Interelectrode Region Research Articles

Electroaerodynamic thrusters: Influence of a freestream on the current, ionic wind, and force produced by a DC corona discharge

In the past few decades, atmospheric plasma propulsion has sustained a growth of interest. Recent studies have demonstrated the feasibility of light air-breathing plasma-propelled aircraft near ground level. Typically, corona discharge actuators are employed. Yet, the effects of the freestream velocity on the discharge current, ionic wind, and thrust must be characterized. The present study focuses on a wire-to-cylinder and a wire-to-airfoil actuators in a wind tunnel in a co-flow configuration. Discharge current and PIV (particle image velocimetry) measurements were used to determine both the differences between the two collecting electrodes and the aforementioned effects of the freestream velocity. The measured current follows the modeling already reported in the literature with a linear increase of the current with the freestream velocity. Besides, an interaction occurs with the charge density between the electrodes, which strengthens the rise of the current with the velocity as the voltage increases. In the study, the charge density increases linearly with the voltage with a slope of 0.75 mC/m2/kV for both collectors. However, the airfoil collector results in a higher current than the cylinder at the same voltage. The local velocity increases in three main regions thanks to the ionic wind. With both actuators, a higher velocity was captured with actuation on the upper and lower surfaces of the collectors. With the cylinder, the interelectrode region experiences a notable rise in velocity as well. In all cases, the air velocity downstream of the actuators is increased by the actuation. The ionic wind is usually less than 1 m/s (around 0.3–0.5 m/s on average) and its effect on the incoming flow decreases when the velocity increases up to 10 m/s. The force was calculated in control volumes around the actuators. For both actuators, the electroaerodynamic (EAD) force is governed by the current, and at constant current, the same EAD force is obtained with the two collectors. Yet, the force decreases with the drag of the collector, leading to a cancellation of the thrust when the drag exceeds the EAD force. At the maximum current tested in the study, the cylinder collector cancels the thrust at around 3 m/s of freestream against 5 m/s with the airfoil, showing that this type of propulsion is currently only applicable to low-speed aircraft.

Фазовый состав и морфология поверхности бронзы БрО10С10 после ее электроискровой обработки анодным материалом аналогичного состава

The coating on the surface of BrO10C10 bronze was studied by scanning electron microscopy and X-ray diffractometry. It was obtained by electrospark processing with an anode material with a composition similar to that of the substrate. X-ray studies of the phase composition revealed the presence of phases of the “copper – tin” system (Cu, α-(Cu; Sn), Cu3Sn, ε-Cu3Sn and Cu5.6Sn) and lead. The incorporation of atmospheric elements and initial bronze components into lead leads to the formation of regions with a distorted crystal lattice. An increase in the energy of a single pulse of an electric spark discharge is accompanied by a decrease in the α-(Cu; Sn) phase and an increase in the content of ε-Cu3Sn and Cu5.6Sn. The ratio of intensities indicates the absence of a predominant orientation of growth (texture) in the electrospark coating. Scanning electron microscopy data indicate the presence of melted areas, pores, spherical and oval inclusions, irregularly shaped particles, cracks, rounded pits, etc. in the surface layer. The main reason for the presence of melted areas is high temperatures in the interelectrode region during an electric spark discharge. The appearance of surface cracks is primarily associated with the occurrence of high thermal stresses and interelectrode mechanical contact. The presence of spherical/oval particles is a consequence of the interaction of liquid droplets with the surface of the cathode substrate. Irregularly shaped particles appear as a result of explosive emission from the edges of the erosion crater of the anode material.

Effect of dilution gas on the distribution characteristics of capacitively coupled plasma by comparing SiH4/He and SiH4/Ar

In this study, we focus on the difference in the spatial distribution of the plasma parameters between SiH4/He capacitively coupled plasma (CCP) and SiH4/Ar CCP. The SiH4/He mixture is modeled using the chemical reactions that were successfully derived in our previous studies. The chemical reaction model of the SiH4/Ar mixture built in this study is based on the detailed set of chemical reactions in Ar. The spatial distribution of the plasma parameters is examined with the aid of a 2D fluid model. The electron and radical densities of SiH4/Ar CCP are higher than those of SiH4/He CCP. In addition, dilution with Ar results in more uniform reaction rates, which leads to a more uniform deposition profile. Because helium requires higher threshold energies for excitation and ionization, dilution with He had little effect on the precursor production. As a result, the concentration of Si2H6 observed in the inter-electrode region when using Ar for dilution was observed to be about ten times higher than the concentration of Si2H6 observed for He. This high concentration played a large role in influencing the formation of important radicals that determine the deposition rate as well as the difference in the deposition rate profile between Ar and He as diluents. The higher concentration of Si2H6 when using Ar means that the production rate of Si2H5 is higher in Ar. An examination of the effect of the dilution gas on the deposition rate profile indicated that the deposition rate profile with Ar is 100% more uniform and the deposition rate nearly 87% higher than for dilution with He.

Efficiency analysis of ignition by Nanosecond Repetitively Pulsed discharges using a low-order model

Plasma-assisted combustion is an interesting method to promote the ignition of a lean reactive mixture instead of conventional spark plugs. Especially, Nanosecond Repetitively Pulsed (NRP) discharges technique is an energy-efficient way to initiate and control combustion processes. Ignition success or failure results from the competition between the discharge energy accumulation, the gas residence time in the discharge region, and the combustion chemistry. Plasma-assisted ignition is modeled here by a Perfectly-Stirred Reactor (PSR) whose volume represents the one surrounding the interelectrode region. NRP discharges are applied inside the reactor to initiate the combustion of the injected mixture. This model numerically identifies a plasma-assisted ignition efficiency map in terms of the amount of energy per pulse and the pulse repetition frequency, at low CPU cost. A criterion based on the residence time, interpulse time, and chemical time is introduced to predict the successful formation of a reactive kernel. This criterion is successfully validated by analyzing the plasma-assisted PSR solutions.

Controlling the structure of a glow discharge by supersonic gas flow

The effect of supersonic gas flow on glow discharge characteristics in interelectrode space is studied. First, gas outflow through a Laval nozzle with a central body was simulated: gas particle density distributions, gas flow velocity, Mach number, and temperature were obtained. Next, these profiles were used for numerical calculations of the parameters of a transverse glow discharge within the framework of an extended fluid description of the plasma. As a result, it was found that the cathode zones can both shrink and stretch depending on the parameters of the supersonic gas flow. The latter creates an inhomogeneous gas density in the interelectrode region and thereby affects the value of E/N, which determines the electrical characteristics of the discharge. Numerical simulation results are confirmed by experimental data.

Modelling and experimental evidence of the cathode erosion in a plasma spray torch

The lifetime of tungsten cathodes used in plasma spray torches is limited by processes leading to a loss of cathode material. It was reported in the literature that the mechanism of their erosion is the evaporation. A model of the ionization layer of a cathode is developed to study the diffusive transport of evaporated tungsten atoms and tungsten ions produced due to ionization by electron impact in a background argon plasma. It is shown that the Stefan–Maxwell equations do not reduce to Fick law as one could expect for the transport of diluted species, which is due to significant diffusion velocities of argon ions. The ionization of tungsten atoms occurs in a distance of a few micrometers from the cathode surface and leads to a strong sink, which increases the net flux of tungsten atoms far beyond that obtained in absence of tungsten ions. This shows that the tungsten ions are driven by the electric field towards the cathode resulting in no net diffusive flux and no removal of tungsten species from the ionization layer even if convection is accounted for. A possible mechanism of removal is found by extending the model to comprise an anode. The extended model resolves the inter-electrode region and provides the plasma parameters for a current density corresponding to the value at the center of the cathode under typical arc currents of 600 A and 800 A. The presence of the anode causes a reversal of the electric field on the anode side, which pulls the ions away from the ionization layer of the cathode. The net flux of tungsten ions can be further fortified by convection. This model allows one to evaluate the loss of cathode material under realistic operating conditions in a quantitative agreement with measured values.

Spatial Structure of Gas Dynamic Characteristics In A Glow Discharge With A Supersonic Axisymmetric Gas Flow

The paper presents the results of theoretical calculations of the gas-dynamic parameters of a supersonic flow in an axisymmetric Laval nozzle for the case when the central body removed from the nozzle serves as one of the glow discharge electrodes. It is shown that the presence of a supersonic flow makes it possible to establish a stationary inhomogeneous density of gas particles between the cathode and the anode of a glow discharge. The density of gas particles near the cathode and anode may differ by tens of times. The results of the study of the glow pattern of a discharge with a supersonic gas flow are presented. The dependence of the characteristics of the glow discharge on the distribution of inhomogeneous gas density in the interelectrode region is revealed. This effect opens up new possibilities for the use of discharges with supersonic gas flow for nanocoating and synthesis of nanostructures.

Effect of Electrode Profile and Polarity on Performance of Pressurized Sparkgap Switch

Sparkgap are most widely used closing switches in various high-voltage pulsed power systems and its reliable operation at desired voltage level is very essential. Conventionally by adjusting the filling gas pressure inside sparkgap switch, breakdown voltage level is altered but switching characteristics such as stability in hold-off voltage at various pressures, breakdown delay, plasma channel formation, and erosion rate are mainly dictated by adopted electrode profile and its dimensions, inter-electrode gap length and polarity. In this paper, experimental results obtained on breakdown characteristics of four different electrode geometries—Plane Parallel, Hemi-spherical, Bruce, and Rogowski and also a generalized criterion for fixing major dimensions of electrode and inter-gap length to ensure uniform electric field in the inter-electrode region are reported. All electrodes are of brass material and have common radius and thickness of 25 mm and 18 mm, respectively (surface finish <1 µm). Experiments performed on various electrode profiles in gap lengths of 2 mm to 5 mm range with pure nitrogen (N2) gas pressurization up to 50 psi reveal that among all profiles, Rogowski performs most reliably having stable hold-off voltage in wide operating range. Hold-off voltage magnitude and breakdown delay was commonly obtained higher for negative polarity in all trials. A comprehensive overview of experimental investigation reported herein compares suitability of various electrode profiles and polarity for reliable switching.

Pseudospark-Driven High-Current Miniaturized Voltage-Tunable Sheet-Electron-Beam Source

For high-power and compact sub-terahertz (THz) sources, a high-current electron beam is a crucial component. In this article, a miniaturized sheet-electron-beam source has been developed for generation of the high-current-density electron beam. It consists of a hollow cathode with a 3-mm diameter circular aperture, an anode with a 1.25 mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times0.25$ </tex-math></inline-formula> mm rectangular aperture, and an interelectrode three-gap region with optimized dimension. The developed electron beam source has been operated in self-breakdown conditions for applied gap voltage ranging between 10 and 30 kV. The sheet electron beam has been successfully propagated for nearly 45 mm inside the drift space without the assistance of an external magnetic field due to the space charge neutralization effect. It has demonstrated good transmission characteristics with more than 50-A beam current and beam current loss of around 25%.

The sputtering of titanium magnetron target with increased temperature in reactive atmosphere by gas injection magnetron sputtering technique

This paper presents a Gas Injection Magnetron Sputtering technique as a target temperature-controlling method. This technique is based on the generation of discrete plasmoids by injecting gas into the inter-electrode region. It provides control over the target temperature with limited heat dissipation by using various frequencies of plasma pulse generation. TiN and TiO2 coatings were selected as test materials for the study. Reactive sputtering processes with limited capability to cool the titanium target were carried out by generating a series of 500-ms plasma pulses at frequencies ranging from 0.5 to 1 Hz. The process variants allowed to achieve target temperatures ranging from 720 to 1210 °C. The results of structural, chemical and phase composition studies showed that the temperature of the target sputtering process did not influence the coatings' chemical and phase state. We have observed slight differences in the structure of TiO2 with the temperature since it is a material with a lower melting temperature than TiN. The differences in the thickness of fabricated coatings have been determined. In all the hot sputtering process cases, the deposited layers’ thickness was increased compared to the cold process. The growth kinetics normalized to the power effectivity decreases with the temperature due to the poisoning of the target material.

Transient modelling and simulation of the forced arc extinguish period of the circuit breakers

Plasma transport during continuous extinction processes from high-current arcs to fully extinction is an important issue in the study of vacuum arcs. In this research a 3D hybrid plasma model has been developed to study the extinguish period of the DC vacuum arc. The model treats the electrons as a massless fluid and ions as macro particles. The ion-neutral collision processes including both charge exchange collisions and momentum exchange collisions are considered with Monte-Carlo method. By this approach, the vacuum arc model consists of plasma jets ejected from multiple cathode spots is established and the behaviours of ions under the collisions with neutral atoms are simulated. It is shown that a large number of low-speed ions are generated in the interelectrode region due to the chare exchange collisions between ions and neutrals. And as the arc current drops to zero, the proportion of low-speed ions to the total ions at each moment becomes higher and higher.

Numerical investigation on the influence of metal particles on the characteristics of a high-current vacuum arc

In the arc-burning process of a high-current vacuum arc (HCVA), the metal particles (MPs) splashed from the active anode will have a significant influence on the plasma characteristics of the arc column. In this paper, the influence of varying MPs on the characteristics of HCVA are studied by establishing a HCVA model containing single or multiple MPs. The simulation results show that when the MPs vaporized metal vapor (MV) enters the interelectrode region and once the arc column plasma cannot ionize all atoms immediately, the ionization layer and the neutral atom vapor area (NAVA) will be formed in the adjacent region of the MPs. When the MP diameter and temperature increase, the number of vaporized metal atoms increases, so that the influence range of MV increases, and the area of ionization layer and the NAVA increases. In addition, when the arc current increases or the MPs are closer to the cathode surface, the greater the ion number density and ion pressure around the MPs are, the stronger the compression on the MV will be, resulting in the decrease of the area of ionization layer and the neutral atom vapor, and the increase of the net ionization rate of the ion number density. When multiple MPs exist in the interelectrode region at the same time, the MV from the MPs will affect each other. In the central region of multiple MPs, the density of MV becomes the largest, while the net ionization rate of ion number density is distributed in the periphery region of the MP group.

Измерение пробивного напряжения вязких жидкостей

The national standards on measuring the breakdown voltage of liquid dielectrics are considered, in particular, the measurement procedure and the measuring cell main parameters. The breakdown voltage of PDMS-1000, PDMS-12500 and PDMS-30000 liquids produced by various manufacturers was measured, and the main problems faced in carrying out measurements were identified. It is shown, using the example of polydimethylsiloxane liquid dielectrics, that after breakdown of viscous liquids, a channel is produced in them, which consists of gas bubbles held by the surface tension force, due to which the channel may in many cases persist for a long time. Theoretical calculations confirmed by an experiment were performed, based on which the velocity of air bubbles in polydimethylsiloxane liquids can be estimated. A conclusion has been drawn about the necessity to increase the time intervals between individual measurements, as well as the time interval before the start of measurements after pouring the test liquid into the measurement cell. It is shown that visual control of the interelectrode region and a special method of mixing the liquid are also necessary. It can be stated based on the accomplished study that the existing standards for breakdown voltage measurements in viscous liquids need to be refined.

Structure optimization of carbon nanotube ionization sensor based on fluid model

Compared with the traditional ionization sensor, triple electrode ionization gas sensor based on carbon nanotubes features excellent performances of small size and low operation voltage, and plays an important role in developing smart grids and the ubiquitous Internet of Things. However, they exhibit the disadvantages of small collecting current and low sensitivity, and need to be optimized in structure. Based on the Townsend discharge mechanism and three governing equations of particle mass conservation, electron energy conservation and Poisson equations, a two-dimensional plasma discharge fluid simulation model of the sensor is established by using the COMSOL Multiphysics software. According to gas composition, component concentration, structure of the sensor and voltage applied to the electrodes, we determine the chemical reaction, reaction rate coefficient, field control equation, initial values and boundary conditions. In order to calculate the interior distribution of charged particles inside the boundary, the inter-electrode space region is meshed. Initial values of mesh are set, such as a dense mesh of nano-meter length around the electrode having micro-nano structure, and a rough mesh of micro-meter length in the other region of inter-electrode space. The initial values of time-step are set for each mesh grid, and the discharge model of each mesh grid is numerically solved by the finite volume method. The three linkage governing equations are discretized and calculated iteratively in time and space, and the mesh values and time-steps are adjusted to make the results converged. The electrostatic field distribution and the average collecting current density of the eight kinds of sensors are obtained in the background gas of nitrogen. The optimal sensor structure is obtained by comparing the electric field intensities and current densities under different structure parameters. The eight kinds of the sensors are prepared for experimentally verifying the optimization method. The optimal structure of the 7# sensor has the highest collecting current density in the eight kind structures, which is consistent with the simulation results, and proves the feasibility of the structure optimization method proposed in the paper. Based on the optimal structure, the sensors, respectively with the electrode spacings of 100 and 120 μm, are fabricated, and the response characteristics of NO/SO&lt;sub&gt;2&lt;/sub&gt; mixture gas are obtained. Compared with other technologies, the sensor with optimal structure exhibits 1－2 orders of sensitivity higher than the others. The optimized triple electrode ionization sensor based on carbon nanotubes exhibits many potential applications.

Elevated OH production from NPHFD and its effect on ignition

This work explores ignition in flowing mixtures of methane and air at 100 kPa and 295 K initial temperature using a nanosecond-pulsed high-frequency discharge ignition source. Simultaneous OH planar laser-induced fluorescence (PLIF) and schlieren imaging were utilized at a frequency of 50 kHz to examine the time dependent radical generation during and after the plasma discharge. The results indicate that a significant volume of OH was generated in the discharge, and the magnitude of the said volume was a strong function of the pulse repetition frequency (PRF). In addition, it is demonstrated that the intensity of the OH-PLIF signal during and shortly after the discharge is elevated for PRF≥10 kHz, indicating that both the volume and concentration of OH are built up at high PRF. This accumulation of OH radicals in the inter-electrode region is directly correlated with high ignition probability, with higher PRF leading to faster OH accumulation in a step-wise fashion. It is shown that in cases in which the PRF is below a certain threshold (<10 kHz), OH accumulation does not occur, and it is believed that this condition, along with lower discharge temperatures, is directly responsible for reduced ignition probability.

Numerical Investigation of Magnetic Field in AMF Vacuum Arc Switch With Transverse Magnetic Field From Three-Phase Structure

When short current is interrupted in three-phase vacuum interrupter in switchgear, the “U”-type loop composed of external busbar, conductive rod, and vacuum interrupter, and its adjacent phases will generate external actual transverse magnetic field (EATMF) in the interrupter. The existence of EATMF will deflect the vacuum arc, which will affect the breaking performance of vacuum switch. In view of this situation, this article establishes two kinds of electrode system simulation models considering and not considering EATMF. The simulation results show that EATMF is not uniformly distributed in the interelectrode region, but is stronger in the contact cup edge area outside the “U”-type loop, weaker in the “U”-type loop, and mainly affects <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${B} _{x}$ </tex-math></inline-formula> . In different short-circuit cases, EATMF is also different due to the difference of phase distance and short current. In the case of three-phase short circuit, the magnetic field in the interelectrode region of the middle phase is most significantly affected by EATMF. EATMF is closely related to the structure size of vacuum switch. By changing the structure size of vacuum switch (e.g., phase spacing <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}$ </tex-math></inline-formula> , conductive rod length <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${h} _{1}$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${h} _{2}$ </tex-math></inline-formula> ), EATMF can be weakened to some extent.

One-Step Synthesis of Graphene, Copper and Zinc Oxide Graphene Hybrids via Arc Discharge: Experiments and Modeling

In this paper, we report on a modified arc process to synthetize graphene, copper and zinc oxide graphene hybrids. The anode was made of pure graphite or graphite mixed with metals or metal oxides. After applying a controlled direct current, plasma is created in the interelectrode region and the anode is consumed by eroding. Continuous and abundant flux of small carbon, zinc or copper species, issued from the anode at a relatively high temperature, flows through the plasma and condenses in the vicinity of a water-cooled cathode leading to few-layered graphene sheets and highly ordered carbon structures. When the graphite rod is filled with copper or zinc oxide nanoparticles, few layers of curved graphene films were anchored with spherical Cu and ZnO nanoparticles leading to a one-step process synthesis of graphene hybrids, which combine the synergetic properties of graphene along with nanostructured metals or semiconducting materials. The as-prepared samples were characterized by Raman spectroscopy, X-ray diffraction (XRD), spatially resolved electron energy loss spectroscopy (EELS), energy filtered elemental mapping and transmission electron microscopy (TEM). In addition to the experimental study, numerical simulations were performed to determine the velocity, temperature and chemical species distributions in the arc plasma under specific graphene synthesis conditions, thereby providing valuable insight into growth mechanisms.

Unified modelling of non-equilibrium microarcs in atmospheric pressure argon: potentials and limitations of one-dimensional models in comparison to two-dimensional models

A recently developed modelling approach is applied in one and two dimensions in order to obtain the plasma properties of direct current microarcs in atmospheric pressure argon under non-equilibrium conditions. The configuration includes a cylindrical cathode made of tungsten and a cooled copper anode. The one-dimensional (1D) description represents the situation on the axis of the two-dimensional (2D) axially symmetric arrangement. The models solve equations for species and energy conservation of electrons and heavy particles and the Poisson equation, which are coupled to the heat conduction in the thermionic cathode. The results obtained show very good agreement between the one- and 2D models provided that the microarc plasma fills and solely occupies the inter-electrode space. When the arc attachment spreads to the lateral surface of the cathode, the 1D model can be applied at a reduced current density to explore the plasma in the inter-electrode region.

Numerical analysis for optimization of the sidewall conditions in a capacitively coupled plasma deposition reactor

Capacitively coupled plasma (CCP) is mainly being used in the semiconductor industry for plasma-enhanced chemical vapor deposition of uniform thin films. Because a discharge volume in the standard configuration of a CCP reactor is surrounded not only with electrode surfaces but also with a sidewall, the sidewall conditions affect the deposition rate profiles noticeably. By toggling the boundary condition from a grounded conductor to dielectrics with the variations of the relative permittivity and the thickness, we compare the spatial profiles for the species densities, ionization rate, power absorption, and particle fluxes in a SiH4/He CCP. Through the SiH4/He CCP fluid model, it is found that a thick and low-permittivity insulator achieves the most uniform plasma density distribution in the interelectrode region and, consequently, the best uniformity in the deposition rate profile of an a-Si:H film. As a validation, experimental results are compared with fluid modeling results, and they match well. For additional validation, a particle-in-cell simulation of pure Ar discharge is also performed. Although simulation conditions are totally different from those of the SiH4/He fluid model, it consistently demonstrates that the dielectric sidewall brings about more uniform distributions of the plasma parameters than the grounded sidewall.

Numerical Study of the Current Constriction in a Vacuum Arc at Large Contact Gap

A classical two-temperature magnetohydrodynamic modeling approach is used to study the influence of a large contact gap (up to 40 mm) on the behavior of a diffuse vacuum arc controlled by a 5-mT/kA uniform external axial magnetic field between two copper electrodes with a 20-mm radius. The current constriction and the energy flux density at the anode surface are more particularly analyzed, considering both supersonic and subsonic flow conditions. In the case of a supersonic arc, simulations show that the constriction of the current develops in the whole interelectrode region, but the constriction level at the anode surface does not evolve monotonically with the contact gap. The constriction is partly correlated with the radial compression of the plasma. In the case of a subsonic arc, the current constriction is related to the presence of the anode sheath. It occurs only close to the anode (from a constant distance from the anode of around 15 mm). Whereas the current constriction at the anode surface increases when the gap length goes from 10 to 20 mm, it no longer evolves when increasing the gap length from 20 to 40 mm. For both flow conditions, the evolution with the gap length of the radial profile of the energy flux density transferred by the plasma to the anode is similar as that followed by the current constriction at the anode.