Dc Discharge In Argon Research Articles

An effective approach for aerosol dynamics modeling in dusty plasma

In this paper, we demonstrate the application of the Dirac-lognormal bimodal moment approach to investigate aerosol dynamics in dusty plasmas. This approach was evaluated by comparison with the detailed and computationally expensive sectional approach for two dusty plasma systems. The first one is a sputtering Argon DC discharge and the second is a capacitively coupled Ar/C2H2 radio-frequency (CCRF) discharge. The results obtained by the bimodal and the sectional approaches are in good agreement for the sputtering DC discharge where the aerosol dynamics is dominated by nucleation and surface deposition growth processes. This agreement is obtained not only on the averaged characteristics of the particle cloud, but also on the detailed particle size distributions. In the case of the CCRF discharge a satisfactory agreement between the two approaches is obtained on the averaged characteristics and on the core distributions of the particle cloud even if a significant difference is observed at low and intermediate particle size. Nevertheless, the bimodal moment approach is accurate enough for the simulation of the dusty plasmas of interest. Its low-computational cost makes it a very effective method as far as dusty plasma simulation is concerned.

Langmuir probe measurements of the secondary electron population across the cathodic pre-sheath of a DC argon discharge

Cylindrical Langmuir probe measurements in a low-pressure DC argon discharge are used to obtain the spatial evolution of ion, cold and hot electron parameters across the cathodic pre-sheath. The cathodic pre-sheath is formed by a stainless steel plate allowed to float negatively with respect to the plasma. The velocity distribution function of the hot electron population in the pre-sheath is shown to be matched by a drifting Maxwellian that thermalizes across it. The source of the hot electron population is hypothesized to arise from secondary electron emission from the plate. A Bayesian estimation routine is proposed to compare and validate different Langmuir probe models as well as calculating the relative uncertainty between models. The results are analyzed using existing pre-sheath theory for which experimental evidence of the spatial influence of energetic electrons is lacking. The data are shown to follow Riemann's pre-sheath model that the ion-neutral mean free path λ i is proportional to the pre-sheath characteristic length l, and that the potential drop ϕ ( x ) with distance x from the sheath/pre-sheath boundary x0 follows ϕ ( x ) ∝ ( x 0 − x ) / l.

Influence of a Nitrogen Admixture on the Value and Radial Profile of the Metastable Argon Atom Density in a DC Glow Discharge in Argon

Results of measurements of the value and radial profile of the density of Ar(3P2) metastables in a dc discharge in pure argon and Ar + 0.1%N2 and Ar + 1%N2 mixtures are presented. The electric field strength in the positive column of the discharge was also measured. The experiments were performed in a 2‑cm-radius discharge tube at gas pressures of 1, 7, and 60 Torr and discharge currents in the range of 10–50 mA. It is found that, at a pressure of 60 Torr, a nitrogen admixture to argon leads to a significant decrease in the electric field strength in the diffuse discharge, while at P = 1 Torr, in contrast, the electric field increases substantially. The degree to which the nitrogen admixture affects the density of Ar(3P2) atoms on the discharge tube axis also depends on the gas pressure. At a pressure of 60 Torr, the Ar(3P2) density decreases substantially (by three orders of magnitude for the 1%N2 admixture and 1.5 orders of magnitude for the 0.1%N2 admixture), while at a pressure of 1 Torr, the Ar(3P2) densities in pure argon and in Ar + N2 mixtures differ less than twice. It is also shown that, at all gas pressures under study, a nitrogen admixture to argon leads to the broadening of the radial profile of the Ar(3P2) density. The experiments were accompanied by numerical and theoretical studies. For pure argon, the calculations were performed in a one-dimensional (along the tube radius) discharge model, while for the Ar + 1%N2 mixture, in a zero-dimensional model, which allows one to calculate the plasma parameters on the tube axis. The calculated results were used to qualitatively explain the experimentally observed effects.

Floating harmonic probe measurements in the low-temperature plasma jet deposition system

The floating harmonic probe is a relatively new plasma diagnostic method, which was proposed for applications at conditions when insulating films are deposited on the probe and, consequently, the classical Langmuir probe method fails. In the floating harmonic probe method a purely sinusoidal AC voltage is applied to the probe constructed in a standard manner via a capacitor. From the spectral components of the measured AC probe current waveforms, the electron temperature and the positive ion density can be obtained. In this contribution we present the comparison of the electron temperature and density acquired by the floating harmonic probe method with those obtained by the classical Langmuir probe. The experiments are performed in the flowing DC discharge in argon. In addition, the results from the floating harmonic probe method obtained during deposition of an insulating iron oxide thin film are shown. All the data is complemented by the qualitative discussion.

Modification of polypropylene in the afterglow of the atmospheric pressure discharges in air and argon

Polypropylene films were modified in the flowing afterglow of the atmospheric pressure DC discharge in argon and in air. The modification was carried out at a discharge current of 15 mA, a gas flow rate of 24 and 105 m/s and a treatment time of 3 - 30 s. Polymer samples were placed on the distance of 5 - 15 mm downstream the plasma Contact angles for water and ATR-FTIR spectra were used for the film surface characterization. Concentrations of oxygen containing groups in modified polymer layer were estimated on the base of ATR-FTIR data. Modification in the both plasma forming gases results in the decrease of the contact angles and in the formation of oxygen containing groups in the polymer surface layer. Dependencies of contact angles on treatment time, gas flow rate and plasma - polymer distance were obtained. Increasing the treatment time and the gas flow rate results in a higher oxidation degree of the PP. Treatment in the afterglow of the argon plasma has been shown to give the less water contact angles and more densities of oxygen containing groups in polypropilene at the gas flow rate of 105 m/s and the treatment time of 30 s.

Inactivation of Candida glabrata by a humid DC argon discharge afterglow: dominant contributions of short-lived aqueous active species

Plasma medicine applications are currently attracting significant interest all over the world. Bactericidal treatments of Candida glabrata cultured in saline suspension are performed in this study by a room-temperature reactive afterglow of a DC-driven argon discharge. Water vapor was added to the discharge to study the inactivation contributions of reactive hydrolytic species including OH and H2O2 transporting along the gas flow to the treated solutions. The inactivation results indicate that the dominant roles in the bactericidal treatments are played by the short-lived aqueous active species, but not the stable species like H2O2aq (aq indicates an aqueous species). Further analysis shows that the ·OHaq radicals play an important role in the inactivation process. The ·OHaq radicals in the suspension are mostly produced from the direct dissolution of the OH species in the reactive afterglow. With the increase of added water vapor content, the ·OHaq production increases and enhances the inactivation efficiency of C. glabrata. Furthermore, it is found that the ambient air diffusion shows essential effects on the bactericidal activity of the remote humid argon discharge. Higher bactericidal effects can be obtained in open-space treatments compared to in a controlled Ar + H2O gas atmosphere. Key active air-byproduct species are believed to be generated in the suspension during the treatments and contributing to the inactivation process. Based on chemical analysis, the peroxynitrous acid ONOOHaq is considered as the key antimicrobial air-byproduct species. These results indicate the important dependence of plasma biomedical effects on the processing environment, which finally relates to the critical contributions of the key reactive species formed therein.

Solvated electrons at the atmospheric pressure plasma–water anodic interface

We present results from a particle-in-cell/Monte Carlo model of a dc discharge in argon at atmospheric pressure coupled with a fluid model of an aqueous electrolyte acting as anode to the plasma. The coupled models reveal the structure of the plasma–electrolyte interface and near-surface region, with a special emphasis on solvated or hydrated electrons. Results from the coupled models are in generally good agreement with the experimental results of Rumbach et al ( Nat. Commun. 6 7248). Electrons injected from the plasma into the water are solvated, then lost by reaction with water within about 10–20 nm from the surface. The major reaction products are OH− and H2. The solvated electron density profile is controlled by the injected electron current density and subsequent reactions with water, and is relatively independent of the external plasma electric field and the salt concentration in the aqueous electrolyte. Simulations of the effects of added scavenger compounds (H2O2, , and H+) on near-surface solvated electron density generally match the experimental results. The generation of near-surface OH− following electron-water decomposition in the presence of bulk acid creates a highly basic region (pH ~ 11) very near the surface. In the presence of bulk solution acidity, pH can vary from a very acidic pH 2 away from the surface to a very basic pH 11 over a distance of ~200 nm. High near-surface gradients in aqueous solution properties could strongly affect plasma-liquid applications and challenge theoretical understanding of this complex region.

Similarities and differences between gliding glow and gliding arc discharges

In this work we have analyzed the properties of a gliding dc discharge in argon at atmospheric pressure. Despite the usual designation of these discharges as ‘gliding arc discharges’, it was found previously that they operate in two different regimes—glow and arc. Here we analyze the differences in both regimes by means of two dimensional fluid modeling. In order to address different aspects of the discharge operation, we use two models—Cartesian and axisymmetric in a cylindrical coordinate system. The obtained results show that the two types of discharges produce a similar plasma column for a similar discharge current. However, the different mechanisms of plasma channel attachment to the cathode could produce certain differences in the plasma parameters (i.e. arc elongation), and this can affect gas treatments applications.

Comparative measurements of plasma potential with ball-pen and Langmuir probe in low-temperature magnetized plasma

The ball-pen probe (BPP) is used for direct plasma potential measurements in magnetized plasma. The probe can adjust the ratio of the electron and ion saturation currents Isat−/Isat+ to be close to one and therefore its I-V characteristic becomes nearly symmetric. If this is achieved, the floating potential of the BPP is close to the plasma potential. Because of its rather simple construction, it offers an attractive probe for measurements in magnetized plasma. Comparative measurements of plasma potential by BPPs of different dimensions as well as one Langmuir probe (LP) in an argon discharge plasma of a cylindrical magnetron were performed at various experimental conditions. An additional comparison by an emissive probe was also performed. All these types of probes provide similar values of plasma potential in a wide range of plasma parameters. Our results for three different BPP dimensions indicate that the BPP can be operated in a cylindrical magnetron DC argon discharge if the value of the ratio of the magnetic field and neutral gas pressure, B/p, is greater than approximately 10 mT/Pa.

Ionization Mechanism and Chemical Composition of an Argon DC Discharge with Water Cathode

This paper reports the results of the experimental study and chemical composition modeling for a DC argon discharge burning above water cathode in the pressure range of 0.1–1 bar and at discharge current of 40 mA. The gas temperature, reduced electric field strength, and emission intensities of some argon lines were obtained. On the base of these data, the modeling chemical composition of plasma was carried out. At modeling, the combined solution of Boltzmann equation for electrons, equations of vibrational kinetics for ground states of N2, O2, H2O and NO molecules, equations of chemical kinetics and plasma conductivity equation was used. The calculations agree well with the measured line intensities. It was shown that discharge burns in diffusion mode, at which the stepwise ionization rate of four lower excited states of argon is equal to the rate of charges diffusion losses.

Modeling of Direct Current Atmospheric Pressure Argon Discharge in Two-Dimensional

The finite element computational package COMSOL multiphysics were used to simulate a bar plate dc discharge in argon at atmospheric pressure. The basic plasma properties such as electron density, ion density, metastable atom density, electron temperature, electric voltage and electric field were studied. The current-voltage (I-V) characteristic of numerical model is in good agreement well with experimental data. This model is simple and insightful as a theoretical tool for argon atmospheric pressure discharges.

Influence of the capillary on the ignition of the transient spark discharge

A self-pulsing negative dc discharge in argon generated in a needle-to-plane geometry at open atmosphere is investigated. Additionally, the needle electrode can be surrounded by a quartz capillary. It is shown that the relative position of the capillary end to the needle tip strongly influences the discharge inception and its spatio-temporal dynamics. Without the capillary for the selected working parameters a streamer corona is ignited, but when the capillary surrounds the needle, the transient spark (TS) discharge is ignited after a pre-streamer (PS) occurs. The time between PS and TS discharge depends on the relative capillary end position. The existence of the PS is confirmed by electro-optical characterization. Furthermore, spectrally and spatio-temporally resolved cross-correlation spectroscopy is applied to show the most active region of pre-phase emission activity as indicators for high local electric field strength. The results indicate that with a capillary in place, the necessary energy input of the pre-phase into the system is mainly reduced by additional electrical fields at the capillary edge. Even such a small change as a shift of dielectric surface close to the plasma largely changes the energy balance in the system.

Field Emission Model of CNT Based Ionization Gas Sensor

A new model to study the gas detection mechanism of carbon nanotube (CNT) based ionization gas sensor has been developed. The model incorporates the effect of electron field emission due to the presence of CNT. The model is then embedded in the standard Particle-In-Cell / Monte-Carlo-Collision (PIC-MCC) codes. This enhanced PIC-MCC codes serve as a tool to optimize CNT based ionization gas sensor. The functionality of the new model is validated by running simulations of DC discharges in argon and comparing the results with published experimental and simulated works. From the simulation, one order of magnitude decrease in the breakdown voltages and three orders of magnitude faster response time was observed.

Numerical model of an Ar/NH3 atmospheric pressure direct current discharge in parallel plate geometry

A one dimensional fluid model is used to investigate the role of ammonia added to an argon DC discharge at atmospheric pressure. The equations solved are the particle balances, assuming a drift-diffusion approximation for the fluxes, and the electron energy balance equation. The self-consistent electric field is obtained from the simultaneous solution of Poisson’s equation. The electron-neutral collision rates are expressed as a function of the average electron energy. The model is comprised of 40 species (neutrals, radicals, ions, and electrons). In total, 75 electron-neutral, 43 electron-ion, 167 neutral-neutral, 129 ion-neutral, 28 ion-ion, and 90 3-body reactions are used in the model. The effects of gas mixing ratio on the densities of plasma species are systematically investigated. The calculated densities of the main plasma species are presented. It is found that in an Ar/NH3 plasma, the main neutrals (Ar*, Ar**, NH3*, NH, H2, NH2, H, and N2) are present at high densities. The Ar2+ and Ar+ ions are the dominant ions in the plasma. Furthermore, the NH3+ ions have a relatively higher density than other ammonia ions, whereas the density of other ions is negligible. Finally, a comparison is made between a pure Ar discharge and dielectric barrier discharge in a mixture of Ar/NH3. It is demonstrated that gas mixing ratio has a significant effect on the densities of plasma species, besides ammonia radical molecules and ammonia ions, and it also affects their ratio. Once the mixing ratio of Ar/NH3 is close to 1:1 at atmospheric pressure, the densities of NH, NH2+ and NH4+ reach to the maximum. The maximum of the different positive ammonia ions corresponds to the different ammonia mixing ratio.

Field Reversal and Particle Growth in DC Discharge

A modeling study of carbon clusters and dust particles formation through carbon graphite sputtering in argon DC discharges is presented. The model combines the description of plasma discharge kinetics, molecular growth and transport of carbon clusters and aerosol dynamics for dust particles. Results show that field reversal is a key effect that ensures trapping and growth of negatively charged molecular carbon clusters, which are the precursors for dust particles. The model enables prediction of the space–time distributions of carbon clusters density as well as dust particle density, average charge and average diameter. Results especially show that dust particles and carbon clusters exhibit a pronounced density maximum in the vicinity of the field reversal position. A parametric study is presented in order to analyze the model sensitivity to some key parameters used in the physical model.

Rapid synthesis of functional oxides by electric discharge assisted mechanical milling method

Here we report application of an electric discharge assisted mechanical milling (EDAMM) technique to synthesize various functional oxides. In this work we studied the effect of electric discharge milling conditions such as AC and DC discharges in helium, argon, nitrogen and oxygen on synthesis of complex oxide MgAl2O4 (i) from pre-mixed oxides as starting materials and (ii) from elemental powders by oxidation in oxygen plasma. Also a compound BaLa2Ti4O12 and CaCu3Ti4O12 was synthesized from elemental oxides by EDAMM and EDAMM and subsequent annealing, respectively. All samples were characterized using X-ray analysis. SEM was performed on MgAl2O4 samples and DTA on nanostructural CaCu3Ti4O12. By using EDAMM, single phase multi-element oxides can be formed in as little as 0.1% of the processing time required in conventional solid-state techniques.

Diffuse, constricted-stratified and constricted modes of a DC discharge in argon. Simulation of transitions between these modes

This paper presents the results of theoretical studies of high-pressure (~10's and 100s of Torr) dc discharges in argon. The diffuse, constricted-stratified, and constricted discharge modes are studied using a developed model. Two transitions were studied. One of them is transition from the constricted-stratified mode to the constricted mode. This transition occurs smoothly, without any jumps of plasma parameters. The other is a hysteresis transition between the diffuse and the constructed-stratified mode. It occurs by a jump and is accompanied by a several orders of magnitude increase in the electron density at the discharge axis and an appearance of moving striations.

Space–time development of low-pressure gas breakdown

In this paper we present an analysis of time- and space-resolved development of several regimes of low-pressure dc discharges in argon. These regimes include low current Townsend discharge, oscillations and constrictions of discharge and high current diffuse glow discharges. Our work is based on ICCD recordings of the discharge structure, synchronized with current and voltage measurements. Measurements were made at three different pressure × electrode gap parameters (pd = 250, 150, 45 Pa cm), at three gaps (d = 1.1, 2.1, 3.1 cm). Special attention is given to radial effects and the influence of dielectric walls during the development of the glow discharge structure. We were able to show how the space time-resolved structure of the discharge can lead to an understanding of the physics of the initial stages of gas breakdown and formation of different regimes of low-pressure discharge. The results also give the basis for interpreting formative time delays associated with gas breakdown.

Fluid Modeling and Analysis of the Constriction of the DC Positive Column in Argon

The glow-to-arc transition of the positive column of a dc discharge in argon in the course of constriction has been investigated on the basis of a self-consistent 1-D axisymmetric fluid model. The model adopts the nonlocal moment method, i.e., the system of balance equations resulting from the moments of the radially dependent Boltzmann equation is solved. The electron transport and rate coefficients are applied in dependence on the mean energy of the electrons, the gas temperature, and the ionization degree. Investigations have been performed for currents from 0.6 to 70 mA and pressures from 100 to 500 torr. The model predictions are compared with experimental and other available modeling results, and they show good agreement with these data in general. The pronounced nonlocal features of the mean electron energy balance are found, and their influence on the constricted argon positive column is analyzed. Different assumptions concerning the electron velocity distribution function have been considered in the present model. In particular, the impact of using a Maxwellian distribution instead of solutions of the steady-state spatially homogeneous electron Boltzmann equation is discussed where the assumption of a Maxwellian distribution for the electrons was found to be inappropriate for describing the constriction effect.

Simulation of diffuse, constricted-stratified, and constricted modes of a dc discharge in argon: Hysteresis transition between diffuse and constricted-stratified modes

This paper presents the results of theoretical studies of high-pressure p (tens and hundreds of Torr) direct current (dc) discharges in argon. The diffuse (D), constricted-stratified (CS), and constricted (C) discharge modes are studied using a developed one-dimensional (radial) model. The model includes the conservation equations for electrons, ions (Ar+ and Ar2+), and excited atoms (metastable and resonant states) for mean electron energy and for the temperature of the high-energy part of the electron-energy distribution function (EEDF), the heat conduction equation for the neutral gas, and Poisson's equation for the radial electric field. The developed model of a dc discharge allowed us, without any artificial assumptions, to obtain periodic oscillations of plasma parameters for the CS mode and to describe a hysteresis transition between the D mode and the CS mode. Direct transition from the D to the CS mode is accompanied by an increase of several orders of magnitude in the electron density at the discharge axis and the appearance of moving striations. It was shown that the experimentally observed hysteresis of the current-voltage characteristic at the transition between the CS and D modes deals with the nonlocal formation of the EEDF, namely, the diffusion of high-energy electrons from the central constricted region. The effect of the nonlocal formation of the EEDF is taken into account by introducing the effective temperature of the high-energy part of the EEDF and solving the equation for the radial profile of this temperature. The transition from the CS mode to the C mode occurs smoothly, without any jumps of the plasma parameters. Plasma parameters and characteristics of all three modes and transitions between these modes are calculated and compared with experimental data.