Positive Column Region Research Articles

Two-dimensional numerical simulation of pre-ionized direct-current glow discharge in atmospheric helium

In this paper, the effect of pre-ionization on the small-gap and large-gap direct-current glow discharge at atmospheric pressure are investigated based on a two-dimensional self-consistent fluid model. For both the discharges, the results show that with the enhancement of pre-ionization, the charged particle distribution gradually shifts toward the cathode along the discharge direction, making the cathode fall zone shrink continuously. The width of the positive column region, negative glow space, and cathode fall zone continuously extend along the vertical discharge direction, and the distribution of electron density and ion density are more uniform. For the electric field, with the enhancement of pre-ionization, the longitudinalal component distribution of the electric field in the cathode fall zone gradually contracts toward the cathode, and the overall electric field near the cathode decreases and becomes more uniformly distributed. The transverse component distribution of the electric field gradually decreases and shrinks toward the wall. The overall electron temperature in the discharge space decreases with the enhancement of the pre-ionization level, and the electron temperature distribution in the cathode fall zone gradually shrinks toward the cathode. In addition, the overall potential of the discharge space also decreases. The introduction of pre-ionization significantly reduces the maintaining voltage and discharge power of the direct-current glow discharge. Furthermore, the potential drop in the small-gap discharge is always concentrated in the cathode fall zone as the pre-ionization increases, while the potential drop in the large-gap discharge is gradually shifted from the cathode fall zone to the positive column region. This simulation shows that the pre-ionization not only effectively enhances the discharge uniformity, but also largely reduces the maintaining voltage and energy consumption of the direct-current glow discharge. This work is an important guideline for further optimizing the electrode configuration and the operating parameters of the plasma source.

Formation and evolution of striation plasma in high-pressure argon glow discharge

The self-organized striation phenomenon in the positive column region of glow discharge plasma is a basic problem in gas discharge physics, which involves rich physics such as electron dynamics, transport process, discharge instability and nonlinear phenomenon. It is an important topic in basic physics and practical application. In this work an argon glow discharge striation plasma at high pressure is reported. The electrical, optical and ionization wave propagation characteristics of the striation plasma, and the evolution of the striation plasma with pressure and impurity gas are investigated experimentally. The generation and quenching mechanism of argon striation plasma are analyzed. The results show that the striation length is about 1.5 mm, and decreases with pressure increasing, and the velocity and frequency of the ionization wave are estimated at 1.87 m/s and 1.25 kHz, respectively. The measurement of optical emission spectrum shows that the generation of striation plasma is probably related to the argon metastable atoms. The stepwise ionization process caused by metastable atoms triggers off an ionization instability. The instability propagates in the form of ionization wave, which leads the plasma parameters to be modulated longitudinally, thus, forming an alternating bright and dark striation plasma. The adding of nitrogen can effectively quench metastable argon atoms and change the electron energy distribution function, which destroys the instability conditions of the plasma, therefore, the striation plasma disappears. This work provides a new insight into the understanding of the formation and annihilation mechanism of glow discharge striation plasma at high pressure.

Numerical simulation of discharge mode conversion with multiple current pulse (MCP) in atmospheric pressure He/N2 dielectric barrier discharge

A one-dimensional fluid simulation model of He/N2 dielectric barrier discharge with parallel plate electrodes was established to study the effects of different parameters (such as gap width, secondary electron emission coefficient γ, and driven frequency) on the characteristics of multiple current pulse (MCP) discharge and the discharge mode conversion. The discharge can be divided into Townsend discharge, transition state, and glow discharge. The results show that with the increase in γ, the number of discharge current pulses increases, making it more difficult to form a glow discharge. When γ is larger, the first discharge approaches the glow discharge mode, but the positive column region is not completely formed, and the subsequent discharge sequence undergoes a transition state to Townsend discharge gradually. Under the condition of larger γ, MCP discharge with a short gap is Townsend discharge. With the increase in the gap width, the transition state will appear in the first discharge, and the subsequent discharge sequence may be converted into Townsend discharge. When the gap width increases further, the discharge can be completely transformed into glow discharge. The pulse number of discharge current decreases with the increase in frequency, and the higher frequency is conducive to the formation of glow discharge.

Numerical simulation on particle density and reaction pathways in methane needle-plane discharge plasma at atmospheric pressure

Methane needle-plane discharge has practical application prospect and scientific research significance since methane conversion heavy oil hydrogenation is formed by coupling methane needle-plane discharge with heavy oil hydrogenation, which can achieve high-efficient heavy oil hydrogenation and increase the yields of high value-added light olefins. In this paper, a two-dimensional fluid model is built up for numerically simulating the methane needle-plane discharge plasma at atmospheric pressure. Spatial and axial distributions of electric intensity, electron temperature and particle densities are obtained. Reaction yields are summarized and crucial pathways to produce various kinds of charged and neutral particles are found out. Simulation results indicate that axial evolutions of CH3+ and CH4+ densities, electric intensity and electron temperature are similar and closely related. The CH5+ and C2H5+ densities first increase and then decrease along the axial direction. The CH3 and H densities have nearly identical spatial and axial distributions. Particle density distributions of CH2, C2H4 and C2H5 are obviously different in the area near the cathode but comparatively resemblant in the positive column region. The CH3+ and CH4+ are produced by electron impact ionizations between electrons and CH4. The CH5+ and C2H5+ are respectively generated by molecular impact dissociations between CH3+ and CH4 and between CH4+ and CH4. Electron impact decomposition between electrons and CH4 is a dominated reaction to produce CH3, CH2, CH and H. The reactions between CH2 and CH4 and between electrons and C2H4 are critical pathways to produce C2H4 and C2H2, respectively. In addition, the yields of electron impact decomposition reactions between electrons and CH4 and reactions between CH2 and CH4 account for 52.15% and 47.85% of total yields of H2 respectively.

Experimental studies on striations in helium glow discharge

This letter reports the detailed observations and investigations of striations in the positive column region of helium glow discharges with a pin-to-plate copper electrode geometry in the pressure range 9–101 kPa. The striations are characterized by several plasma layers with alternate brightness and darkness. In this discharge, features such as negative glow, Faraday dark space, and striated positive column regions can be clearly observed. The evolution of a striation structure in the positive column region was found to be sensitive to gas pressure and electrode spacing. The striated discharge propagates in the form of an ionization wave with a velocity of 20.78 m/s and a frequency of 5.2 kHz.

Characterization of the DC glow discharge in longitudinal flowing helium at atmospheric pressure

A one-dimensional self-consistent fluid model was developed to study the direct-current (DC) glow discharge in longitudinal flowing helium (He) at atmospheric pressure. Distinct dynamical features of charged particles are observed at different He flow velocities, which is accompanied by a dramatic change of spatial discharge structure. The positive He flow leads to the reduction of charged particle densities in the positive column and negative glow regions, and even makes the negative glow region disappear. The negative He flow allows for the enhancement of charged particle densities in the negative glow regions. These observations above mainly lie in the fact that the convection effects due to the He gas flow influence the transport properties of charged particles through different ways by comparing the He flow velocity with the ion drift one. These findings are of guiding significance for the improvement of the chemical activity and work efficiency of plasma source by controlling the gas flow in practical industrial fields.

Influence of longitudinal argon flow on DC glow discharge at atmospheric pressure

A one-dimensional self-consistent fluid model was employed to investigate the influence of longitudinal argon flow on the DC glow discharge at atmospheric pressure. It is found that the charges exhibit distinct dynamic behaviors at different argon flow velocities, accompanied by a considerable change in the discharge structure. The positive argon flow allows for the reduction of charge densities in the positive column and negative glow regions, and even leads to the disappearance of negative glow. The negative argon flow gives rise to the enhancement of charge densities in the positive column and negative glow regions. These observations are attributed to the fact that the gas flow convection influences the transport of charges through different manners by comparing the argon flow velocity with the ion drift velocity. The findings are important for improving the chemical activity and work efficiency of the plasma source by controlling the gas flow in practical applications.

Elongated dust clouds in a uniform DC positive column of low pressure gas discharge

Experimental investigations of the formation of elongated dust clouds and their influence on the plasma glow intensity of the uniform direct current (DC) positive column (PC) have been performed under microgravity conditions. For the axial stabilization of the dust cloud position a polarity switching DC gas discharge with a switching frequency of 250 Hz was used. During the experiment, a spontaneous division of one elongated dust cloud into two smaller steady state dust clouds has been observed. Quantitative data on the dust cloud shape, size and dust number density distribution were obtained. Axial and radial distributions of plasma emission within the 585.2 nm and 703.2 nm neon spectral lines were measured over the whole discharge volume. It has been found that both spectral line intensities at the dust cloud region grew 1.7 times with respect to the undisturbed positive column region; in this the 585.2 nm line intensity increased by 10% compared to the 703.2 nm line intensity. For a semi-quantitative explanation of the observed phenomena the Schottky approach based on the equation of diffusion was used. The model reasonably explains the observed glow enhancement as an increasing of the ionization rate in the discharge with dust cloud, which compensates ion-electron recombination on the dust grain surfaces. In this, the ionization rate increases due to the growing of the DC axial electric field, and the glow grows directly proportional to the electric field. It is shown that the fundamental condition of the radial stability of the dusty plasma cloud is equal to the ionization and recombination rates within the cloud volume that is possible only when the electron density is constant and the radial electric field is absent within the dust cloud.

Physical and chemical properties of low-pressure argon-chlorine dielectric barrier discharge

The influence of adding chlorine on the characteristics of a dielectric barrier discharge in Argon is investigated on the basis of a one-dimensional fluid model. The spatio-temporal characteristics of the discharge are obtained by applying a sinusoidal voltage with a frequency and amplitude of 7 kHz and 350 V, respectively. The study shows that the discharge has a homogeneous feature across the electrodes and has only one current pulse per half cycle of the applied voltage. The calculated electric field and electric potential as well as species number densities indicated that the discharge is in glow mode, and adding chlorine as electronegative gas up to 50% does not change its mode. It is observed that the most abundant negative species are Cl− ions even in low additive chlorine. As a result, the maximum of plasma electronegativity takes place at 30% amounts of chlorine additive. The study of plasma radiations on the discharge gap shows that the main spontaneous emissions are observed at the wavelengths of 128.5 nm and 258 nm due to de-excitation of ArCl* and Cl2* molecules, respectively. Between different Ar/Cl2 mixtures, 0.99Ar−0.01Cl2 has the nearly uniform radiation in the positive column region.

Absolute OH density measurements in an atmospheric pressure dc glow discharge in air with water electrode by broadband UV absorption spectroscopy

Spatially resolved absolute OH radical density measurements are performed in an atmospheric pressure glow discharge generated in ambient air with water electrode by broadband UV absorption spectroscopy. The radial distributions of OH density and gas temperature are obtained for the positive column, anode and cathode regions both for water-cathode and water-anode discharges. It is found that for both polarities of the water electrode the radial profiles of the ground state OH density and gas temperature are significantly broader than the total discharge emission intensity and the emission intensity originating from OH() only. Exceptional large OH densities exceeding 1023 m−3 are found. The OH kinetics are discussed in detail.

Realization of DC atmospheric pressure glow discharge without external airflow

A new needle-to-cylinder electrode structure with small gap (1.5 mm) was designed to realize the stable gas discharge in ambient air. The stable corona discharge and the stable glow discharge were realized without external airflow, which was proved by the discharge waveform stored in the oscilloscope, the discharge pictures recorded by the digital camera and the voltage–current plot. The image of the glow discharge showed that there was a negative glow region, Faraday dark space, a positive column region and an anode spot. Also the experiment and the COMSOL simulation showed that the needle with a diameter of 56.4 μm can realize the glow discharge while the needle with a diameter of 626 μm can not. The reasons for the realization of the glow discharge by the small gap between the needle and the cylinder were analyzed. It also shows that the glow discharge is easily realized with the black spot on the cylinder surface and a proper ballast resistor value (larger than 2 MΩ and less than 30 MΩ). The needle-to-cylinder electrode structure is easy to fabricate by MEMS technology, which can be used in portable analytical instruments.

Instantaneous Reduction of Graphene Oxide Paper for Supercapacitor Electrodes with Unimpeded Liquid Permeation

Scanning positive column of atmospheric-pressure glow discharge (AGD) plasma is proposed as an eco-friendly and cost-effective strategy toward the instantaneous (∼2 s) reduction of graphene oxide (GO) paper, further leading to a reduced graphene oxide (rGO) paper with unimpeded liquid permeation for high performance supercapacitors. Unlike previous finding that AGD plasma has no effect on GO, a well-designed AGD plasma containing scanning positive column region is demonstrated to be capable of converting GO paper to rGO paper. With the synergy of energetic electrons (electron density 1.03 × 1016 cm–3) and surface touch heating (translational temperature ∼800 K), restoration of graphene’s excellent electrical properties is realized, evidenced by a substantially (5 orders of magnitude) improved electrical conductivity. Moreover, the ultrafast reduction of oxygen functionalities results in the creation of exposed open channels and the expansion of graphene layers, leading to a rGO paper supercapacitor electrode with unimpeded liquid permeation. Taking the above advantages, the as-obtained rGO paper exhibits obviously better capacitive and rate performance than the parent GO and chemically reduced rGO counterparts in terms of higher specific capacitance, better charge/discharge rate response, and faster ion diffusion, holding a great promise for energy storage applications.

Distributions of Electron Density and Electron Temperature in Magnetized DC Discharge

The distribution of electron temperature Te and density Ne in a dc glow discharge that is created by a pair of circular parallel electrodes is studied using double probes. The measurement are carried out for the following two modes: in the presence and in the absence of the magnetic field. In the absence of the magnetic field, for the radial distribution, temperatures have same values with small variation, in the same discharge region [cathode fall or negative glow or positive column region], except at the edge. For the axial distribution, the distribution profiles of Te shows that Te is decreased, and Ne continues to increase toward the anode. In the presence of the magnetic field, the radial distribution of the electron density has its highest value at the edge where the magnetic field is maximum. The electron temperature changes very little. The density begins to increases also from the cathode fall region, at the edge, to the negative glow region and then it decreases sharply at the positive column region. The plasma is very intense in the cathode fall region (very thin bright ring) and very brighter and sharp in the negative glow region (extends several mm from the cathode fall region). But in the positive column, the density begins to decrease.

Electrical and Optical Characteristics of a Color Alternating Current Plasma Display Panel With Floating Electrodes

A new electrode configuration was proposed to improve the performance of color ac plasma display panels (PDPs). ITO electrodes on the front plate of a PDP were used as auxiliary sustain electrodes, which were electrically isolated from the metal electrodes by a dielectric isolation layer. When this type of PDP operates, the sustain voltage waveforms are only applied to the metal electrodes, so the ITO electrodes always lie in floating potentials. By numerical simulation, the microdischarge process in a PDP cell with floating auxiliary electrodes was analyzed, and the influence of the insulation layer thickness on the cell electrical parameters was investigated. It was found that a discharge in the cell was firstly generated at the narrow gap between a pair of floating electrodes, and then it grew into a long-path discharge including a positive column region between a pair of metal electrodes. Two 12-inch test panels having this floating-electrode-type cell structure were fabricated, together with two other test panels as references having a conventional cell structure and a regular long-gap cell structure, respectively, and their electrical and optical characteristics were measured and compared to each other. The results indicate that the PDP with floating electrodes can achieve a high luminance and a high luminous efficacy at a relatively low sustain voltage.

Effect of hydrogen ratio on plasma parameters of N2-H2 gas mixture glow discharge

A dc plane glow discharge in a nitrogen-hydrogen (N2-H2) gas mixture has been operated at discharge currents of 10 and 20 mA. The electron energy distribution function (EEDF) at different hydrogen concentrations is measured. A Maxwellian EEDF is found in the positive column region, while in both cathode fall and negative glow regions, a non-Maxwellian one is observed. Langmuir electric probes are used at different axial positions, gas pressures, and hydrogen concentrations to measure the electron temperature and plasma density. The electron temperature is found to increase with increasing H2 concentration and decrease with increasing both the axial distance from the cathode and the mixture pressure. At first, with increasing distance from the cathode, the ion density decreases, while the electron density increases; then, as the anode is further approached, they remain nearly constant. At different H2 concentrations, the electron and ion densities decrease with increasing the mixture pressure. Both the electron and ion densities slightly decrease with increasing H2 concentration.

Modelling study of dielectric barrier glow discharge in Ar/NH3 mixture at atmospheric pressure

In order to investigate the mechanism of dielectric barrier glow discharge in Ar/NH3 mixture at atmospheric pressure, a multiple particles self-consistent coupled fluid model is proposed. And the finite-element method is used in the numerical calculation model, so the periodically varying waveforms of gas voltage, dielectric surface charge density and discharge current density are investigated. The spatial and temporal distributions of charged and neutral particles density and space electrical field strength are also obtained. The simulation results show that the periodic breakdown process of the gas gap is controlled by the gas voltage, and affected by dielectric surface charges. The spatiotemporal distributions of charged particle density and electrical field strength show that the discharge under conditions considered in this model is a typical atmospheric pressure glow discharge, and that in the discharge process there exist an obvious cathode fall region, a negative glow region, a Faraday dark space, and a positive column region. In the Ar/NH3 plasma, the dominant positive ions are NH3+, and the next ions are Ar2+; the dominant negative ions are NH2-; the main radical molecule products of ammonia decomposition are NH, NH2, and N2H3, but the main final stable products are N2 and H2.

One-dimensional simulation of dielectric barrier glow discharge in atmospheric pressure Ar

In order to investigate the mechanism of dielectric barrier atmospheric pressure glow discharge(APGD) in Ar, an one-dimensional multiple particle self-consistent coupled fluid model is proposed. And the finite-element method (FEM) is used in the numerical calculation model, so the periodic evolvement waveforms of gas voltage, barrier surface charge density and discharge current density are investigated. The spatio temporal distributions of electrons, ions, metastable particles density and space electrical field are also obtained. The simulation results show that the charges accumulated on the barrier dielectric surface play an important role in ignition and extinguishment of the discharge. With the increase of applied voltage amplitude, the DBD breakdown occurs ahead of time, and discharge current density and the surface charge density increase gradually, which indicate that the discharge process becomes fierce. Furthermore, with the increase of relative permittivity of dielectric material, the discharge current density also gradually increases. The spatio temporal distributions of the particles density and the space electrical field show that the DBD breakdown occurs every half the AC period and the discharge under conditions considered in this model is a typical atmospheric pressure glow discharge(APGD), having an obvious cathode fall region, a negative glow region, and a positive column region.

A global model of micro-hollow cathode discharges in the stationary regime

This paper presents a global model of micro-hollow cathode discharges working in argon gas. Inspired by experiments and two-dimensional numerical simulations, the discharge is decomposed into two main regions, labelled the cathodic region and the positive column region. The first is composed of a cathode sheath (surrounding the cathode) and a plasma in the centre, while the positive column region is essentially filled with plasma. A cathode sheath model is developed to calculate the sheath size and the power dissipated by ions and electrons in the sheath. Charged-particle transport equations are solved in one-dimensional cylindrical coordinates, in the quasi-neutral plasma region, in order to determine the radial density profiles and the electron temperature in the microhole. A global power balance is then performed in the two distinct regions in order to determine the absolute electron density. We found that the electron density is one order of magnitude higher in the cathodic region than in the positive column, showing that the power dissipation is mainly located in the cathode sheath. The calculated electron density in the cathodic region is in reasonable agreement with experiments.

Electrical characteristics of (N2-H2) gas mixture DC glow discharge

The electrical characteristics of DC Nitrogen-Hydrogen mixture glow discharge were investigated. The discharge current-voltage (I-V) characteristic curves of the discharge were measured at different gas pressures and gas mixture percentage. The axial distribution of both potential and electric field were studied at different H2percentage. The existence of the reversal electric field in the negative glow region was confirmed. Using the axial electric field distribution, the cathode fall thickness (Xc) was calculated. Second derivative of the single Langmuir probe electron current method was used to measure the electron energy distribution function (EEDF) in (N2-H2) gas mixture DC glow discharge at different H2 percentage. In the positive column region, a Maxwellian EEDF was found at different conditions, while in both cathode fall and negative glow regions, a non-Maxwellian EEDF was observed. Two groups of electrons were detected in these two regions. Key words: Glow discharge, axial potential and electric field distributions, Paschen's curves, Langmuir probes.

DC normal glow discharges in atmospheric pressure atomic and molecular gases

DC glow discharges were experimentally investigated in atmospheric pressure helium, argon, hydrogen, nitrogen and air. The discharges were characterized by visualization of the discharges and voltage and current measurements for current of up to several milliamperes. Significant differences are seen in the gas temperature; however all the discharges appear to operate as temperature and pressure scaled versions of low pressure discharges. In the normal glow discharges, features such as negative glow, Faraday dark space and positive column regions are clearly observable. In hydrogen and to a lesser degree in helium and argon standing striations of the positive column were visible in the normal glow regime. Normal glow characteristics such as normal current density at the cathode and constant electric field in the positive column are observed although there are some unexplained effects. The emission spectra for each of the discharges were studied. Also the rotational and vibrational temperature of the discharges were measured by adding trace amounts of N2 to the discharge gas and comparing modeled optical emission spectra of the N2 2nd positive system with spectroscopic measurements from the discharge. The gas temperatures for a 3.5 mA normal glow discharge were around 420 K, 680 K, 750 K, 890 K and 1320 K in helium, argon, hydrogen, nitrogen and air, respectively. Measured vibrational and excitation temperatures indicate non-thermal discharge operation. Mixtures of gases achieved intermediate temperatures.