Plasma Shielding Effect Research Articles

The ablation behavior of diamond/Cu composites by infrared nanosecond pulsed laser

This paper investigates the material removal behavior of diamond/Cu composites by multi-passes infrared nanosecond pulsed laser ablation, focusing on the formed surface cracks, periodic ripples, profile evolution and deposition. Combining nanosecond laser ablation experiments with simulations based on thermophysical properties is then undertaken to effectively reduce the maximum thermal stress and inhibit crack initiation and propagation. Three types of cracks were identified in nanosecond laser ablation, which were influenced by the diverse thermophysical properties of the material and the temperature gradient. The periodic ripples of 300–1800 nm on the sidewalls of the machined groove on diamond grains were induced when the laser fluence approached the ablation threshold, which did not even appear on the copper matrix as the thermal effects dominate. The groove profile showed a limited change as the ablation passes further increase to be over 40 due to the plasma shielding effect inside the groove, the laser defocusing phenomenon and the increased difficulty of sputtering the molten material. In addition, the different ablation mechanisms of diamond and copper resulted in the formation of overlapping deposition layers with changed surface characteristics at varying laser energy densities. The uneven island-like recast layer affected the uniformity of the ablated grooves, which was dependent on the material microstructure and accumulated laser energy.

The ablation behavior and modification mechanism of SiC under different laser energy

The laser modification pretreatment method is an effective way to address the machining difficulties of silicon carbide (SiC), a typical hard-to-machine material. Therefore, the ablation behavior and modification mechanism of SiC under different laser energy were explored in this paper. A new multi-scale model of laser irradiation SiC that couples heat transfer and fluid motion is first established. Then, a series of experiments is carried out to evaluate the model's accuracy, and the interaction between laser and SiC is discussed in detail. The results show that the dominant modification mechanism changes from coulomb explosion to multiphoton absorption, incubation effect, and heat accumulation with the laser energy increase. This leads the surface topography of SiC to transition from nanoparticle formation to disorder to a melting state. In ablative state, micro/nano porous and humps are formed at the edge of ablation groove due to surface tension, generation and rupture of bubbles, respectively. Furthermore, the surface roughness is not proportional to the laser energy due to the plasma shielding effect, and the surface roughness can be reduced by enhancing the flow of the molten material. Amorphous Si–O–C, Si and spherical SiO2 exist in deposition area, leading to SiC elastic modulus decreases from 347 GPa to 103.82 GPa, and the shear strength decreases from 20.9 GPa to 17.25 GPa. The results of this study can provide references for parameters selection and theoretical support for improving the machinability of SiC.

Lagrangian perspective on the expansion dynamics and shielding effect of femtosecond laser-induced copper plasma plumes

Femtosecond laser ablation of metals generates a strongly ionized plasma plume near the irradiated surface. The resulting plasma shielding effect can reduce subsequent laser energy deposition and lower nanomachining efficiency, especially during multi-pulse irradiation. Understanding the spatiotemporal evolution of the laser-induced plasma and its associated shielding effect is, therefore, crucial. A hybrid two-temperature and direct simulation Monte Carlo (TTM-DSMC) computational model is developed in this study, which synergistically couples the ultrafast laser–metal interaction physics and the plasma collisional transport. The model simulates the plasma properties including electron density, temperature dynamics, reflectivity, and energy attenuation throughout the plume expansion process from femtosecond to nanosecond timescales. A complex “penguin-shaped” plasma plume with internal shockwaves is observed due to the effects of double-pulse irradiation. Significantly enhanced plasma reflectivity and reduced laser energy deposition demonstrate the accumulated shielding effect, which increases with higher plasma density accumulation when the pulse separation is insufficient. Our model provides valuable theoretical guidance for optimizing processing parameters to enhance efficiency and precision in femtosecond laser machining. The integrated TTM-DSMC approach could also facilitate the study of laser-induced plasmas in other contexts like material characterization and nanoparticle synthesis.

Development of laser-ablated aluminum plasma plume during the irradiation of laser pulse

In this paper, we theoretically simulate the dynamic evolution of plasma from the interaction between a nanosecond pulsed laser and aluminum targets. The self-luminous image of the plasma plume during the laser loading process was experimentally obtained using high-speed photography. Theoretical and experimental results demonstrated that the plasma shielding effect and plasma lateral expansion have a significant effect on the plasma plume. It will affect laser–target coupling and further affect the evolution of plasma plumes. The detonation wave contained in the early stage of the laser-induced plasma plume directly affects the subsequent shock wave.

Influence of ambient pressure and laser energy on the plasma dynamics and parameters of laser-irradiated polyvinyl chloride

The dependence of propulsion performance generated by laser ablation of polyvinyl chloride on laser energy and pressure is investigated using Q-switched Nd: YAG laser with the wavelength of 1064 nm. When the pressure is decreased, the impulse and coupling coefficient rise first and then decline. Such a trend is also reflected in the variation of coupling coefficient with laser energy in the whole pressure range. However, the change in impulse with laser energy at atmospheric pressure is not completely consistent with that at low pressure levels. The dynamic behavior and duration of plasma plume are considered to be the factors for the difference in propulsion performance. By capturing the fast exposure images of plume, the separation at atmospheric pressure and severe expansion accompanied by rapid quenching in near vacuum are observed. Moreover, the plasma plume lasts longer time at high pressures. It is ascribed to the higher electron temperature, which promotes background gas to excite and ionize. Since the electron density increases with the improvement of laser energy and pressure, the absorption of electrons to laser energy becomes stronger through the inverse bremsstrahlung mechanism. Accordingly, the shielding effect of plasma is enhanced, causing the weak laser-target interaction. The result is that the crater depth and ablative mass increase with decreasing pressure. This work is important for understanding the energy conversion mechanism and optimizing the laser propulsion performance.

Tensile strength and crack tip stress distribution of modified glass in the composite laser beam separation

The tensile strength of the modified cross-section (MCS) determines to a certain extent whether the composite laser can complete the separation of glass, and the quality and efficiency of the separation. In this work, the effect of the picosecond Bessel laser on the tensile strength of soda-lime glass was systematically investigated for the first time. The relationship between the tensile strength and residual stress was not negatively correlated absolutely. The tensile strength was also related to the homogeneity and micro-structure of MCS, including micro cavities, cracks, and voids. Meanwhile, it was pointed out that by selecting appropriate modified parameters, the plasma shielding effect can be suppressed and the micro-morphology of MCS can be controlled to obtain smaller tensile strength and better cutting quality. Secondly, the stress field distribution at the crack tip was simulated to analyze the mechanism when the continuous wave (CW) laser interacted with the modified glass. The simulation results showed that only the stress perpendicular to the laser incidence direction and the scanning direction exceeded the tensile strength of glass material, ensuring that the cracks propagated only along the MCS. Finally, one-time separation of glass was achieved by using a composite laser beam separation (CLBS) technology with a speed of 50 mm/s, which was comparable to the industrial level, and the roughness of separated sidewall was less than 0.5 μm.

The effect of different laser head angles and shielding gas supply systems to maximize the depth of penetration by minimizing the plasma shielding effect in fiber laser welding of 3-mm thick NiTinol sheet

Bead-on-plate laser welding of 3 mm thick NiTinol sheet was tried with the help of 2.3 kW Yb:Fiber laser. It was found that the laser power was insufficient to get the full penetration with narrow weld-bead by varying the scan speed when shielding gas was provided from the side of the bead and the laser head was kept perpendicular to the plate. It was observed that when gas was supplied from the top of the plate and moving with the laser head, fully penetrated bead was obtained at much lesser value of heat input (P/v) because of the reduction of length of plasma plume. It was also detected that with increase in rotation of laser head angle the depth of penetration and laser bead width decreases because of the increase in laser spot area, reduction of laser intensity and increase in length of plasma plume. The increase in length of plasma plume increased the plasma shielding effect and reduced the absorptivity of the laser in the NiTinol sheet. According to the study, the maximum depth of penetration at a constant laser heat input was obtained for minimum value of plasma plume length and maximum value of laser intensity.

Research on Reflection Mechanism of Nonuniform Plasma

The surface of the near-space hypersonic vehicle is covered with a plasma sheath (PSh) with fluid properties. During the radar detection of hypersonic vehicles, the spatial distribution characteristics of PSh parameters affect the reflected wave intensity and the position of the maximum reflection layer of plasma in the vehicle’s different areas, resulting in significant differences in the intensity and frequency offset of echo signals in various areas. In order to further study the reflection characteristics of nonuniform plasma, this article uses the equivalent wave impedance method to calculate the reflection and transmission coefficients of each layer of plasma, analyzes the reflection intensity distribution mechanism of nonuniform plasma, and reveals the EM shielding effect (ES-effect) of plasma. Based on the typical carrier frequency, collision frequency, electron density, and electron density distribution gradient, the variation law of maximum reflection position and maximum reflection intensity in nonuniform plasma is revealed. The research results lay a theoretical foundation for analyzing echo signal characteristics of the PSh-covered target, radar detection anomaly suppression, and EM stealth.

Reducing plasma shielding effect for improved nanosecond laser drilling of copper with applied direct current

Laser drilling of copper faces challenges due to its high reflectivity and high thermal conductivity of the laser beam. To enhance laser drilling efficiency and hole surface quality, the copper substrate was applied with a direct current (DC) during a 355 nm nanosecond laser drilling process. It was found that introduction of the DC current played a noticeable role in reducing the entrance hole diameter, increasing the depth of the blind hole, decreasing the taper angle, improving the inner surface finish, and enhancing the hole integrity with less amount of debris at the hole exit. Explanation of the processing mechanism is provided by investigation effects of key processing parameters and modeling of DC current-induced magnetic field on the laser-induced plasma. The applied DC current through the copper substrate during laser drilling generated an induced magnetic field and the Lorentz force of the induced magnetic field limited the expansion of the laser-induced plasma in the direction parallel to the laser beam, which weakened the shielding effect of the plasma and thus improved the processing efficiency and hole quality of the nanosecond laser drilling. The study results may open a new path for efficient and high quality laser machining of reflective, high thermal and electrical conductive materials.

Simulation of Laser-Heating and Energetic Plasma Plume Expansion in Pulsed Laser Deposition of Y3Fe5O12

In the present study, numerically iterative models are employed to study two processes involved in the pulsed laser deposition of an Y3Fe5O12 target. The 1D conduction heat model is used to evaluate the temperature of the target irradiated by a nano-second pulse laser, taking into account the plasma shielding effect. Further, the gas dynamics model is employed to simulate the kinetic of plasma plume expansion. The results may be important in obtaining high-quality Y3Fe5O12 thin films.

Study of spectral intensity of the laser ablated tungsten plasma and ablation mass at various laser spot sizes and laser fluence in vacuum environment

The ablation features such as spot size and crater volume are essential parameters for the in-situ laser-induced breakdown spectroscopy (LIBS) technique to be well applied in the quantitative diagnosis of the plasma-wall interaction (PWI) processes on the first wall or divertor in tokamak devices. The spectroscopic characteristics and the ablation features as well as the plume dynamics of laser-induced tungsten plasma in vacuum environment at various spot sizes were investigated in this work. A nano-second laser pulse (5 ns, 1064 nm) with a maximum energy of 200 mJ was focused on the tungsten target with different spot sizes (from 0.42 mm to 2.71 mm) by adjusting the distance from focal point to sample surface. A paraxially collective configuration (15°) and time-integrated spectral collection mode were applied, similar to the present in-situ LIBS system on Experimental Advanced Superconducting Tokamak (EAST). It was observed that the spot size affects the spectral intensity significantly, and the optimal spot size is 1.16 mm both for the observed tungsten ionic and atomic lines at the laser energy of 200 mJ. The electron temperature (Te), electron density (ne) and ablation mass also reach maxima at the optimal spot size. Temporally spectral dynamic investigation shows the tungsten line emission time at the optimal spot size lasts longer and decays more slowly than other sizes, and the fast photography investigation also verified this. It was found that the central intensity profile of the plasma plume image from the target surface along with the opposite direction of the laser incident at the early stage (< 8 ns, during the laser pulse) is stronger at the smaller spot size while it decays faster. This was attributed to the stronger plasma shielding effect leading to the relatively less total ablation mass. Moreover, the optimal spot size was found to increase with the laser energy in the energy range from 65 mJ to 200 mJ, and the corresponding optimal laser fluence almost keeps the constant value of ∼18 J/cm2, which provides valuable information for optimizing the current in-situ LIBS for quantitative diagnosis of the PWI processes such as fuel retention and impurity deposition in EAST.

Subwavelength Quasi-Periodic Array for Infrared Antireflection.

Infrared antireflection of a zinc sulfide (ZnS) surface is important to improve performance of infrared detector systems. In this paper, double-pulse femtosecond laser micro-machining is proposed to fabricate a subwavelength quasi-periodic array (SQA) on ZnS substrate for infrared antireflection. The SQA consisting of approximately 30 million holes within a 2 × 2 cm2 area is uniformly formed in a short time. The double-pulse beam can effectively suppress the surface plasma shielding effect, resulting in obtaining a larger array depth. Further, the SQA depth is tunable by changing pulse energy and pulse delay, and can be used to readily regulate the infrared transmittance spectra as well as hydrophobicity. Additionally, the optical field intensity distributions of the SQA simulated by the rigorous coupled-wave analysis method indicate the modulation effect by the array depth. Finally, the infrared imaging quality captured through an infrared window embedded SQA is evaluated by a self-built infrared detection system.

Experimental study on the dynamics and parameters of nanosecond laser-induced aluminum plasma

In this paper, impulse measurement, spectral diagnostics, temporal evolution images and target ablation are employed to investigate the dynamic behaviors and parameters of Nd:YAG nanosecond laser-induced aluminum plasma at different pressures and laser fluences. The impulses and coupling coefficients generated by laser ablation increase firstly and then reduce with the decrease in pressure for the laser fluences of 17.22 J cm−2 and 20.94 J cm−2, but they only drop at 0.5 Torr for a laser fluence of 6.19 J cm−2. The fast exposure images captured by the high-speed camera and ICCD show that the plasma plumes present the separation at atmospheric pressure and expansion near vacuum, but last longer time at pressures of 150 Torr and 22 Torr. The duration and dynamic property of plasma are responsible for the impulse, as well as are mainly dependent on the characteristics of plasma parameters. Therefore, the electron density and electron temperature are obtained by Stark broadening method and Boltzmann plot, respectively. The change in the electron density is proportional to pressure and laser fluence. The variation trend of the crater depth and ablative mass with pressure is opposite to that of the electron density, which is ascribed to the plasma shielding effect caused by the inverse bremsstrahlung absorption mechanism. Besides, the electron temperature varies inversely with pressure in the detection range due to various recombination processes. This study provides an insight for clarifying the energy conversion mechanism and improving the laser propulsion performance.

Dependence on the Lens-to-Target Distance and with the Laser Energy at Constant Irradiance of the Laser-Induced Breakdown Spectroscopy Signal.

The emission signal-to-noise ratio (S/N) of a laser-produced plasma on an aluminum target at different focusing distances and at fixed irradiances was investigated. The plasma was produced by a 1064nm nanosecond-pulsed laser and the energy and irradiances were varied in the 6-110mJ and 0.4-700GWcm-2 ranges, respectively. Regardless of the applied laser energy, adjusting the lens-to-target distance, best emission values were obtained for an irradiance of nearly 8GWcm-2. At lower irradiances, the signal decreases due to less matter removal, while at higher values, the plasma shielding effect prevents the laser from reaching the sample. This mechanism is surpassed when the lens-to-sample distance is close to the nominal focusing value at about 100GWcm-2. The enhancement of the signal with the focusing distance is due to a combination of an increment of the plasma temperature, electron density, and atomized mass. When the irradiance is kept fixed changing simultaneously the laser energy and the ablated area, an increment of the emission was observed. This is basically due to an increment of the ablated mass while both electron density and temperature do not show significant changes, even though the laser energy increased by more than one order of magnitude.

Spatio-temporal multi-scale observation of the evolution mechanism during millisecond laser ablation of SiCf/SiC

Millisecond laser processing has advantages in terms of depth, efficiency, and cost. The evolution mechanism of the laser ablation of SiCf/SiC fabricated through melt infiltration remains unclear. Therefore, spatio-temporal multi-scale observation is proposed. The information of gas, plasma, and droplets is collected and analysed. The results reveal that laser power and the type of assist gas considerably affect the material removal, hole morphology, and efficiency. Typical topographies, such as crack propagation, fibre fracture, recasting, and debonding are clarified with various melting temperatures and internal stress induced by temperature gradient. Additionally, the distribution of elements Si and C is exclusive, which is clarified with surface tension, melting temperature difference, chemical reaction, and intervention of assist gas. The simulation model based on heat transfer laws and the birth-death element method verifies the rationality of the evolutionary model of the hole morphology. Moreover, the simulation model established for first pulse ablation in N2 indicates that a limited increase in maximum depth at high power can be attributed to the plasma shielding effect. This study can guide the processing of parts in the aerospace and nuclear industries.

Improvement of laser welded joint properties of AZ31B magnesium alloy to DP590 dual-phase steel produced by external magnetic field

External magnetic field was utilized to enhance laser welded joint properties of AZ31B magnesium alloy to DP590 dual-phase steel based on the joint structure with adding Sn powder prefabricated concave groove. The influence mechanism of external magnetic field on the plasma behavior of metal vapor, shape and size of grain in molten pool and improvement of joint performance were also revealed. The results suggested that the changes of joint property were closely related to the thermoelectric magnetic force (TEMF) generated by the interaction of metal vapor plasma and thermal current in molten pool. External magnetic field suppressed the shielding effect of metal vapor plasma on laser energy, increasing the absorption rate of laser energy, and the “bi-directional” metallurgical bonding of magnesium/steel interface was achieved. Simultaneously, grain size and distribution morphology were also reasonably controlled, hence, the mechanical performance of magnesium/steel joint was significantly improved by magnetic field.

Numerical simulation of the effect of laser wavelength on nanosecond laser ablation and plasma characteristic

Based on the heat conduction equation, hydrodynamics equations, and radiation transport equation, a two-dimensional axisymmetric radiation hydrodynamics model is developed. The charge state distribution and energy level population in the plasma are solved by the collisional-radiative model using screened hydrogenic levels. The model is used to study the effect of excitation laser wavelength at 1064 and 266 nm on aluminum target evolution, plasma generation, laser absorption in the plasma, and the plasma characteristic during laser ablation in the presence of atmospheric pressure. For 1064 nm radiation, the evaporation of the target surface stops earlier and the plasma formation time is later. The plasma has higher temperature as well as density and the hottest region is at the forefront of the plasma. The plasma shielding effect resulted in a sharp decrease in the laser transmissivity of 1064 nm radiation to about 0.1%, while the transmissivity of 266 nm radiation only decreased to about 30%. The inverse bremsstrahlung is the most important laser absorption mechanism for 1064 nm, whereas photoionization dominates the entire absorption process in the case of 266 nm radiation. The effect of the plasma model on optical breakdown has been present. The results show that neither breakdown nor plasma formation is encountered if the local thermodynamic equilibrium model is used in 266 nm radiation.

Spatiotemporal evolution of the morphology of multi-pulse laser ablated metals considering plasma shielding

With the laser ablation of metals, ultrafast lasers have high peak power density and significant nonlinear absorption, but plasma shielding and large taper often exist during ablation, which seriously affects the quality and efficiency of ablation. In this paper, the heat conduction equation of the lattice system is rewritten into the dual-temperature model, the time and space terms in the femtosecond laser source equation are superimposed to calculate, and the plasma shielding effect is incorporated into the ablation model using multi-pulse laser ablation iterative calculations. The constructed 3D improved dual-temperature model uses the finite difference method to investigate the spatio-temporal evolution of the ablation morphology of the metal target under the influence of different laser parameters using the critical point phase separation mechanism. In the numerical simulation, the error of considering plasma shielding is controlled within 8.24% compared with that of not considering plasma shielding, the ablation process has obvious layering phenomenon, the actual ablation experimental results are basically consistent with the calculation results of the proposed model, and the prediction error of the ablation depth can be controlled within 13.28%, which indicates that the model proposed in this paper has the ability to more accurately describe the spatial and temporal evolution of metal ablation by femtosecond laser.

A fractional dual-phase-lag generalized thermoelastic model of ultrashort pulse laser ablation with variable thermal material properties, vaporization and plasma shielding

The ultrashort pulse laser is widely used in micro scale non-contact machining of nonmetallic materials because of its high energy density, short pulse duration and small heat affected zone. Ablation is the main material removal mechanism in ultrashort pulse laser processing. This motivates us to establish the fractional dual-phase-lag (FDPL) generalized thermoelastic ablation model and investigate the transient responses of silicon ablated by the picosecond pulse laser. The temperature-dependent material properties, surface recession velocity and plasma shielding effect on the subsequent laser beam have been paid more attention. In calculation, the temperature distributions at the surface melting moment and plasma formation moment are taken as the initial conditions of the next Phases. The coupled governing equations containing fractional order parameter, lag times as well as spatial nonlocal parameter are formulated and solved by Laplace transform together with its numerical inversion. The temperature, displacement and stress with different laser intensity, pulse duration, fractional order parameter, lag time ratio as well as times are obtained and illustrated graphically. The accurate thermoelastic coupling description of silicon processed by the ultrashort pulse laser is obtained, which provides a reliable theoretical guide for high-quality laser processing of nonmetallic materials.

Electron-impact ionization of hydrogen and hydrogen-like ions within a magnetized quantum plasma

We suggest an approach within the fundamental framework of variational theory to calculate the dynamics of electron-impact ionization of hydrogen and hydrogen-like ions within a magnetized quantum plasma. This model can be used to predict the magnetic field effect and the plasma shielding effect on the atomic structure and collision dynamics. As an application, the binding energies of atoms/ions in the magnetized quantum plasma environment(B∼109 G) are calculated. The electron-impact ionization processes are studied. Our results show that, compared to the inner shell, the outer shell’s binding energy is greatly affected by plasma number density and magnetic field. The present results can be used not only to test other theoretical predictions and Hamiltonian models, but also be useful for the modeling of astrophysical plasmas.