Plasma Shielding Research Articles

Microstructure, strength, and fiber texture evolutions in arc-based casting using low-carbon steel wire

The present investigation introduced a modified casting route for steel component manufacturing that uses an electric arc to melt the filler wire and transfer the molten metals as tiny droplets into the sand mould. To study the influence of heat input on microstructure, phase percentage, interlamellar spacing, tensile properties, wear resistance, residual stress, and texture fiber evolutions, samples are cast on a wide range of wire feed speed (WFS) (3 to 9 m/min). It has been observed that higher WFS increases the heat input that transfers small-sized molten droplets into the mould cavity at a faster rate and suitably reduces the mould filling time. Heat input shows a positive response with the microstructural evolutions, i.e., large-sized grains with increased pearlite concentration and interlamellar spacing are encountered under high heat input (low cooling rate). It is attributed to improved carbon atom diffusion with the ferrite phase and increased pearlite transformation time. This increment in pearlite concentration and compressive residual stress further enhanced the tensile performances. Wear resistance varies inversely with the heat input rate. A confirmation of the oxide layer formation due to frictional heat generation during wear testing has been authenticated from Raman spectroscopy analysis. Due to its drop-by-drop deposition nature, high WFS raises the heating rate and droplet detachment frequency. Hence, the degree of recrystallisation increases that boost up ϒ-fiber development with intensified θ-fiber components like {001}〈110〉 and {001}〈100〉. Due to the controlled metal transfer under plasma and argon gas shielding, the casted sample has fewer chances of porosity.

Shannon entropy for hydrogen atom in Debye and quantum plasma environment

The plasma screening effect on Shannon entropy values is studied for atomic states of hydrogen under the more general exponential cosine screened Coulomb (MGECSC) potential, which can be used to model Debye and quantum plasmas. The wavefunctions used in the calculation of Shannon entropy are obtained by solving the Schrödinger equation employing the efficient Numerov technique. Shannon entropy is calculated for hydrogen atom quantum levels using various sets of screening parameters to account for the four different potential forms present in the MGECSC potential. The electron density distributions are considerably altered due to the plasma shielding influence on the embedded hydrogen atoms, and this effect is measured by the shift in Shannon entropy. A greater screening influence on entropy is observed in quantum plasma modeled by the MGECSC potential than that in Debye plasma due to the significant combined effects of screening parameters. Excellent convergence is obtained on comparing our results for plasma-free hydrogen atom with the currently available literature. This study is the first to examine the effects of shielding on Shannon entropy of hydrogen atoms in plasmas modeled by the MGECSC potential. These findings will be important for theoretical and experimental research in the disciplines of atomic physics and plasma diagnostics.

Mechanism analysis of space debris removal by nanosecond pulsed laser

In this study, a numerical model is used to study the mechanism of laser ablation for space debris removal. The model couples the physical processes involved in laser ablation, including the heating, melting and evaporation of target (i.e. Aluminum), the formation of plume and plasma, and plasma shielding. Validation of the model is performed by comparing the simulated impulse density and impulse coupling coefficient with experimental data, showing the availability of the model in accurately describing the processes involved in laser ablation for removing space debris. The effects of laser irradiance, wavelength and pulse width on laser ablation were systematically studied by the model. In addition, the continuous laser ablation is explored and the effect of the laser pulse time interval is demonstrated. The results show that laser irradiation and pulse width have a significant impact on laser ablation, while the impact of wavelength is much smaller compared to the former two. In addition, using two short duration laser ablation instead of a long duration can significantly improve the impulse density obtained by the target and the corresponding impulse coupling coefficient, but the time interval between two laser pulses should not be too long.

Ionization dynamics of sub-micrometer-sized clusters in intense ultrafast laser pulses

Sub-micrometer-sized targets are found in intense laser–cluster interaction experiments and laser-based material processing. Here, we investigate the internal field localization due to Mie scattering and its effect on ionization dynamics in sub-micrometer-sized clusters using Mie calculation and particle-in-cell simulations. As a result of intertwined processes of pulse propagation and ionization, sub-micrometer nanofocusing dominates at lower intensity and gives rise to an ionization hotspot at the rear of the targets while plasma shielding wins over at a higher intensity, stopping further rear side ionization. As ionization is often the precursor of other processes, understanding the ionization dynamics of ultrafast laser pulses with wavelength-sized nanostructure can be relevant for intense laser-cluster experiments and femtosecond laser micro-/nanomachining.

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.

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.

Dynamics of filamentary damage induced by nanosecond laser in fused silica

The phenomena and mechanisms of energy deposition and plasma shielding during laser breakdown of transparent dielectrics deserve further discussion. In this work, the formation characteristics of 1064 nm ns pulsed laser-induced fused silica filamentary damage behavior during the bulk damage process were studied using the time-resolved pump-probe technique. The role of plasma shielding in the damage process was also further studied. The results indicate that the filamentary damage formed before the core damage. With the enhancement of laser energy, the filamentary damage length is stable in the initial stage and then begins to grow dramatically, and the core damage location moves towards the front surface of the fused silica. However, the plasma scattering and shielding effects in air and fused silica severely affect the damaged region when the laser energy is further enhanced. It causes rapid decay of the filamentary damage length, while the core damage moves along the optical axis to the rear surface of the fused silica. It was also found that the diameter of the filamentary damage was not significantly affected by the relatively small laser energy.

Portable nanosecond laser for handheld laser-induced breakdown spectroscopy instruments

The portable laser is indispensable for handheld laser-induced breakdown spectroscopy (LIBS) instruments. We developed a passive, Q-switched pulsed, Nd:YAG laser system with small size and low cost. The maximum pulse energy of the laser was 10 mJ and the pulse duration was <2 ns. The peak power was >5 MW. The laser exhibited very high stability without any cooling system and its relative standard deviation of the pulse energy was <0.2 % . The laser was also used as a light source of LIBS to measure the concentrations of Cr, Cu, Mn, and Ni in alloys. The experimental results show that the determination coefficient of the calibration curves for these four elements is all better than 0.99. For its shorter pulse duration, the laser can decrease the plasma shield effect to improve the coupling of laser pulse and the target and stability of spectra intensity. The portable laser developed is a good laser source for handheld LIBS instruments.

Electron-impact excitation of atoms or ions with the screened Coulomb potential

This paper reports the development of a novel distorted wave approach based on the relativistic configuration interaction theory for calculating atomic structures, radiative properties, and collision dynamics of atoms or ions under external confinement. Due to its physical interpretations, the Hulthén potential, valid for many important areas (widely plasma environments) in physics and chemistry, such as atomic physics, solid-state physics, nuclear physics, particle physics, and astrophysics, is used to include the effect of plasma background. The Breit interaction and quantum electromagnetic dynamics effects are perturbatively included in the multi-electron Hamiltonian. Wave functions of bound and continuous states are obtained, which are analyzed by means of the Dirac equations. Interparticle interactions in the scattering matrix element of the system are described by the screened Coulomb interactions. As a test desk, the present approach is applied to the calculation of atomic structures and collision dynamics of an exemplary He-like S14+ ion embedded in a plasma. The energy eigenvalues, transition probabilities, and excitation cross sections are analyzed in detail in terms of the screening parameter. The results of the verification cases are compared to references from the literature, which show a good agreement. The present study not only opens up an opportunity to provide a simple and effective way for understanding the plasma shielding models for many-electron systems but also has important implications for a wide range of applications in astrophysics and laboratory plasma experiments.

Theoretical studies of the spectral characteristics and electron impact dynamics of Ti XXI placed in the hot dense regimes

We describe a theoretical approach to calculate the electron impact dynamics of atoms/ions placed in a dense plasma. The model takes into consideration that the continuous electron and the guest atom/ion are affected by a recently proposed (dense) plasma shielding parameter potential. As a test case, the present numerical method is employed to study the electron-impact excitation of He-like Ti XXI by using the distorted-wave wavefunction in the framework of the fully relativistic theory. The finite temperature and density effects as well as solid density plasma effects on the excitation energies, transition properties, and integrated cross sections are discussed in detail, which shed light on the shielding parameter potential in resolving practical problems. Results are compared with the available measurements and previous computations. The present work is not only providing a new implementation of the finite-temperature method by a self-consistent methodology, but also of great interest for the high energy density physics community, i.e., our results are of interest to the fusion researches.

Mechanistic Insight of Synthesized 1,4-Dihydropyridines as an Antidiabetic Sword against Reactive Oxygen Species.

The pharmacologically privileged DHP derivatives were synthesized using the pragmatic multicomponent Hantzsch synthesis to screen the antidiabetic activity. Initially, the candidates were screened using an in vivo blood glucose test, where compound 8b showed the most prominent antidiabetic effect (% potency = 218%) compared to glimepiride. Then, a propositioned structure-activity relationship study was executed to reveal that longer side chains decreased the DHP's antidiabetic action. Mechanistically, compound 8b diminished ROS in β-cells and muscle cells simultaneously, which was proved by enhanced serum biochemical markers. Also, compound 8b decreased blood glucose by α-glucosidase inhibition (IC50 = 4.48 ± 0.32 μM), compared to acarbose (7.40 ± 0.41 μM), based selectively on the plasma window of 8b. Acarbose demonstrated auspicious inhibitor activity according to the binding affinity (ΔGbinding), which was slightly lower than that of compound 8b (-54.7 and -46.8 kcal/mol, respectively). During the 100 ns molecular dynamics simulations, the structural and energetic assessments exposed the high consistency of compound 8b to bind to the α-glucosidase.

Effect of Surface Roughness on the Surface Texturing of 316 l Stainless Steel by Nanosecond Pulsed Laser

Stainless steel 316L is an austenitic alloy that is widely used in varying industries due to its outstanding corrosion resistance, high strength, and ductility properties. However, the wear and friction resistance properties are low. Laser surface texturing can improve the wear and friction resistance of the material via the functionalisation of the surface. The laser surface texturing efficiency and the texture quality are defined by the material’s surface properties and laser parameters. The surface roughness is an important material property having an effect on laser surface texturing. This paper reports on a study of the material’s surface roughness influence on the texturing of 316L stainless steel with 1064 nm nanosecond pulsed laser. Single pulse shots were employed to avoid the topographic influence of the previous laser shots. The surface shape and the topography of the textures were assessed using optical microscopy and profilometry. It was observed that the textures produced were dimples of U-type and sombrero-like type geometries depending on surface roughness and pulse energy. The overall quality of the texture shape was better for smoother surfaces. The energy fluence necessary to generate textures is lower on surfaces of lower roughness than surfaces with high roughness. The surface at 24 nm of average roughness is the best surface for creating deep textures. The ablation mechanisms associated with high pulse energy, including plasma shielding, are produced at lower pulse energies for the 100 nm roughness, compared with other roughness samples.

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.

Raw signal improvement using beam shaping plasma modulation for uranium detection in ore using laser-induced breakdown spectroscopy

Real-time quantitative detection of uranium in ores is one of the major challenges for uranium exploration. Laser-induced breakdown spectroscopy (LIBS) has been regarded as a most promising technique for this application. However, due to the matrix complexity as well as low uranium concentration of ore, the detection sensitivity of LIBS for uranium in ores is still unsatisfactory. This work explored the potential of a beam-shaping plasma modulation method to improve the limit of detection of uranium in ores. By shaping the profile of laser beam from normally Gaussian distribution to flat-top, the plasma was modulated to be more excited with reduced peak electron density at the laser-plasma interaction point for plasma shielding reduction especially at high laser energy as well as to be more morphologically stable for LIBS signal repeatability improvement. It was further found that this method enhanced LIBS signal intensity mainly by increasing the plasma temperature, while the electron density was almost unchanged, which was very attractive for uranium detection in ore since one of the major problem for uranium detection was that it is hard to find clear uranium spectral lines for analysis due to the high dense emission lines of ore samples in real cases and lower electron density, indicated less line broadening and less line overlap or interferences. A clear uranium emission line has been found in crowded ore spectra, and the intensity of U II 409.013 nm based on flat-top beam was about 5 times higher than that of Gaussian beam, and the relative standard deviation (RSD) of the signal was reduced by about 50%. Moreover, the LOD of uranium in ores was estimated to be 21.2 ppm with flat-top beam, indicating that beam shaping is a promising method for rapid and accurate detection of uranium in ores.

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.

On the development of relativistic distorted wave approximation for the energies and collision dynamics of atoms or ions subjected to the outside plasma

In this manuscript, we present the development of a relativistic distorted wave method for determining the energies and collision dynamics of plasma-immersed atoms or ions. The methodology is based on the Dirac–Coulomb Hamiltonian, in which contributions from relativity and higher order effects, such as quantum electrodynamics and Breit interaction, are incorporated. The key element in this method is that a modified Debye–Hückel approximation is employed to represent the effect of plasma screening. In order to correctly describe the (bound and continuous state) wave functions, a self-consistent field calculation incorporating the shielding potential is performed within the fully relativistic framework. The particle interaction within the scattering matrix element of the excitation process is described by the shielded Coulomb interaction. The present technique is illustrated by calculations of energy, line shift, transition probability, electron-impact excitation/ionization cross section, and photoionization cross section of a few-electron system confined in plasma environments. The present model is tested and validated against a number of known cases (simulations are made for the He-like Al11+ ion) in the literatures. Numerical results demonstrate that the modifications to the Coulomb potential proposed in the spatial and temporal criteria of the Debye–Hückel approximation allow us to improve the theoretical description of the plasma shielding and thus the dynamical processes in dense plasmas. Comparisons of our computational predictions and the recent experimental measurements are performed. The current work not only has far-reaching implications for our understanding of the dense plasma screening, but also has potential applications in fusion, laboratory astrophysics, and related disciplines.

Plasma-enabled microwave modulation for continuous beam scanning

The continuous beam scanning capability is a key characteristic of the reconfigurable antenna in wireless communication. In this work, we theoretically imply that the plasma columns are capable of smoothly manipulating the transmission of the electromagnetic waves. And then we propose a plasma-based microwave modulation method for obtaining a high precision beam scanning over 360∘ on the azimuthal plane by tailoring the operating currents in 22-tube plasma window antenna system. Through this simple current modulation approach, a continuous beam scanning is achieved at the reconfiguration time of 0.6 ms without any detriment to the lobe shape.

Experimental studies of a novel one-dimensional plasma window

The plasma window is a windowless vacuum sealing device that utilizes a cascade arc discharge to isolate regions with different pressures. It was proposed as an alternative to the traditional windowless sealing technology of differential pumping, which greatly improves the sealing efficiency and has been successfully applied in electron beam welding. The discharge channel of the existing plasma window is a small circular hole, referred to as a zero-dimensional plasma window, which has limited applications to its dimensions. In this study, we suggested a one-dimensional (1D) plasma window with a slit discharge channel (cross section: 3 × 35 mm2, length: 60 mm). An arc discharge is realized with the support of an 80 A DC power supply, which can maintain a pressure drop of more than 10 times between the high-pressure and low-pressure ends of the plasma window. A COMSOL-based magneto-hydrodynamic model of the plasma window was established, and simulations were in good agreement with the experimental results. The pressure drop in the plasma window caused by argon plasma has also been theoretically analyzed and discussed. The feasibility of a 1D plasma window to achieve vacuum isolation was verified theoretically, numerically, and experimentally.

Modelling the initial plasma dynamics in a laser triggered vacuum switch

The numerical simulations of the initial plasma dynamics induced by 1064 nm laser irradiation on a KCl-Ti mixture target in a laser-triggered vacuum switch are performed. The simulation results showed that laser irradiance strongly affects the dynamics of the initial plasma. With higher laser fluence, the plasma plume will have higher temperatures and densities, and the shock front will also move faster. During the laser ablation, the plasma shielding is a non-negligible process. As the degree of ionization increases, the absorption coefficient of the plume increases and the it begins to absorb the laser energy. The temperature and velocity of the plume increase dramatically as it absorbs laser energy. When laser fluence gets higher, the plasma shielding starts with a shorter time and a larger proportion of the laser energy is absorbed by the plume.