Plasma Shielding Research Articles

Vapour shielding of liquid-metal CPS-based targets under ELM-like and disruption transient loading

This paper presents experimental studies of plasma-surface interactions during powerful plasma impacts of a quasi-stationary plasma accelerator (QSPA) on the Sn capillary porous systems (CPSs) in conditions simulating disruption and edge localized modes (ELM) like loads. Experiments were carried out using two QSPA devices. ELM-like plasma exposures were performed with QSPA-M test-bed facility. A large-scale QSPA Kh-50 device was used to simulate plasma disruptions and giant ELMs. Variation of the plasma stream energy density has been performed to study the onset of vapour shield. It is shown that during plasma exposures of a Sn-CPS target with the QSPA plasma load <1 MJ m−2, single dust particles traces have been registered. A further increase in the heat load leads to the splashing of the eroded material. For ELM-like impacts, a rather weak melt motion was observed on the target surface. A post-mortem analysis has shown that the CPS structure was not destroyed in the course of many repetitive ELM-like pulses. Surface morphology has changed from a smooth surface to corrugation structures with the formation of some cavities in mesh cells due to the influence of the surface tension and capillary effects. Spectral lines of Sn I and Sn II have been identified by optical emission spectroscopy in the near-surface plasma. A plasma shield, that consists mostly of Sn neutrals appears at Q ∼ 0.1 MJ m−2. An increase in the surface heat load resulted in the intensive emission of Sn II lines, which started to be observed at Q ∼ 0.3 MJ m−2. The plasma electron density near the surface increases significantly at Q > 0.5 MJ m−2, which corresponds to the strong vapour shielding of the exposed surface. A comparison between the obtained results on the vapour shielding of Sn CPS and available numerical simulation using the TOKES code has been performed.

Investigation of ablation efficiency during the pulsed laser ablation of a zinc metal target in a distilled water environment

In this study, the effect of Nd:YAG laser energy with a 9 ns time duration as one of the important parameters on the laser ablation of a zinc metal target in a distilled water environment has been investigated both experimentally and theoretically. The influence of plasma shielding on the ablation rate for 100–600 mJ energies is also considered. After investigating XRD, SEM, and UV-Vis results of each sample, it is determined that synthesized nanostructures are ZnO nanoparticles, and also the morphologies of the synthesized nanostructures are formed in two forms of hexagonal crystals and amorphous nanosheets. By increasing the laser energy to 400 mJ, the ablation rate on the zinc target increases. However, with increasing energy up to 600 mJ, the plasma shielding effect is evident and the ablation rate is reduced. Also, the theoretical two-fluid heat transfer model of nanosecond pulsed laser ablation is developed to visualize the evolution of temperature distributions, crater profile, and ablation depth.

Effects of a negative corona discharge on subsequent positive streamers

Positive streamers can be affected significantly by preceding discharges such as earlier streamers or a corona discharge. In this paper, we primarily discuss the effect of such a corona discharge on subsequent positive streamers with different intervals between them (0/500/990 ms) in air and pure nitrogen at 80 mbar with a repetition frequency of 1 Hz. We found that in air, when the interval time is 500 or 990 ms, a preceding discharge leads to shorter and weaker streamers or even prevents streamer inception altogether. This is likely due to negative ions which have converted to species that are harder to detach. The weaker streamers are also caused by more stable inception clouds which branch out later. When a corona discharge immediately precedes a streamer discharge in air, the streamers become longer, brighter while some branches develop from the side of the electrode instead of its tip. These effects are likely all related to plasma shielding caused by the corona discharge. In nitrogen, inception is primarily caused by electrons instead of negative ions. When the interval time is 500 or 990 ms in this gas, there is nearly no difference between streamers with and without a preceding corona discharge, because the inception time in nitrogen is the formative time that is not sensitive to the initial electron density. For near-zero intervals between corona and streamer discharges in nitrogen, streamers become smoother and thicker which can be attributed to a higher background ionization left by the corona discharge.

Charge‐Current Output in Plasma‐Immersed Hydrogen Atom with Noncentral Interaction

AbstractA theoretical investigation on the charge‐currents and induced magnetizations of hydrogen atoms with noncentral interaction, immersed in a plasma environment, under the influence of spherical encompassing is performed. The Hartmann potential for noncentral interaction is considered, but it is modified due to the plasma‐embedded hydrogen atom. The interactions in plasma are delineated by keeping in view the more general exponential cosine screened Coulomb (MGECSC) potential which reproduces more general and detailed outcomes. The shielding effect of the plasma environment is regarded by modifying the Hartmann potential through the MGECSC potential. Thus, the hydrogen atom is considered to be under the influence of the plasma modified Hartmann (PMH) potential. While the numerical tridiagonal matrix method (TMM) is employed due to the difficulty of analytical solution to the radial wave equation in nonrelativistic scheme including PMH potential, a hybrid solution technique is preferred by using asymptotic iteration method (AIM) to solve the angular wave equation. This study provides a detailed analysis of the plasma shielding, spherical confinement, and noncentral effect on the production and control of charge‐currents and induced magnetizations. In this analysis, the altenativeness to each other of the plasma environment, spherical encompassing, and noncentral effect is also evaluated, and suggestions are made for related application studies.

Dynamics of laser produced plasma from foam targets for future nanolithography devices and X-ray sources

Foam targets are expected to be more efficient candidates than solid targets for laser produced plasma (LPP) for extreme ultraviolet (EUV) and X-ray radiation sources due to the expected plasma conditions that can be optimized regarding plasma opacities, volumetrics heating effects, and the produced ions debris characteristics. In this paper, a comparison of ion dynamics between low-density foam and solid Ni plasma was systematically investigated at CMUXE. The foam Ni target (density 0.6 g/cm3) and solid Ni target (density 8.9 g/cm3) were irradiated with 1064 nm Nd:YAG laser in vacuum. A Faraday cup (FC) was used to record the ion flux and time-of-flight (TOF) signals. A lower and wider TOF signal was observed for foam Ni plasma on the time scale. The average ion energy and peak of the TOF signal of solid Ni plasma were much higher than that of the foam Ni plasma. However, the total charge values between foam and solid Ni plasma were comparable indicating a more volumetric absorption of laser energy for foam Ni. The average ion energy and peak of the TOF signal of solid Ni showed a stronger angular and laser energy dependence than that of foam Ni. The plume shape of the solid Ni plasma appeared as an oblong ellipse at each time, while that of foam Ni plasma tended to be more circular, especially at early times. The results of mass ablation rate were consistent with the FC signals and showed a more intense plasma shielding for solid Ni.

Density and temperature dependence of the cross sections after excitation of Ar XVIII by electron impact

We propose a novel distorted wave approach within the fundamental framework of relativity theory to calculate the dynamics of magnetic sublevels excitations of highly charged ions by electron impact within a quantum plasma. These cross sections can be used to predict the alignment parameter and subsequent angular pattern of the photoemission. Two kinds of modified Shukla and Eliasson [Phys. Rev. Lett. 108, 165007 (2012)] models, based on the quantum hydrodynamic theory, are adopted to determine the effects of screened Coulomb interaction among charged particles. The bound and continuous state (radial) wave functions are addressed through the one-dimensional Dirac equations, where the numerical convergence is checked. As an application, the 1s1∕2 → 2p3∕2 transition in Ar XVIII is used as a probe for the assessment of the quantum plasma shielding effects and the exchange-correlation effects on the energies, excitation cross sections, alignment parameters, and angular patterns of x-ray emission vs electron densities, from 1019 to 1024 cm−3, and compared to existing data. Our results highlight that the inclusion of the effect of plasma density and temperature not only leads to a significant reduction in the magnetic sublevels cross section, but also produces a change in the alignment parameter and angular pattern of the photoemission. The exchange-correlation effect is found to be crucial to the atomic structure, yet it weakens in the scattering process. In addition to insights into collision dynamics, this study opens the door to a further understanding of the polarization and angular distribution properties of x-ray radiation under extreme conditions. The present study is of interest for the warm density matter physics and high-energy density physics applications. The obtained atomic structure, sublevel cross section, and angular pattern of the photoemission are essential fundamental properties for advanced diagnostics and equation of state calculations in planet interior, inertial confinement fusion, and Z pulsed power facility.

Pattern-Steerable Endfire Plasma Array Antenna

The modification of a conventional plasma window antenna (PWA) into a novel endfire plasma array antenna (E-PAA) and the comparison of their performances are first made. It will be shown that the E-PAA provides better performance than the PWA does. Then, the gain improvement and more importantly the radar cross section (RCS) reductions for stealth properties of the E-PAA are described. A type of feeding and generating system of plasma antenna is given. Simulated and measured results at 300 MHz are discussed, indicating the potential applications of the E-PAA in high-level integrating of radio transceivers onboard for wireless communications.

Ultrafast dynamics control on ablation of Cu using shaped femtosecond pulse trains

The ablation processes of Cu film are investigated using temporal shaped femtosecond pulse trains. The depth is modulated by changing the number and interval of the sub-pulses. The underlying ultrafast dynamic processes are discussed based on plasma shielding and electron multiple heating mechanisms. When the sub-pulse interval is less than 0.4 ps electron multiple heating is the dominant mechanism, while the plasma shielding dominates the subsequent ablation processes when the sub-pulse interval is larger than 0.4 ps. The curve of depth obtained by three pulse trains shows more significant oscillation as the function of sub-pulse interval under the low-fluence. We propose that the oscillation of depth is due to the coherent phonon oscillation excited by the pulse train. The study provides a basis for giving insight into the ultrafast dynamics for improving micromachining and nano-fabrications using shaped femtosecond pulse trains.

Transient thermal model of nanosecond pulsed laser ablation: Effect of heat accumulation during processing of semi-transparent ceramics

Nanosecond ablation of semi-transparent ceramics represents a particularly interesting scenario where non-linear absorption mechanism allows absorption of laser energy for photon energy below the band gap of the material. Yttria Stabilized Zirconia (YSZ) ablated by infrared radiation represents as an example of such scenario. Ablation threshold experiments reveal a stark difference between the behaviour observed during ablation of single pulses compared to multiple pulses. During single pulses, a statistical occurrence of ablation is observed. For laser intensity below the dielectric breakdown threshold, this phenomenon has been explained on the basis of a defect-mediated thermal runaway which causes an exponential increase of the bulk material absorption coefficient as function of temperature. To investigate this phenomenon, and its effect during multi-pulse ablation, a 1D modelling framework has been developed and validated. The framework focuses on simulation of ablation of trenches (i.e. partially overlapped pulses) considered as convolution of single pulses, and account for the governing equation of heat diffusion, vaporization, plasma shielding, volumetric absorption and temperature-dependent absorption coefficient by use of a simplified defect mediated absorption model which represents the main novelty of the paper.The application of the model for various laser processing parameters reveals the importance of heat accumulation on the process. A positive feedback mechanism is established over successive pulses which leads to the material becoming progressively more absorbing, and eventually results in material removal through ablation. This phenomenon explains the sharp ablation threshold observed experimentally during ablation of trenches. The modelling framework also reveals interesting dynamics caused by the temperature dependence of the material absorption behaviour, such as a delay in the starting time for material removal, and drastic changes in the energy distribution within the sample throughout the duration of a laser pulse.Ultimately, the effect of heat conservation, which is often neglected, has been shown to represent a key dynamic during multi-pulse ablation, and should be accounted for during simulations of real-world laser processes, in particular for low diffusivity semi-transparent materials.

Spatio-temporal evaluation of Zr plasma parameters in a single-beam-splitting double-pulse laser-induced plasma

The plasma shielding effect is one of the major weaknesses of laser-induced breakdown spectroscopy (LIBS) as it causes non-linearity in signal strength. Although LIBS is typically carried out in constant laser energy, this non-linearity causes a reduction in sensitivity. In this work, we systematically examine laser-induced plasma, formed by two different excitation source modes, i.e. single pulse (SP)-excitation and single-beam-splitting double-pulse (SBS-DP)-excitation over Zr-2.5% Nb alloy. The two most important plasma parameters influencing the emission line intensity, plasma temperature (T e) and electron density (N e) were studied and compared for both modes of laser excitation. Comparison of the results conclusively demonstrates that due to the splitting of the laser energy in the SBS-DP mode, the plasma shielding effect is significantly reduced. The reduced plasma shielding translates to an increased laser–sample coupling under SBS-DP mode. Temporal imaging of the total intensity of the laser-induced plasma in both excitation modes was also studied. The study shows how the plasma shielding effect can be reduced to improve the analytical quality of the LIBS methodology.

Plasma irradiation experiment on the metal pebble flow in the TPDsheet-U

A new divertor concept REVOLVER-D2 that uses metal pebbles as a divertor target has been proposed for the LHD-type helical fusion reactor FFHR-c1. Demonstration of sufficient plasma shielding rate is one of the important issues to show the feasibility of the REVOLVER-D2. For this purpose, an experiment on the plasma irradiation to the metal pebble flow has been conducted in the sheet plasma device TPDsheet-U in Tokai University. Reduction of the ion flux and electron density in the metal pebble flow was observed and it was confirmed that the metal pebble flow can play the role of the divertor target. The measured shielding rate is consistent with the prediction by the model based on the Lambert-Beer-Bouger’s law. Achievement of the shielding rate of > 72 % is expected in the FFHR-c1 using the nozzle with the size of > 310 mm × 310 mm and the pebble flow with the diameter of 5 mm.

Systematic simulation studies on the penetration of resonant magnetic perturbations in an Experimental Advanced Superconducting Tokamak

The penetration properties of the n = 1 resonant magnetic perturbations (RMPs) with toroidal rotation are systematically studied by the upgraded three-dimensional toroidal magnetohydrodynamic code CLTx, through both linear and nonlinear simulations. It is found that in the presence of toroidal plasma rotation, the saturation state for high resonant harmonics is obtained in linear simulations due to the mode becoming unlocked from the internal magnetic islands. In nonlinear simulations, nonlinear effects become important when the toroidal plasma rotation is not included. The zonal component resulting from the nonlinear mode coupling is necessary for the saturation of the whole system including the internal kink mode and the m /n = 2/1 tearing mode. The simulations of RMP penetration demonstrate that the mode coupling is associated with the toroidal effect rather than nonlinear effects. With a low resistivity close to the experimental value, the single-harmonic-RMP is hard to penetrate the mode-rational surface because of the plasma screening effects, resulting in a truncation of the radial mode structure. On the other hand, the non-resonant components in the multiple-harmonic-RMP could largely reduce the effect of the plasma shielding, which leads to that the RMP is able to penetrate deeply into the central plasma region through the poloidal harmonic coupling.

Three-Dimensional Spatio-Temporal Evolution of Femtosecond Laser Multi-Pulse Ablation of Metals Considering Plasma Shielding

Three-Dimensional Spatio-Temporal Evolution of Femtosecond Laser Multi-Pulse Ablation of Metals Considering Plasma Shielding

Plasma and nanoparticle shielding during pulsed laser ablation in liquids cause ablation efficiency decrease

Understanding shielding cross-effects is a prerequisite for maximal power-specific nanosecond laser ablation in liquids (LAL). However, discrimination between cavitation bubble (CB), nanoparticle (NP), and shielding, e.g., by the plasma or a transient vapor layer, is challenging. Therefore, CB imaging by shadowgraphy is performed to better understand the plasma and laser beam-NP interaction during LAL. By comparing the fluence-dependent CB volume for ablations performed with 1 ns pulses with reports from the literature, we find larger energy-specific CB volumes for 7 ns-ablation. The increased CB for laser ablation with higher ns pulse durations could be a first explanation of the efficiency decrease reported for these laser systems having higher pulse durations. Consequently, 1 ns-LAL shows superior ablation efficiency. Moreover, a CB cascade occurs when the focal plane is shifted into the liquid. This effect is enhanced when NPs are present in the fluid. Even minute amounts of NPs trapped in a stationary layer decrease the laser energy significantly, even under liquid flow. However, this local concentration in the sticking film has so far not been considered. It presents an essential obstacle in high-yield LAL, shielding already the second laser pulse that arrives and presenting a source of satellite bubbles. Hence, measures to lower the NP concentration on the target must be investigated in the future.

Parameter optimization of the spectral emission of laser-induced tungsten plasma for tokamak wall diagnosis at different pressures

The ambient pressure influences the plume expansion and spectral emission, and two pressure regions are observed to distinguish the enhanced plasma shielding effect.

Water-assisted femtosecond laser drilling of alumina ceramics

In this study, we used a water-assisted femtosecond laser drilling method to drill holes in alumina ceramics. The morphology, diameter, taper angle, cross-section area, and sidewall characteristics of the holes were investigated, and the hole characteristics obtained in the presence of air were compared with those obtained under water assistance. Finally, the mechanism underlying the water-assisted femtosecond laser drilling of alumina ceramics was investigated. The results revealed that the drilling efficiency and quality of the holes improved significantly by using the water-assisted femtosecond laser drilling method, and the effect was considerably obvious at lower pulse repetition rates. At the pulse repetition rate of 25 kHz, the taper angle decreased by 64.19% to 4.20° and the hole cross-section area increased by 46% under water assistance. In addition, water assistance could reduce the amount of the residual debris and redeposition of ablated material on the hole sidewalls, and there were almost no residual debris and redeposition of ablated material on the hole sidewalls near the hole exit. Compared to the case when the drilling was carried out in the presence of air, the composition of the hole sidewalls changed slightly when the drilling was carried out in the presence of water. With an increase in the pulse repetition rate, more plasma was formed and the plasma shielding effect strengthened, which weakened the effect of water assistance. Therefore, in order to obtain high drilling efficiency and quality during the water-assisted femtosecond laser drilling of alumina ceramics, it is necessary to use a low pulse repetition rate. The results of this study will find potential application in the field of ceramic manufacturing.

Expansion dynamics and radiation absorption in plumes induced by irradiation of a copper target by single and multiple nanosecond laser pulses in the doughnut beam mode

The two-dimensional kinetic simulations of the plume expansion process induced by irradiation of a copper target in argon background gas with nanosecond laser pulses in the doughnut beam mode are performed. The target is irradiated by single and double laser pulses, as well as by bursts consisting of up to 35 individual pulses. The simulations are based on a hybrid computational model that includes the direct simulation Monte Carlo method, where the ionization and absorption of laser radiation are considered. The numerical results are systematically compared with the results obtained for a Gaussian beam at the same pulse energy, peak fluence, and peak intensity. The structure of the plumes induced by a doughnut beam is found to drastically differ from the structure of the plumes induced by a Gaussian beam. The doughnut beam initially induces a toroidal shock wave. The self-interaction of this shock at the axis of symmetry results in the formation of multiple internal shock waves that strongly increase the plume temperature and density. The interaction between plumes induced by a double pulse in the doughnut beam mode occurs in two distinct regimes depending on the beam size and inter-pulse separation time. At a relatively small peak laser fluence, the effect of plasma shielding in the plumes induced by a burst of pulses in the doughnut beam mode is found to be smaller compared to the case of a Gaussian beam. At a relatively large fluence, the simulations reveal the opposite trend, but the amount of vaporized material is found to be always larger for a doughnut beam than for a Gaussian one. The obtained results suggest that the absorption of laser radiation in plasma plumes induced by nanosecond laser pulses can be tuned by changing the radial distribution of energy in the incident laser beam.

Silicon twisted cone structure produced by optical vortex pulse with structure evaluation by radiation hydrodynamic simulation

We demonstrate a radiation hydrodynamic simulation of optical vortex pulse-ablated microcone structures on silicon (Si) substrates. Doughnut-shaped craters were formed by single pulse irradiation on the Si substrate, and a twisted cone structure with a height of 3.5 µm was created at the center of the irradiation spot by the circularly polarized optical vortex pulse. A two-dimensional (2-D) radiation hydrodynamic simulation reproduced the cone structure well with a height of 3 µm. The central part of the incident laser power was lowered from the initial profile due to plasma shielding over the laser pulse duration for an inverted double-well laser profile. The acute tip shape of the silicon surface can survive over the laser irradiation period.

Characterization of multilayer coatings of aluminium, tungsten and molybdenum on steel substrate using laser-induced breakdown spectroscopy

Laser induced breakdown spectroscopy (LIBS) has been applied for the depth profiling of multilayer metallic coating structure which is simulated to the ITER-like wall materials in fusion devices such as EAST tokamak. The coatings were produced by magnetron sputtering and characterized by LIBS using a Nd:YAG laser at 1064 nm with pulse duration of 5 ns. The light from the resulting plasma was analyzed as a function of both number of laser pulse (N) and elemental spectral line intensity. The depth profile of the multilayer coating was determined by three different spectral signal processing methods; intensity background subtraction, intensity normalization and linear Pearson correlation. The intensity normalization method provides the best outcomes/solutions for thickness measurement and analysis of interfaces of multilayer coatings among other methods. The evolution of spectral intensity representing different materials was properly examined for 90 consecutive laser shots at the same spot. The ablation behaviour of the coating materials was analyzed in terms of ablation rate under the laser pulse of various fluences. In a much deep crater, some factors originating from laser ablation, such as material re-deposition and plasma shielding effect were analyzed. As a comparison, the depth, shape and diameter of craters were also measured using SEM and a profilometer. The influence of laser power density on the ablation rate and crater profile was investigated. The elemental composition, inside the crater, was verified by the EDX technique. The results indicate that LIBS has a potential power for in-situ depth profiling of multilayer coatings on the wall of the EAST.

Plasma Parameters Generated from Iron Spectral Lines By Using LIBS Technique

An interesting LIBS technique was used to investigate the optical emission spectral plasma of iron metal properties. To generate plasma from the surface Fe, Q-switched Nd-YAG laser with wavelength 532 nm and a focal length 10 cm was used with different energies (500-800) mJ. Then plasma parameters were calculated; electron density ne ranged between (0.92-1.4) × 1018 cm−3, the electron temperature Te was in the range of (2.19-2.59) eV. These calculations were done using Boltzmann’s plot and the Stark broadening respectively depending on the experimental spectrum, and followed up to estimate the others plasma parameters, Debye length (λD), frequency (fp ) and the Debye sphere (ND). Results indicate that plasma parameters are proportional to the energy of laser due to the increase in the intensity of spectral lines energy, and that plasma shielding of iron increases with laser energy in the range of (3.2-4.3).