Plasma Formation Research Articles

Percolation leads to finite-size effects on the transition temperature and center-of-mass energy required for quark-gluon plasma formation

We investigate the finite-size effects on the transition temperature associated with the quark-gluon plasma (QGP) formation. From a percolation perspective, the onset of the QGP in high-energy collisions occurs when the spanning cluster of color strings emerges. The principal result presented here is the finite-size effects on the transition temperature expressed as a power law in terms of the nucleon number. We found that the transition temperature is higher for small systems than for large ones. It means that minimal triggering conditions events in pp collisions require about 20 times higher energies than AuAu-PbPb collisions. We also estimate the center-of-mass energy required for the QGP formation as a function of the nucleon number. Our results are consistent with the minimal center-of-mass energies at which the QGP has been observed.

Plasma Dynamics Characterization for Improvement of Resonantly Enhanced Harmonics Generation in Indium and Tin Laser-Produced Plasmas

In this study, we characterize the properties of indium and tin laser-induced plasmas responsible for efficient high-order harmonics generation of the ultrashort pulses propagating through these media. The optimally formed plasma was determined using the analysis of the time-resolved variations in the spectral and morphological features of spreading indium and tin plasma components under different regimes of laser ablation. We report the measurements of plasma velocities under different regimes of ablation and correlate them with the optimal delay between the heating and probe laser pulses for the generation of harmonics with the highest yield. Electron temperatures and densities are determined using the integrated and time-resolved spectral measurements of plasmas. The resonance-enhanced harmonics are compared with other harmonics from the point of view of the modulation of plasma characteristics. The harmonics of 800 and 1200–2200 nm lasers and their second-harmonic fields were analyzed at optimal conditions of Sn and In plasma formation. The novelty of this work is the implementation of the diagnostics of the dynamics of plasma characteristics for the determination of the optimal plasma formation for harmonics generation. Such an approach allows for the demonstration of the maximal harmonic yield from the studied plasma and the definition of the various resonance-induced harmonic generation conditions.

Основные результаты и направления дальнейших исследований по низкотемпературному плазменному формированию композитных структур на рабочих поверхностях геометрически сложных металлических изделий

The attained results and valid directions of further research on improving the efficiency of low-temperature plasma for-mation of composite structures on the functional surfaces of geometrically complicated metal products (in particular, metal-cutting tools) are conducive to improving their durability and expanding areas of application

Excitonic optical properties and lasing mode shifts in square CsPbBr3 nanoplate cavities

Cesium lead bromide (CsPbBr3) perovskite micro/nanostructures have potential applications in miniaturized integrated light sources due to their confined modal volume and high luminous efficiency. In this work, we synthesized CsPbBr3 nanoplates with regular square shapes and smooth surface by the chemical vapor deposition method. The temperature-dependent and excitation power-dependent photoluminescence spectra reveal the excitonic photoluminescence properties. The whispering gallery mode (WGM)-like lasing in CsPbBr3 nanoplates was observed at room temperature. We systematically investigated the pump fluence-dependent multi-mode and single-mode lasing actions in square microcavities. The blueshift of individual lasing modes can be attributed to the variation of refractive index resulting from the formation of the electron-hole plasma (EHP) state under highly injected carrier density. The results reveal the dependence of emission properties on excitation intensity, providing further understanding of the carrier-induced mode shift mechanism in the WGM-like lead halide perovskite microcavities.

Atmospheric air plasma sustainment by semiconductor microwave for hydroxyl radical production and powder metal element analysis

A semiconductor microwave device that generates a series of burst microwaves at a sub-microsecond duration has been successfully used in a breakdown plasma spectrometer in atmospheric conditions. Microwave delivery has been changed to couple the microwave with laser sparks and electric sparks which are typical plasma ignition sources in laser-induced breakdown spectroscopy (LIBS) and spark-induced breakdown spectroscopy (SIBS). A helical antenna was used for the laser spark, while a coaxial antenna was considered more appropriate for the electric spark. The weak and transient sparks in LIBS and SIBS are enlarged by the microwaves which are stably sustained in the air. The microwave's output power and pulse duration are easily controllable, resulting in tunable plasma intensity and sustained production of hydroxyl radicals (OH radicals). Even in continuous-wave operation by microwave, the low-energy system prevented the formation of high-temperature thermal plasma (>10,000 K) without any mechanical cooling system. The microwave-enhanced LIBS (MW-LIBS) and microwave-enhanced SIBS (MW-SIBS) could be applied to optical emission spectroscopy analyses. In analytical applications, MW-SIBS produces no shockwave in contrast with MW-LIBS which is a great advantage in powdered samples. The MW-SIBS successfully analyzed the direct introduction of copper metal powders.

Probing neuronal functions with precise and targeted laser ablation in the living cortex: comment

The paper [Optica 8, 1559 (2021)OPTIC82334-253610.1364/OPTICA.433562] is devoted to probing neuronal functions by targeted cell ablation in living cortex. It demonstrates the advantage of cell ablation by single high-energy pulses from an amplified femtosecond (fs) laser system, which leaves adjacent structures intact. The single-pulse effects rely on mechanical disruption by laser-induced microcavitation, which goes along with a lack of diffusive collateral damage. In comparison, cell damage by low-energy pulses from fs laser oscillators is shown to be associated with a damage zone with tens of micrometer radius around the targeted cell. The collateral damage is attributed to heat accumulation and diffusion. We demonstrate through numerical simulations of plasma formation and temperature evolution for the experimentally employed irradiation conditions that a large number of free electrons is produced but the temperature rise at focus is only 0.3 K. Therefore, the damage is caused by nonlinear photochemistry and plasma-mediated chemical effects rather than by thermal effects.

Laser-Induced Optical Breakdown of an Aqueous Colloidal Solution Containing Terbium Nanoparticles: The Effect of Oxidation of Nanoparticles.

The influence of the number of oxidized terbium nanoparticles on the intensity of physicochemical processes occurring during optical breakdown in aqueous colloidal solutions of nanoparticles has been studied. It is shown that the effect of the number of oxidized terbium nanoparticles on the physicochemical processes occurring during optical breakdown depends significantly on the fluence of laser radiation. At a fluence of less than 100-110 J/cm2, plasma formation processes occur more intensively on less-oxidized (metal) nanoparticles. At a fluence of more than 100-110 J/cm2, the processes of plasma formation during optical breakdown occur much more intensively on more-oxidized nanoparticles. It has been established that the dependence of the rate of laser-induced decomposition of water on the concentration of nanoparticles is two-phase. The rate of generation of water decomposition products increases with an increase in the concentration of nanoparticles up to 109 NP/mL. With a further increase in the concentration of nanoparticles, the rate of generation of water decomposition products decreases. In this case, more than 99% of the decomposition products of water are formed due to the action of plasma, and the share of ultraviolet and ultrasound formed during optical breakdown is approximately 0.5% on each.

Plasma Formation And Its Sustainment In Time And Space In Microwave Enhanced Laser Induced Breakdown Spectroscopy

The transient and unstable plasma generation in the laser-induced breakdown is potentially resolved by the addition of microwaves. The laser and microwave technology combination extends the plasma lifetime and enhanced its emission intensity and volume. The expansion of the plasma started a few seconds after the initial formation of the plasma seed and the absorption of the microwave by the plasma also started to take effect. Large dome-shaped plasma is then briefly sustained until the end of the microwave duration. As the plasma expands, the plasma drifts away from the sample surface which is a good indication of the non-ablation effect of the microwave energy. The expansion of the plasma by microwaves enhances the breakdown of the plasma but potentially regulates the formation of toxic ablation fumes.

Laser-Induced Molecular Plasma: A Medium for High-Order Harmonics Generation of Ultrashort Pulses.

I present a study of laser-induced plasmas (LIPs) produced on the surfaces of molecular targets to create optimal conditions for high-order harmonics generation during the propagation of femtosecond pulses through the LIP. The resonance enhancement of a harmonic, two-color pump of plasma, quasi-phase-matching and nanoparticle-induced growth of the harmonic yield are analyzed, which allows for the formation of sources of coherent extreme ultraviolet radiation based on molecular plasma formation.

Metal additive manufacturing by laser-powder bed fusion:Guidelines for process optimisation

Additive manufacturing is growing rapidly in the last decades due to the significant development in the laser technology and metal powder manufacturing. It offers a way to rapidly create near-net-shape engineered parts starting from a digital CAD file without the need for dies or molding. It also offers the ability to manufacture parts designed in a complex geometry and made of metals which are difficult to produce by using the traditional methods. One of the main obstacles in Laser-Powder Bed Fusion (L-PBF) for example is the dissimilarity in the produced parts’ properties. The main reason behind this is the non-stationary nature due to the turbulence formation above the melt-pool area accompanied by weld fumes, metal evaporation, plasma and sparks formation. These effects increase within the individual printer as the fumes suspending and circulating with the inert gas increasing with the time. Also, these effects differ from one printer to another depending on the build chamber size and gas flow arrangement. This article is focused on exploring the (L-PBF) process in a step-by-step principle and present it to the researchers, technicians and even manufacturers especially those who have novice or lack of information about this technology. The study puts the main issues which must be considered and learned in a successful Additive Manufacturing (AM) lab to produce a functioning metal part.

Characterization and investigation of the unique plasma behavior caused by variable driving frequencies in the formation of cold atmospheric plasma

The study presents the characterization of a novel variable frequency (11–50 kHz) Atmospheric Pressure Plasma Jet (APPJ) source and comparison of the results with 13.56 MHz source, in terms of species generation and species temperature. The behavior of variable frequency APPJ at different frequency regimes was investigated using optical emission spectroscopy to understand the interaction with the ambient environment. The quantitative dependence of the radical generation on driving frequency and time of treatment was also analyzed by direct synthesis of H2O2 by plasma-water interaction. The plasma formed with a kHz driving frequency had a low treatment temperature, which was suitable for biological species, but the plasma generated with a 13.56 MHz driving frequency had a substantially higher radical density than the kHz plasma. As a result, the APPJ device's ability to tune the radical density and treatment temperature with a change in kHz frequency has been demonstrated.

Arc hopping dynamics induced by interfacial negative differential resistance.

Pattern formation in plasma-solid interaction represents a great research challenge in many applications from plasma etching to surface treatment, whereby plasma attachments on electrodes (arc roots) are constricted to self-organized spots. Gliding arc discharge in a Jacob's Ladder, exhibiting hopping dynamics, provides a unique window to probe the nature of pattern formation in plasma-surface interactions. In this work, we find that the existence of negative differential resistance (NDR) across the sheath is responsible for the observed hopping pattern. Due to NDR, the current density and potential drop behave as activator and inhibitor, the dynamic interactions of which govern the surface current density re-distribution and the formation of structured spots. In gliding arc discharges, new arc roots can form separately in front of the existing root(s), which happens periodically to constitute the stepwise hopping. From the instability phase-diagram analysis, the phenomenon that arc attachments tend to constrict itself spontaneously in the NDR regime is well explained. Furthermore, we demonstrate via a comprehensive magnetohydrodynamics (MHD) computation that the existence of a sheath NDR can successfully reproduce the arc hopping as observed in experiments. Therefore, this work uncovers the essential role of sheath NDR in the plasma-solid surface pattern formation and opens up a hitherto unexplored area of research for manipulating the plasma-solid interactions.

Inhibition of Radix Scutellariae flavones on carboxylesterase mediated activations of prodrugs

AimsCarboxylesterase (CES) plays an essential role in the hydrolysis of ester prodrugs. Our study explored the inhibitions of Radix Scutellariae flavones, including baicalein (B), baicalin (BG), wogonin (W), wogonoside (WG), oroxylin A (OXA) and oroxylin A-7-O-glucuronide (OAG), on CES-mediated hydrolysis of seven prodrugs (capecitabine, clopidogrel, mycophenolate mofetil, dabigatran etexilate, acetylsalicylic acid, prasugrel and irinotecan). Main methodsIn vitro screenings were developed by incubating the flavones with prodrugs in rat plasma, intestine S9 and liver S9. Docking simulations were conducted using AMDock v1.5.2. In vivo evaluations were performed in rats co-administered with the selected flavone and prodrug via oral gavage/intravenous administration for five consecutive days. Key findingsThe in vitro investigation showed that B and OXA demonstrated strongest inhibitions on the hydrolysis of irinotecan followed by dabigatran in rat plasma, intestine S9 and liver S9. Consistent results showed in the molecular docking analyses. Additionally, in rats receiving irinotecan, B/OXA intravenous and oral pre-treatments both led to reduction trends on the active metabolite SN-38 formation in plasma. Besides, significant decreases of SN-38/irinotecan plasma concentration ratios were found in the B/OXA oral pre-treatment group with quicker and stronger inhibition potential in OXA pre-treatment than that from B pre-treatment. OXA oral pre-treatment was also found to be able to significantly inhibit intestinal CES2 activities at 0.5 h and 5 h after irinotecan administration. SignificanceOur current findings for the first time alert on potential CES-mediated HDIs between RS flavones and prodrugs, which provide a constructive information referring to rational drug combinations in clinical practice.

Electron-to-ion Bulk Speed Ratio as a Parameter Reflecting the Occurrence of Strong Electron-dominated Current Sheets in the Solar Wind

Current sheets (CSs) are preferred sites of magnetic reconnection and energy dissipation in astrophysical plasmas. Electric currents in them may be carried by both electrons and ions. In our prior theoretical studies of the CS formation in turbulent plasmas, we utilized fully kinetic and hybrid code simulations with ions considered as particles and electrons—as a massless fluid. We found that electron-dominated CSs in which electrons become the main carriers of the electric current and contributors to energy dissipation may form inside or nearby ion-dominated CSs. These structures represent a distinguished type of CSs and should not be mixed up with so-called electron-scale CSs. Current simulations show that such CSs are characterized by the electron-to-ion bulk speed ratio (u e /u i ) increases that can be seen at ion scales according to theoretical predictions and high-resolution observations from the Magnetospheric Multiscale mission. Therefore, applying the u e /u i parameter to the solar wind data may allow locating the strongest electron-dominated CSs with an ordinary spacecraft resolution of 1−3 s. This study shows that, indeed, electron-dominated CSs observed during a period of quiet solar wind conditions at 1 au impact the surrounding plasma, which may be reflected in sharp changes of u e /u i . Electron-dominated CSs are found to be localized in the vicinity of ion-dominated CSs identified via changes in the magnetic field and plasma parameters, displaying the same clustering. We conclude that u e /u i may be used as one of the key parameters for statistical studies of CSs in the solar wind and analyzing the role of electrons in them.

Effect of plasma boundary and electrode asymmetry in planar DC discharge system

This paper present presents a detailed characterization and analysis of plasma formation using different anode sizes in two contrasting configurations in a planar DC discharge system. One configuration has a conducting boundary (CB) formed by the conducting wall of the vacuum chamber that acts as an extended cathode. The second configuration, the Small Volume Insulated Boundary (SVIB) with a volume 22.5 times smaller than the CB system, is realized by confining the plasma completely within a fully insulating boundary. Anode sizes may be equal to the cathode size (symmetric electrodes) or smaller (asymmetric electrodes). In general, CB discharges require much lower applied voltages, showing very little variation with the pressure. Although the symmetric CB discharges have only single electron population, the asymmetric electrode discharges exhibit two electron populations, a high-density bulk population (Te ∼ 2–3 eV) and a very low-density warm population (Tw ∼ 40 eV) that serves to enhance ionization and compensate for reduced anode size. In contrast, the SVIB discharges require high voltages, show considerable variation in discharge voltage both with pressure and anode size, and have higher densities. In addition, one finds two electron populations for all anode sizes. From estimates of the anode sheath drop, it is possible to show that all CB discharges have an electron-rich anode sheath for all anode sizes. In contrast, the SVIB discharges exhibit ion-rich anode sheaths for all anode sizes, although for small-sized anodes and high pressures the sheaths transform to an electron-rich sheath.

Effect of Coaxial Electrode Geometry on the Electric Field Enhancement Factor for a High Voltage Vacuum Gap

We present an experimental analysis of the change in the electric field enhancement factor with varying gap size and penetration depth (P.D) of cathode into anode for a coaxial vacuum gap, diagnosed using Fowler–Nordheim analysis and optical imaging via scanning electron microscope (SEM) and time integrated Digital single lens reflex camera (DSLR). Data were collected on the Coaxial Gap Breakdown Machine (240 A, 25 kV, 150 ns, ~0.1 Hz). Experiments using five different gap sizes at nine different P.Ds are compared over runs comprising 50 shots for each case. The results show a strong link between enhancement factor and gap size, with P.D and surface topology. For large gap sizes, 150, 330, and 700 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \mathrm {m}$ </tex-math></inline-formula> , the average enhancement factor value increases with increasing P.D. For smaller gap sizes, 50 and 100 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \mathrm {m}$ </tex-math></inline-formula> , the average enhancement factor decreases with P.D. SEM imaging before and after plasma formation for each gap size allows for quantifying surface finish, microprotrusion growth, average blast diameter, and an estimation of the surface area breakdowns occupy. Time integrated DSLR imaging analysis of the gap at each shot allows for a determination of the distribution of breakdowns about the circumference of the gap for each case tested. The Fowler–Nordheim analysis allows for a quantitative analysis of the surface roughness of all gap sizes tested. Results show that for large gap sizes, the gap geometry and increasing area of breakdown is the main cause for increasing average enhancement factor. For small gap sizes, the dominant driving factor for small average enhancement factors—that subsequently decrease with P.D—is significant changes in surface topology due to an increased number of breakdowns.

Cross-polarized common-path temporal interferometry for high-sensitivity strong-field ionization measurements.

Absolute density measurements of low-ionization-degree or low-density plasmas ionized by lasers are very important for understanding strong-field physics, atmospheric propagation of intense laser pulses, Lidar etc. A cross-polarized common-path temporal interferometer using balanced detection was developed for measuring plasma density with a sensitivity of ∼0.6 mrad, equivalent to a plasma density-length product of ∼2.6 × 1013 cm-2 if using an 800 nm probe laser. By using this interferometer, we have investigated strong-field ionization yield versus intensity for various noble gases (Ar, Kr, and Xe) using 800 nm, 55 fs laser pulses with both linear (LP) and circular (CP) polarization. The experimental results were compared to the theoretical models of Ammosov-Delone-Krainov (ADK) and Perelomov-Popov-Terent'ev (PPT). We find that the measured phase change induced by plasma formation can be explained by the ADK theory in the adiabatic tunneling ionization regime, while PPT model can be applied to all different regimes. We have also measured the photoionization and fractional photodissociation of molecular (MO) hydrogen. By comparing our experimental results with PPT and MO-PPT models, we have determined the likely ionization pathways when using three different pump laser wavelengths of 800 nm, 400 nm, and 267 nm.

Low Frequency Oscillations of Plasma Injected into an Open Magnetic Trap from an Independent Ultrahigh Frequency Source

Methods of plasma heating by electromagnetic waves, interaction of electro-magnetic waves with magnetoactive plasma, plasma turbulence and transport processes are investigated in the trap of the linear plasma device OMT2. An ultrahigh frequency (UHF) contactless method is used to fill the open magnetic trap by plasma. We propose a new method of open magnetic trap filling with plasma whereby plasma is injected in the trap along the magnetic field from an independent stationary UHF source. The source is located outside the trap and plasma formation in it takes place in the strongly nonuniform magnetic field, in the electron cyclotron resonance (ECR) regime by means of the UHF power. In this paper we study the efficiency of open magnetic trap filling by plasma and plasma low frequency (LF) oscillations in the trap.

Runaway electron generation during tokamak start-up

Tokamak start-up is characterized by low electron densities and strong electric fields, in order to quickly raise the plasma current and temperature, allowing the plasma to fully ionize and magnetic flux surfaces to form. Such conditions are ideal for the formation of superthermal electrons, which may reduce the efficiency of ohmic heating and prevent the formation of a healthy thermal fusion plasma. This is of particular concern in ITER where engineering limitations put restrictions on the allowable electric fields and limit the prefill densities during start-up. In this study, we present a new 0D burn-through simulation tool called STREAM (STart-up Runaway Electron Analysis Model), which self-consistently evolves the plasma density, temperature and electric field, while accounting for the generation and loss of relativistic runaway electrons. After verifying the burn-through model, we investigate conditions under which runaway electrons can form during tokamak start-up as well as their effects on the plasma initiation. We find that Dreicer generation plays a crucial role in determining whether a discharge becomes runaway-dominated or not, and that a large number of runaway electrons could limit the ohmic heating of the plasma, thus preventing successful burn-through or further ramp-up of the plasma current. The runaway generation can be suppressed by raising the density via gas fuelling, but only if done sufficiently early. Otherwise a large runaway seed may have already been built up, which can avalanche even at relatively low electric fields and high densities.

High-order harmonics generation in the laser-induced lead-free perovskites-containing plasmas

High-order harmonics generation in the laser-induced plasmas produced on the surfaces of lead-free perovskites is studied. We analyze the harmonics generation in (CH3NH3)2CuCl4 and (CH3NH3)2CuBr4 plasmas during their ablation by the femtosecond, picosecond, and nanosecond pulses. The modifications of the high-order harmonics spectra are studied using the -color pump scheme (800 nm and 400 nm, 40 fs pulses). The influence of the variations of laser chirp and pulse duration on the dynamics of high-order harmonics generation is examined. The spectral shift, chirp-related harmonic cutoff scaling, and the role of the pulse duration of converting and heating laser radiation are examined at different conditions of plasma formation and harmonic generation. The dependencies of the pulse duration and the fluence of heating pulses on the harmonic’s blue shift are found. The effect of harmonics broadening and splitting on the two red- and blue-shifted components is demonstrated.