Showing papers in "Plasma Sources Science and Technology in 2012"
TL;DR: In this article, the development of APNP-J is reviewed and a discussion on the interaction between plasma jets is presented. And the most interesting phenomena of the plasma bullet is discussed and its behavior described.
Abstract: Atmospheric-pressure non-equilibrium plasma jets (APNP-Js), which generate plasma in open space rather than in a confined discharge gap, have recently been a topic of great interest. In this paper, the development of APNP-Js will be reviewed. Firstly, the APNP-Js are grouped based on the type of gas used to ignite them and their characteristics are discussed in detail. Secondly, one of the most interesting phenomena of APNP-Js, the ?plasma bullet?, is discussed and its behavior described. Thirdly, the very recent developments on the behavior of plasma jets when launched in a controlled environment and pressure are also introduced. This is followed by a discussion on the interaction between plasma jets. Finally, perspectives on APNP-J research are presented.
664 citations
TL;DR: An overview of the general characteristics of atmospheric pressure plasmas and a brief summary of their biomedical applications, and a wide range of these sources developed for biomedical applications are reviewed in this paper.
Abstract: Atmospheric-pressure plasmas (APPs) have attracted great interest and have been widely applied in biomedical applications, as due to their non-thermal and reactive properties, they interact with living tissues, cells and bacteria. Various types of plasma sources generated at atmospheric pressure have been developed to achieve better performance in specific applications. This article presents an overview of the general characteristics of APPs and a brief summary of their biomedical applications, and reviews a wide range of these sources developed for biomedical applications. The plasma sources are classified according to their power sources and cover a wide frequency spectrum from dc to microwaves. The configurations and characteristics of plasma sources are outlined and their biomedical applications are presented.
419 citations
TL;DR: In this article, the authors review the new functions of plasmas as metamaterials, including a photonic-crystal-like behavior, a negative refractive index state and a nonlinear bifurcated electric response.
Abstract: When we form a structure of plasmas distributed in a certain space in which electromagnetic waves propagate, such a plasma structure serves as a different medium from a homogeneous bulk plasma. We can also enhance or generate novel functions of the plasmas when we add other structural materials such as functional components. That is to say, when we estimate such a medium from the material properties such as permittivity, permeability and conductivity, it shows extraordinary and/or functional effects that arise from the synthesis of the structure. We call such an artificial material a plasma metamaterial. In this review, starting from a fundamental understanding of electromagnetic wave propagation in and around plasmas, we review the new functions of plasmas as metamaterials, including a photonic-crystal-like behavior, a negative refractive index state and a nonlinear bifurcated electric response, by describing specific plasma structures. In addition, we survey some specific applications of such media and predict a feasible scientific expansion of this field in the near future.
258 citations
TL;DR: In this article, the influence of humid air on reactive species in radio frequency (rf)-driven atmospheric-pressure helium?oxygen mixture plasmas (He?O2, helium with 5000?ppm admixture of oxygen) for wide air impurity levels of 0?500ppm with relative humidities of from 0% to 100% was explored.
Abstract: In most applications helium-based plasma jets operate in an open-air environment. The presence of humid air in the plasma jet will influence the plasma chemistry and can lead to the production of a broader range of reactive species. We explore the influence of humid air on the reactive species in radio frequency (rf)-driven atmospheric-pressure helium?oxygen mixture plasmas (He?O2, helium with 5000?ppm admixture of oxygen) for wide air impurity levels of 0?500?ppm with relative humidities of from 0% to 100% using a zero-dimensional, time-dependent global model. Comparisons are made with experimental measurements in an rf-driven micro-scale atmospheric pressure plasma jet and with one-dimensional semi-kinetic simulations of the same plasma jet. These suggest that the plausible air impurity level is not more than hundreds of ppm in such systems. The evolution of species concentration is described for reactive oxygen species, metastable species, radical species and positively and negatively charged ions (and their clusters). Effects of the air impurity containing water humidity on electronegativity and overall plasma reactivity are clarified with particular emphasis on reactive oxygen species.
251 citations
TL;DR: In this article, a simulation study of streamer propagation in a cold, atmospheric-pressure, helium jet in ambient air is presented, where a self-consistent, multi-species, mult-temperature plasma model with detailed finite-rate chemistry and photoionization effects is used to provide fundamental insights into the structure and dynamics of the streamers.
Abstract: A computational modeling study of streamer propagation in a cold, atmospheric-pressure, helium jet in ambient air is presented. A self-consistent, multi-species, multi-temperature plasma model with detailed finite-rate chemistry and photoionization effects is used to provide fundamental insights into the structure and dynamics of the streamers. A parametric study of the streamer properties as a function of important discharge geometric and operating conditions is performed. The fluid mechanical mixing layer between the helium jet core and the ambient air is instrumental in guiding the propagation direction of the streamer and gives the plasma jet a visibly collimated appearance. The key chemical reactions which drive the streamer propagation are electron-impact ionization of helium neutral and nitrogen molecules. Photoionization plays a role in enhancing the propagation speed of the streamer, but is not necessary to sustain the streamer. The streamer yields a large radical concentration through chemical reactions in the streamer head and the body. The streamer propagation speed increases with reduced helium jet radius and increased helium–air mixing layer width. Impurities in the helium jet result in a significant increase in the discharge propagation speed within the tube through photoionization, but not after the streamer propagates into the open ambient region. It is also observed that thinner electrodes produce stronger electric-field concentrations that increase discharge propagation speeds within the tube but have a smaller influence on the discharge after it emerges out of the tube as a streamer.
221 citations
TL;DR: In this paper, the authors used laser scattering on an argon atmospheric pressure microwave plasma jet operating in an air environment to determine profiles of the electron temperature, electron density, gas temperature, partial air pressure and the N2/O2 ratio with a spatial resolution of 50 µm.
Abstract: Laser scattering provides a very direct method for measuring the local densities and temperatures inside a plasma. We present new experimental results of laser scattering on an argon atmospheric pressure microwave plasma jet operating in an air environment. The plasma is very small so a high spatial resolution is required to study the effect of the penetration of air molecules into the plasma. The scattering signal has three overlapping contributions: Rayleigh scattering from heavy particles, Thomson scattering from free electrons and Raman scattering from molecules. The Rayleigh scattering signal is filtered out optically with a triple grating spectrometer. The disentanglement of the Thomson and Raman signals is done with a newly designed fitting method. With a single measurement we determine profiles of the electron temperature, electron density, gas temperature, partial air pressure and the N2/O2 ratio, with a spatial resolution of 50 µm, and including absolute calibration.
197 citations
TL;DR: In this article, the conversion of CO2 to carbon monoxide (CO) and oxygen in an atmospheric pressure microwave plasma/catalyst system was investigated, where the CO2 gas was first treated with plasma alone, and then treated with a combination of plasma and rhodium (Rh) catalyst material.
Abstract: The continual and increasing use of fossil fuels throughout the world has advanced concerns of atmospheric carbon dioxide (CO2) concentrations, causing a swell of scientific interest to ease the predicted effects of global warming. This work experimentally investigates the conversion of CO2 to carbon monoxide (CO) and oxygen in an atmospheric pressure microwave plasma/catalyst system. Diagnostics such as mass spectrometry and optical emission spectroscopy are used to identify the gas species present after plasma treatment and to measure plasma temperatures. The CO2 gas is first treated with plasma alone, and is then treated with a combination of plasma and rhodium (Rh) catalyst material. While the plasma system alone is able to achieve a 20% energy efficiency, the Rh catalyst actually causes a drop in efficiency due to reverse reactions occurring on the surface. The plasma temperature measurements indicate thermal equilibrium between Tr and Tv around 6000–7000 K.
167 citations
TL;DR: In this paper, an experimental study of atmospheric-pressure rare gas plasma propagation in a high-aspect-ratio capillary is reported, where the plasma is generated with a plasma gun device based on a dielectric barrier discharge (DBD) reactor powered by either nanosecond or microsecond rise-time high-voltage pulses at single-shot to multi-kHz frequencies.
Abstract: An experimental study of atmospheric-pressure rare gas plasma propagation in a high-aspect-ratio capillary is reported. The plasma is generated with a plasma gun device based on a dielectric barrier discharge (DBD) reactor powered by either nanosecond or microsecond rise-time high-voltage pulses at single-shot to multi-kHz frequencies. The influence of the voltage waveform, pulse polarity, pulse repetition rate and capillary material have been studied using nanosecond intensified charge-coupled device imaging and plasma-front velocity measurements. The evolution of the plasma appearance during its propagation and the study of the role of the different experimental parameters lead us to suggest a new denomination of pulsed atmospheric-pressure plasma streams to describe all the plasma features, including the previously so-called plasma bullet. The unique properties of such non-thermal plasma launching in capillaries, far from the primary DBD plasma, are associated with a fast ionization wave travelling with velocity in the 107?108?cm?s?1 range. Voltage pulse tailoring is shown to allow for a significant improvement of such plasma delivery. Thus, the plasma gun device affords unique opportunities in biomedical endoscopic applications.
153 citations
TL;DR: In this article, numerical investigations of dielectric barrier discharges (DBDs) have been performed in three basic configurations: in the volume, coplanar and surface discharge arrangements.
Abstract: Based on experimental results, numerical investigations of dielectric barrier discharges (DBDs) have been performed in three basic configurations: in the volume, coplanar and surface discharge arrangements. It is shown that the DBD dynamics is the same in all arrangements and it is determined by the development of a few principal constituents, i.e. cathode- and anode-directed streamers, discharge channel, cathode layer and surface charges. It is found that the anode- and cathode-directed streamers appear with a highly conductive channel in between. The interaction of the streamers with conductive and dielectric surfaces determines the filamentary or homogeneous appearance of the discharge and its properties. The cathode-directed streamer is a self-sustaining phenomenon, which moves in a gas gap or along an electrode driven by a positive loop-back between photoemission and electron multiplication. The anode-directed streamer plays a subsidiary role. Depending on the kind of gas (electronegative or electropositive) and/or the degree of development of the cathode-directed streamer, the field strength in the conductive channels changes significantly. When the cathode-directed streamer touches the electrode surface, a cathode layer appears with parameters close to those of normal glow discharges. In volume discharge arrangements the movement of the streamers results in the appearance of Lichtenberg figures on dielectric surfaces.
146 citations
TL;DR: In this article, a numerical investigation of the fundamental properties of ionization waves generated by nanosecond voltage pulses inside a 15cm long, 600 µm wide (aspect ratio 250), flexible dielectric channel was conducted.
Abstract: Atmospheric pressure ionization waves (IWs) propagating in flexible capillary tubes are a unique way of transporting a plasma and its active species to remote sites for applications such as biomedical procedures, particularly in endoscopic procedures The propagation mechanisms for such IWs in tubes having aspect ratios of hundreds to thousands are not clear In this paper, results are discussed from a numerical investigation of the fundamental properties of ionization waves generated by nanosecond voltage pulses inside a 15cm long, 600 µm wide (aspect ratio 250), flexible dielectric channel The channel, filled with a Ne/Xe = 999/01 gas mixture at 1atm, empties into a small chamber separated from a target substrate by 1cm The IWs propagate through the entire length of the channel while maintaining similar strength and magnitude Upon exiting the channel into the chamber, the IW induces a second streamer discharge at the channel-chamber junction This streamer then propagates across the chamber and impinges upon the target The average speeds of the capillary-bounded IW are about 5 × 10 7 cms −1 and 1 × 10 8 cms −1 for positive and negative polarities, respectively The propagation speed is sensitive to the curvature of the channel In both cases, the peak in ionization tends to be located along the channel walls and alternates from side-to-side depending on the direction of the local instantaneous electric field and curvature of the channel The ionization region following the IW extends up to several centimeters inside the channel, as opposed to being highly localized at the ionization front in unconstrained, atmospheric pressure IWs The maximum speed of the IW in the chamber is about twice that in the channel (Some figures may appear in colour only in the online journal)
138 citations
TL;DR: In this article, a self-similar, local ionization kinetic model predicting energy coupling to the plasma in a surface ionization wave discharge produced by a nanosecond voltage pulse has been developed.
Abstract: Nanosecond pulse discharge plasma imaging, coupled pulse energy measurements, and kinetic modeling are used to analyze the mechanism of energy coupling in high repetition rate, spatially uniform, nanosecond pulse discharges in air in plane-to-plane geometry. Under these conditions, coupled pulse energy scales nearly linearly with pressure (number density), with energy coupled per molecule being nearly constant, in good agreement with the kinetic model predictions. In spite of high-peak reduced electric field reached before breakdown, E/N ∼ 500–700 Td, the reduced electric field in the plasma after breakdown is much lower, E/N ∼ 50–100 Td, predicting that a significant fraction of energy coupled to the air plasma, up to 30–40%, is loaded into nitrogen vibrational mode.A self-similar, local ionization kinetic model predicting energy coupling to the plasma in a surface ionization wave discharge produced by a nanosecond voltage pulse has been developed. The model predicts key discharge parameters such as ionization wave speed and propagation distance, electric field, electron density, plasma layer thickness, and pulse energy coupled to the plasma, demonstrating good qualitative agreement with experimental data and two-dimensional kinetic modeling calculations. The model allows an analytic solution and lends itself to incorporation into existing compressible flow codes, at very little computational cost, for in-depth analysis of the nanosecond discharge plasma flow control mechanism. The use of the model would place the main emphasis on coupling of localized thermal perturbations produced by the discharge with the flow via compression waves and would provide quantitative insight into the flow control mechanism on a long time scale.
TL;DR: In this article, two-photon absorption laser-induced fluorescence (TALIF) spectroscopy measurements are performed in a cold argon/oxygen atmospheric pressure plasma jet.
Abstract: By investigating the atomic oxygen density in its effluent, two-photon absorption laser-induced fluorescence (TALIF) spectroscopy measurements are for the first time performed in a cold argon/oxygen atmospheric pressure plasma jet. The measurements are carried out in ambient air and quenching by inflowing air species is considered. We propose a novel absorption technique in the VUV spectral range, where emission originating from within the discharge is used as light source to determine the inflow of atmospheric oxygen into the effluent. Furthermore, we propose a modelling solution for the on-axis density of inflowing ambient air based on the stationary convection?diffusion equation.
TL;DR: In this paper, relative and absolute optical emission spectroscopy (OES) was performed for the helium and hydrogen lines for the determination of electron density through the Stark broadening mechanism, and a superposition fitting method composed of two component profiles corresponding to two different electron densities was developed to fit the investigated lines.
Abstract: The characteristics of plasma temperatures (gas temperature and electron excitation temperature) and electron density in a pulsed-dc excited atmospheric helium plasma jet are studied by relative and absolute optical emission spectroscopy (OES). High-resolution OES is performed for the helium and hydrogen lines for the determination of electron density through the Stark broadening mechanism. A superposition fitting method composed of two component profiles corresponding to two different electron densities is developed to fit the investigated lines. Electron densities of the orders of magnitude of 1021 and 1020 m−3 are characterized for the center and edge regions in the jet discharge when the applied voltage is higher than 13.0 kV. The atomic state distribution function (ASDF) of helium demonstrates that the discharge deviates from the Boltzmann–Saha equilibrium state, especially for the helium lower levels, which are significantly overpopulated. Local electron excitation temperatures T13 and Tspec corresponding to the lower and upper parts of the helium ASDF are defined and found to range from 1.2 eV to 1.4 eV and 0.2 eV to 0.3 eV, respectively. A comparative analysis shows that the Saha balance is valid in the discharge for helium atoms at high excited states.
TL;DR: In this article, a numerical model for the simulation of the ionization wave dynamics is presented, where the dominant interactions between electron and main ions present in He-air mixtures with their associated basic data are taken into account.
Abstract: This work is devoted to fluid modeling based on experimental investigations of a classical setup of a low-temperature plasma jet. The latter is generated at atmospheric pressure using a quartz tube of small diameter crossed by helium gas flow and surrounded by an electrode system powered by a mono-polar high-voltage pulse. The streamer-like behavior of the fast plasma bullets or ionization waves launched in ambient air for every high-voltage pulse, already emphasized in the literature from experimental or analytical considerations or recent preliminary fluid models, is confirmed by a numerical one-moment fluid model for the simulation of the ionization wave dynamics. The dominant interactions between electron and the main ions present in He–air mixtures with their associated basic data are taken into account. The gradual dilution of helium in air outside the tube along the axis is also considered using a gas hydrodynamics model based on the Navier–Stokes equation assuming a laminar flow. Due to the low magnitude of the reduced electric field E/N (not exceeding 15 Td), it is first shown that consideration of the stepwise ionization of helium metastables is required to reach the critical size of the electron avalanches in order to initiate the formation of ionization waves. It is also shown that a gas pre-ionization ahead of the wave front of about 109 cm−3 (coming from Penning ionization without considering the gas photo-ionization) is required for the propagation. Furthermore, the second ionization wave experimentally observed during the falling time of the voltage pulse, between the powered electrode and the tube exit, is correlated with the electric field increase inside the ionized channel in the whole region between the electrode and the tube exit. The propagation velocity and the distance traveled by the front of the ionization wave outside the tube in the downstream side are consistent with the present experimental measurements. In comparison with the streamer dynamics in a classical corona discharge, it is shown that under the same gas composition the plasma jet ionization waves propagate with a lower velocity (about 5 times), and have a higher diameter (at least 10 times) and a lower plasma density (at least 100 times).
TL;DR: In this article, the authors used a very high resolution spectrometer (2.6 pm resolution) to estimate the absolute density of atmospheric pressure plasmas by broadband UV absorption in diffuse atmospheric-pressure RF glow discharges.
Abstract: The measurement of radical densities in atmospheric-pressure plasmas has gained increasing importance in recent years in view of their crucial role in many applications. In this paper we present absolute OH density measurements by broadband UV absorption in diffuse atmospheric-pressure RF glow discharges in mixtures of He and H2O. The use of a 310 nm light-emitting diode as a light source and a very high resolution spectrometer (2.6 pm resolution) made the estimation of the total OH density possible by simultaneously measuring the absorption rates of different spectrally resolved rotational lines of the OH(A–X) transition. For different RF powers and water concentrations, OH densities and gas temperatures ranging between 6 × 1019and 4 × 1020 m−3 and 345 and 410 K, respectively, were obtained. The gas temperature Tg was also measured by three different methods. Tg deduced from the rotational temperature of N2(C–B) emission, nitrogen being present as a trace impurity, provided the most reliable value. The rotational temperature Tr of the ground state OH(X) presented values with a maximum deviation of 25 K compared with Tg. To obtain the gas temperature from the emission intensities of OH(A–X) rotational lines, the recorded intensities of different lines must be corrected for the effect of self-absorption inside the plasma.
TL;DR: In this article, the presence of plasma bullets in a MHz argon atmospheric pressure plasma jet was confirmed by phase-resolved optical emission measurements, and the ozone production rates were investigated by two independent diagnostic techniques yielding complementary results.
Abstract: This study for the first time confirms the presence of plasma bullets in a MHz argon atmospheric pressure plasma jet. Bullet characteristics are investigated by phase-resolved optical emission measurements. Regarding the jet's reactive component output, its ozone production rates are investigated by two independent diagnostic techniques yielding complementary results. The first method—UV-absorption spectroscopy in the Hartley band—determines space-resolved distribution of the ozone concentration in the jet effluent. The second method—quantum cascade laser-absorption spectroscopy in the mid-infrared spectral region—yields high sensitivity results of the average ozone concentration in a multipass cell, in which the effluent is directed. The results of both diagnostic techniques show excellent agreement.
TL;DR: In this paper, a nanosecond surface dielectric barrier discharge (SDBD) initiated by negative or positive polarity pulses 10?15?kV in amplitude in a cable, 25?30?ns FWHM, 5?ns rise time, in the regime of a single shot or 3?Hz repetitive frequency was studied.
Abstract: We study a nanosecond surface dielectric barrier discharge (SDBD) initiated by negative or positive polarity pulses 10?15?kV in amplitude in a cable, 25?30?ns FWHM, 5?ns rise time, in the regime of a single shot or 3?Hz repetitive frequency. Discharge parameters, namely spatial structure of the discharge and time- and space-resolved electric field are studied in a N2?:?O2?=?4?:?1 mixture for P?=?1?5?atm. The possibility of igniting a combustible mixture with the help of an SDBD is demonstrated using the example of a stoichiometric C2H6?:?O2 mixture at ambient initial temperature and at 1?atm pressure. Flame propagation and ignited volume as a function of time are compared experimentally for two discharge geometries: SDBD and pin-to-pin configurations at the same shape and amplitude of the incident pulse. It is shown that the SDBD can be considered as a multi-point ignition system with maximum energy release near the high-voltage electrode. Numerical modeling of the discharge and subsequent combustion kinetics for the SDBD conditions is performed. The discharge action leads to the production of atoms and radicals as well as to fast gas heating, due to the relaxation of electronic and vibrational degrees of freedom. The calculated ignition delay time is in reasonable agreement with the experimental results.
TL;DR: In this paper, a review of work performed over the past ten years in France, centered on the utilization of laser-induced fluorescence (LIF) spectroscopy to diagnose the low-pressure magnetized dc discharge of a Hall thruster (HT), is presented.
Abstract: This article presents a review of work performed over the past ten years in France, centered on the utilization of laser-induced fluorescence (LIF) spectroscopy to diagnose the low-pressure magnetized dc discharge of a Hall thruster (HT). The latter is a gridless electric propulsion device in a crossed electric and magnetic field configuration, which is used onboard satellites and space probes for various types of maneuvers. Although the design of a HT is relatively simple, the physical mechanisms that govern thrust generation and efficiency are not yet fully understood. Characterization of the ion and atom velocity distribution function (VDF) appears to be a powerful way to obtain insights into the underlying physics. The VDF of xenon and krypton—the most common propellants—is therefore locally interrogated by means of LIF on excited levels. In this review emphasis is placed on time-averaged and time-resolved continuous-wave LIF measurements, associated quantities and recent outcomes. Results will be presented concerning a variety of phenomena: velocity vector field structuring, ion population interaction, electric field generation, ion magnetic drift, apparent atom acceleration, interaction of the plasma plume with background gas and low-frequency electric field oscillations, to name only a few.
TL;DR: In this article, the authors reported insights into the dynamics of atmospheric-pressure low-temperature plasma jets (APLTPJ) using a Pearson current monitor for different operating conditions.
Abstract: In this study, we report insights into the dynamics of atmospheric-pressure low-temperature plasma jets (APLTPJs). The plasma jet current was measured by a Pearson current monitor for different operating conditions. These jet current measurements confirmed a proposed photo-ionization model based on streamer theory. Our results are supported by intensified charged-couple device camera observations. It was found that a secondary discharge ignition, arising from the positive high-voltage electrode, causes the inhibition of plasma bullet propagation. Our observations also showed the existence of an ionization channel between the APLTPJ reactor and the plasma bullet. In addition, the maximum electron density along the plasma jet was estimated using Ohm's law, and an empirical relationship was derived between the plasma bullet velocity and the plasma bullet area.
TL;DR: MAGPIE as discussed by the authors is a helicon source plasma device operating in a magnetic hill configuration designed to support a broad range of research activity and is the first stage of the Materials Diagnostic Facility at the Australian National University.
Abstract: The Magnetized Plasma Interaction Experiment (MAGPIE) is a versatile helicon source plasma device operating in a magnetic hill configuration designed to support a broad range of research activity and is the first stage of the Materials Diagnostic Facility at the Australian National University. Various material targets can be introduced to study a range of plasma–material interaction phenomena.Initially, with up to 2.1 kW of RF at 13.56 MHz, argon (1018–1019 m−3) and hydrogen (up to 1019 m−3 at 20 kW) plasma with electron temperature ∼3–5 eV was produced in magnetic fields up to ∼0.19 T. For high mirror ratio we observe the formation of a bright blue core in argon above a threshold RF power of 0.8 kW. Magnetic probe measurements show a clear m = +1 wave field, with wavelength smaller than or comparable to the antenna length above and below this threshold, respectively. Spectroscopic studies indicate ion temperatures <1 eV, azimuthal flow speeds of ∼1 km s−1 and axial flow near the ion sound speed.
TL;DR: Time-resolved tunable diode-laser absorption spectroscopy measurements were performed on the argon metastable (Ar-m) level 3s(2)3p(5)(P-2(3/2)degrees)4s excited at 801.478 nm, in the dense plasma r...
Abstract: Time-resolved tunable diode-laser absorption spectroscopy measurements were performed on the argon metastable (Ar-m) level 3s(2)3p(5)(P-2(3/2)degrees)4s excited at 801.478 nm, in the dense plasma r ...
TL;DR: In this paper, a two-temperature chemically non-equilibrium model describing in a selfconsistent manner the heat transfer, the plasma chemistry, the electric and magnetic field in a high-current free-burning arc in argon has been developed.
Abstract: A two-temperature chemically non-equilibrium model describing in a self-consistent manner the heat transfer, the plasma chemistry, the electric and magnetic field in a high-current free-burning arc in argon has been developed. The model is aimed at unifying the description of a thermionic tungsten cathode, a flat copper anode, and the arc plasma including the electrode sheath regions. The heat transfer in the electrodes is coupled to the plasma heat transfer considering the energy fluxes onto the electrode boundaries with the plasma. The results of the non-equilibrium model for an arc current of 200 A and an argon flow rate of 12 slpm are presented along with results obtained from a model based on the assumption of local thermodynamic equilibrium (LTE) and from optical emission spectroscopy. The plasma shows a near-LTE behaviour along the arc axis and in a region surrounding the axis which becomes wider towards the anode. In the near-electrode regions, a large deviation from LTE is observed. The results are in good agreement with experimental findings from optical emission spectroscopy.
TL;DR: Model studies of 400 mu s long discharge pulses in high-power impulse magnetron sputtering have been made to study the gas dynamics and plasma chemistry in this type of pulsed processing plasma as discussed by the authors.
Abstract: Model studies of 400 mu s long discharge pulses in high-power impulse magnetron sputtering have been made to study the gas dynamics and plasma chemistry in this type of pulsed processing plasma. Da ...
TL;DR: In this paper, the effluent of a He/O2 microplasma jet has been analyzed in a helium and in an air atmosphere by molecular beam mass spectrometry.
Abstract: Microplasma jets are commonly used to treat samples in ambient air. The effect of admixing air into the effluent may severely affect the composition of the emerging species. Here, the effluent of a He/O2 microplasma jet has been analyzed in a helium and in an air atmosphere by molecular beam mass spectrometry. First, the composition of the effluent in air was recorded as a function of the distance to determine how fast air admixes into the effluent. Then, the spatial distribution of atomic oxygen and ozone in the effluent was recorded in ambient air and compared with measurements in a helium atmosphere. Additionally, a fluid model of the gas flow with reaction kinetics of reactive oxygen species in the effluent was constructed. In ambient air, the O density declines only slightly faster with distance compared with a helium atmosphere. In contrast, the O3 density in ambient air increases significantly faster with distance compared with a helium atmosphere. This unexpected behavior cannot be explained by simple recombination reactions of O atoms with O2 molecules. A reaction scheme involving the reaction of plasma-produced excited species of unknown identity with ground state O2 molecules is proposed as a possible explanation for these observations.
TL;DR: In this article, a simplified configuration with planar symmetry was chosen as a vehicle to develop a physics-based model of nanosecond-pulse discharges, including realistic air kinetics, electron energy transport, and compressible bulk gas flow.
Abstract: Recent experiments with a nanosecond-pulse, dielectric barrier discharge at the stagnation point of a Mach 5 cylinder flow have demonstrated the formation of weak shock waves near the electrode edge, which propagate upstream and perturb the bow shock. This is a promising means of flow control, and understanding the detailed physics of the conversion of electrical energy into gas motion will aid in the design of efficient actuators based on the concept. In this work, a simplified configuration with planar symmetry was chosen as a vehicle to develop a physics-based model of nanosecond-pulse discharges, including realistic air kinetics, electron energy transport, and compressible bulk gas flow. A reduced plasma kinetic model (23 species and 50 processes) was developed to capture the dominant species and reactions for energy storage and thermalization in the discharge. The kinetic model included electronically and vibrationally excited species, and several species of ions and ground state neutrals. The governing equations included the Poisson equation for the electric potential, diffusion equations for each neutral species, conservation equations for each charged species, and mass-averaged conservation equations for the bulk gas flow. The results of calculations with this model highlighted the path of energy transfer in the discharge. At breakdown, the input electrical energy was transformed over a time scale on the order of 1?ns into chemical energy of ions, dissociation products, and vibrationally and electronically excited particles. About 30% of this energy was subsequently thermalized over a time scale of 10??s. Since the thermalization time scale was faster than the acoustic time scale, the heat release led to the formation of weak shock waves originating near the sheath edge, consistent with experimental observations. The computed translational temperature rise (40?K) and nitrogen vibrational temperature rise (370?K) were of the same order of magnitude as experimental measurements (50?K and 500?K, respectively), and the approach appears promising for future multi-dimensional calculations. The effectiveness of flow control actuators based on nanosecond-pulse, dielectric barrier discharges is seen to depend crucially on the rapid thermalization of input energy, in particular the rate of quenching of excited electronic states and the rate of electron?ion recombination.
TL;DR: In this article, the transient spark (TS) is a streamer-to-spark transition discharge with short spark duration, based on charging and discharging of the internal capacity of the electric circuit with repetition frequency 1?10?kHz.
Abstract: We present a study of the streamer-to-spark transition in a self-pulsing dc-driven discharge called a transient spark (TS). The TS is a streamer-to-spark transition discharge with short spark duration (?10?100?ns), based on charging and discharging of the internal capacity of the electric circuit with repetition frequency 1?10?kHz. The TS can be maintained under relatively low energy conditions (0.1?1?mJ?pulse?1). It generates a very reactive non-equilibrium air plasma applicable for flue gas cleaning or bio-decontamination.Thanks to the short spark current pulse duration, the steady-state gas temperature, measured at the beginning of the streamers initiating the TS, increases from an initial value of ?300?K only up to ?550?K at 10?kHz. The streamer-to-spark transition is governed by the subsequent increase in the gas temperature in the plasma channel up to ?1000?K. This breakdown temperature does not change with increasing repetition frequency f. The heating after the streamer accelerates with increasing f, leading to a decrease in the average streamer-to-spark transition time from a few ?s to less than 100?ns.
TL;DR: In this article, particle-in-cell simulations and semi-analytical modeling were used to investigate the electric field and ionization dynamics in capacitively coupled radio frequency (RF) atmospheric pressure microplasmas operated in helium.
Abstract: Electron heating and ionization dynamics in capacitively coupled radio frequency (RF) atmospheric pressure microplasmas operated in helium are investigated by particle-in-cell simulations and semi-analytical modeling. A strong heating of electrons and ionization in the plasma bulk due to high bulk electric fields are observed at distinct times within the RF period. Based on the model the electric field is identified to be a drift field caused by a low electrical conductivity due to the high electron?neutral collision frequency at atmospheric pressure. Thus, the ionization is mainly caused by ohmic heating in this ??-mode?. The phase of strongest bulk electric field and ionization is affected by the driving voltage amplitude. At high amplitudes, the plasma density is high, so that the sheath impedance is comparable to the bulk resistance. Thus, voltage and current are about 45? out of phase and maximum ionization is observed during sheath expansion with local maxima at the sheath edges. At low driving voltages, the plasma density is low and the discharge becomes more resistive, resulting in a smaller phase shift of about 4?. Thus, maximum ionization occurs later within the RF period with a maximum at the discharge center. Significant analogies to electronegative low-pressure macroscopic discharges operated in the drift-ambipolar mode are found, where similar mechanisms induced by a high electronegativity instead of a high collision frequency have been identified.
TL;DR: The lower deposition rate for high power impulse magnetron sputtering (HiPIMS) compared with direct current magnetron (CCM) sputtering for the same average power is often reported as a drawback as discussed by the authors.
Abstract: The lower deposition rate for high power impulse magnetron sputtering (HiPIMS) compared with direct current magnetron sputtering for the same average power is often reported as a drawback The ofte
TL;DR: In this paper, the authors describe relevant models and their applications and address the importance of electron-impact excitation cross sections, towards gaining a quantitative understanding of the phenomena in question.
Abstract: Electrons from the Sun, often accelerated by magnetospheric processes, produce low-density plasmas in the upper atmospheres of planets and their satellites. The secondary electrons can produce further ionization, dissociation and excitation, leading to enhancement of chemical reactions and light emission. Similar processes are driven by photoelectrons produced by sunlight in upper atmospheres during daytime. Sunlight and solar electrons drive the same processes in the atmospheres of comets. Thus for both understanding of planetary atmospheres and in predicting emissions for comparison with remote observations it is necessary to simulate the processes that produce upper atmosphere plasmas. In this review, we describe relevant models and their applications and address the importance of electron-impact excitation cross sections, towards gaining a quantitative understanding of the phenomena in question.
TL;DR: The experimental evidence presented in this work does not support the model of the electrically insulated streamer head, but it is shown that the electrode potential is transferred to the streamers head along the streamer column to which it is attached with no significant voltage drop.
Abstract: This work presents a simple method for the characterization of streamers developing in cold atmospheric plasma jets. The method is based upon stopping (‘scattering’) of streamers by means of an external dc potential in order to determine the potential of the streamer head. The experimental evidence presented in this work does not support the model of the electrically insulated streamer head. In fact, it shows that the electrode potential is transferred to the streamer head along the streamer column to which it is attached with no significant voltage drop. Based on the proposed method, we determine various streamer parameters such as head charge ((1–2) × 108 electrons), electrical field in the head vicinity (about 100 kV cm−1), average conductivity (10−2 Ω−1 cm−1) and plasma density of the streamer column (2 × 1013 cm−3).