maximum applied field of 1.12 T. The supplementary investigations will be conducted towards further improvements of the purposeful analysis of each component in confined plasma by magneto-LIBS technique in order to find new approaches in the field of magnetic confinement.

Pulsed laser deposited [formula omitted] thin films: Influence of target to substrate distance

The influence of target to substrate distance (dts) on the structural, morphological and photoluminescence (PL) properties of commercially obtained KY3F10:Ho3+ phosphor thin films prepared by pulsed laser deposition is investigated for dts values in the range of 4–7 cm. Analysis of XRD indicates that high crystalline quality film with largest grain size is obtained for target to substrate distance of 4 cm. Decrease in the thickness of the films is observed at larger target to substrate distances. This is attributed to the increased hemispherical expansion of the laser induced plasma plume at larger distances reducing the particle flux of the target species over a substrate area. Main PL emission at 540 nm due to the 5F4−5I8 and 5S2−5I8 transitions of Ho3+ is investigated using four excitation wavelengths namely: 362, 416, 454 and 486 nm. The highest PL intensity occurs at excitation of 454 nm for all target to substrate distances considered. All the films are characterized by low reflectance and high absorption in the visible region.

Pressure and temperature dependence of the laser-induced plasma plume dynamics

The influence of different background gases and substrate heating on the plasma plume dynamics from silver ablation is investigated by species selected time and space resolved imaging. The results provide a time-resolved understanding on how those process parameters affect the expansion: from a free expansion in vacuum with velocities exceeding 20 000 m/s to a very slow expansion in Ar at 1 × 10−1 mbar with arrival velocities of 280 m/s. In addition, we observe a rebound of the ablated material on the substrate holder leading to a re-coating of the ablated target. At 1 × 10−1 mbar, it seems that the expansion of the plasma plume displaces a considerable portion of the background gas and traps it against the frontal area of the substrate holder. This leads to a transient high local pressure just above the substrate. In the case of Ar, the rebound is enhanced due to inelastic scattering, whereas for an O2 background, an area of high reactivity/emission in addition to the rebound is created. Imaging of selected species shows that the light emission in this area is mainly due to excited Ag and metal oxygen species. There is a clear influence of substrate heating on the plasma expansion due to the background gas density gradients, reducing the stopping ability of the background gas and already detectable 2 cm away from the substrate. Both rebound and excitation effects are reduced in intensity due to the substrate heating.

Study on the Two-Dimensional Density Distribution for Gas Plasmas Driven by Laser Pulse**supported by National Natural Science Foundation of China (Nos. 11264036, 11465016 and 11364038)

We perform an experimental study of two-dimensional (2D) electron density profiles of the laser-induced plasma plumes in air by ordinarily laboratorial interferometry. The electron density distributions measured show a feature of hollow core. To illustrate the feature, we present a theoretical investigation by using dynamics analysis. In the simulation, the propagation of laser pulse with the evolution of electron density is utilized to evaluate ionization of air target for the plasma-formation stage. In the plasma-expansion stage, a simple adiabatic fluid dynamics is used to calculate the evolution of plasma outward expansion. The simulations show good agreements with experimental results, and demonstrate an effective way of determining 2D density profiles of the laser-induced plasma plume in gas.

Space-dependent characterization of laser-induced plasma plume during fiber laser welding

The role of a plasma plume in high power fiber laser welding is of considerable interest due to its influence on the energy transfer mechanism. In this study, the space-dependent plasma characteristics including spectrum intensity, plasma temperature and electron density were investigated using optical emission spectroscopy technique. The plasma temperature was calculated using the Boltzmann plot of atomic iron lines, whereas the electron density was determined from the Stark broadening of the Fe I line at 381.584 nm. Quantitative analysis of plasma characteristics with respect to the laser radiation was performed. The results show that the plasma radiation increases as the laser power increases during the partial penetration mode, and then decreases sharply after the initiation of full penetration. Both the plasma temperature and electron density increase with the increase of laser power until they reach steady state values after full penetration. Moreover, the hottest core of the plasma shifts toward the surface of the workpiece as the penetration depth increases, whereas the electron density is more evenly distributed above the surface of the workpiece. The results also indicate that the absorption and scattering of nanoparticles in the plasma plume is the main mechanism for laser power attenuation.

Ablation enhancement by femtosecond laser irradiation assisted with a microtorch for microgrooves fabrication in PMMA

This study proposes an ablation enhancement approach to fabricate microgrooves in PMMA by femtosecond laser irradiation assisted with a microtorch. The influences of pulse energy and scanning speed on the groove depth and removal area of groove are investigated. It is demonstrated that the improvement of groove depth has a close relationship with the scanning speed. When the scanning speed was less than 50 µm/s, the ablated groove depth is considerably improved with various pulse energies, up to 100 %. Moreover, the removal area of groove has significant enhancements of up to 250 % in various processing parameters. It is suggested that the ablation enhancement of microgrooves fabrication is related to the status of plasma plume and substrate heating. With the assistance of the microtorch, laser-induced plasma plume is confined and its density at center region is raised, which results in the increment of the central plasma’s temperature and more energy deposited on the PMMA surface, ultimately leading to the ablation enhancement. Meanwhile, the instantaneous substrate heating also plays a crucial role on enhanced microgrooves fabrication.

Micro torch assisted nanostructures' formation of nickel during femtosecond laser surface interactions

In this letter, we perform a comprehensive study of micro torch effect on the formation of femtosecond laser-induced nanostructures on nickel. Under identical experimental conditions, laser induced nanostructures and periodic surface patterns exhibit distinctly different level of morphology with and without the micro torch. In addition, assisted by the micro torch, the ablation threshold is considerably reduced and the content of oxygen in the textured nanostructures keeps a stable low level. It is suggested that the change on the surface directly relates to the status of plasma plume and substrate heating. With the assistance of the micro torch, laser induced plasma plume is confined and its density at center region is raised, which results in the increase of the central plasma's temperature, more energy deposited on the nickel surface, and ultimately leading to the changes in the nanostructures' morphology and ablation threshold.

Stimulated emission and lasing in laser-induced plasma plume

The lasing effect is demonstrated in laser plasmas induced on various metal targets and pumped by a laser tuned in resonance with a strong optical transition of a metal. The intense, polarized and low-divergent radiation is emitted from a longitudinally pumped plasma plume in forward and backward directions with respect to the pump beam. Lasing occurs only within duration of the pumping pulse. The effect is found for elements of 13th and 14th groups and for Ca, Ti, Zr, Fe and Ni. The Einstein coefficients for spontaneous emission of all lasing transitions are higher than 107s−1. For some elements like Al and In, a three-level lasing scheme is realized. For others, like Tl, both three- and four-level lasing schemes are realized. It is found that the longitudinal pump geometry is more efficient than the transversal one.

Non-epitaxial pulsed laser deposition of Ag2Se thermoelectric thin films for near-room temperature applications

Non-epitaxial growth of Ag2Se thermoelectric thin films on (La, Sr) (Al, Ta)O3 or glass substrates at a relatively low temperature of 100°C has been achieved by using pulsed laser deposition (PLD). The deposition kinetics has been optimized by varying the background interactions with laser induced plasma plume. The shockwave plasma expansion is to be critical for the structure, composition and morphologies of thin films. As-grown Ag2Se thin films under the optimized deposition condition exhibit comparable transport performances as compared to the reported Ag2Se bulk materials at similar carrier concentrations. The non-epitaxial correlation between the Ag2Se thin films and their substrates is expected to benefit for the further fabrication of in plan thermoelectric micro-devices for room temperature applications.

High power laser welding of thick steel plates in a horizontal butt joint configuration

In this investigation, two laser-based welding techniques, autogenous laser welding (ALW) and laser welding assisted with a cold wire (LWACW), were applied to join thick plates of a structural steel (A36) in a horizontal narrow gap butt joint configuration. The main practical parameters including welding method and laser power were varied to get the sound weld with a requirement to achieve a full penetration with the reinforcement at the back side of weld in just one pass. The weld-bead shape, cross-section and mechanical properties were evaluated by profilometer, micro-hardness test and optical microscope. In order to investigate the stability of laser-induced plasma plume, the emitted optical spectra was detected and analyzed by the spectroscopy analysis. It was found that at the laser power of 7kW a fully penetrated weld with a convex back side of weld could be obtained by the LWACW. The microstructural examinations showed that for the ALW the acicular ferrite and for the LWACW the pearlite were formed in the heat affected zone (HAZ). The prediction of microstructure based on continuous cooling transformation (CCT) diagram and cooling curves obtained by thermocouple measurement were in good agreement with each other. According to the plasma ionization values obtained from the spectroscopy analysis the plume for both processes was recognized as dominated weakly ionized plasma including the main vaporized elemental composition. At the optimum welding condition (LWACW at the laser power of 7kW) the fluctuation of the electron temperature was reduced. The spectroscopy analysis demonstrated that at the higher laser power more of the elemental compositions such as Mn and Fe were evaporated.

Physical insights of cavity confinement enhancing effect in laser-induced breakdown spectroscopy.

Using cavity confinement to enhance the plasma emission has been proved to be an effective way in LIBS technique while no direct visual evidence has been made to illustrate the physical mechanism of this enhancing effect. In this work, both laser-induced plasma plume images and shockwave images were obtained and synchronized for both flat surface case and rectangular cavity case. Phenomena of shockwave reflection, plasma compression by the reflected shockwave and merge of the reflected shockwave into plasma were observed. Plasma emission intensities recorded by ICCD in both cases were compared and the enhancement effect in the cavity case was identified in the comparison. The enhancement effect could be explained as reflected shockwave "compressing" effect, that is, the reflected shockwave would compress the plasma and result in a more condensed plasma core area with higher plasma temperature. Reflected shockwave also possibly contributed to plasma core position stabilization, which indicated the potential of better plasma signal reproducibility for the cavity case. Both plasma emission enhancement and plasma core position stabilization only exist within a certain temporal window, which indicates that the delay time of spectra acquisition is essential while using cavity confinement as a way to improve LIBS performance.

Characterization of SnO2thin films grown by pulsed laser deposition under transverse magnetic field

ABSTRACT : SnO 2 thin films were deposited on fused silica substrate by pulsed laser deposition under transverse magnetic field. We have explored the effects of magnetic field and ablation laser wavelength on the optical properties of laser-induced plasma plume and structural characteristics of the deposited SnO 2 films. Optical emission from the plume was monitored using an optical fiber to examine the influence of magnetic field on the population of the excited neutral and ionic species and their decay with times after laser ablation. Also, we employed photoluminescence, x-ray diffraction, and UV-Vis absorption to characterize SnO 2 films. SnO 2 is n-type semiconducting material with a wide bandgap of 3.6 eV at room temperature. Since SnO 2 has high transparency in visible region and electric conductivity, it has diverse applications in gas sensors, solar cells, transparent conducting electrodes, and optoelectronic devices