High-voltage Nanosecond Discharge Research Articles

Numerical simulation of a partially anisotropic electron distribution function in a pulsed discharge with a hollow cathode

In this paper, we propose a convenient method for calculating the electron energy distribution function (EEDF) in a gas-discharge plasma, where the presence of high-energy electrons results in the formation of a partial EEDF anisotropy. A detailed description of this method is provided. The performance of the proposed method is demonstrated to calculate the EEDF of plasma column of a high-voltage nanosecond discharge in argon with an extended hollow cathode. The applicability assessment of the proposed method calculating the EEDF in the plasma column of pulsed discharge sustained by the 50 Hz pulse and gas pressure from 5 to 40 Torr is given. It was shown that high-energy electrons were effectively formed in this type of discharge with an increase in the EEDF in the high-energy region. This technique can also be usefully employed for other types of discharges where fast electrons are formed.

Characteristics and Parameters of Plasma of a High-Voltage Nanosecond Discharge in Argon at Atmospheric Pressure with an Ectonic Mechanism of Copper Vapor Introduction into Plasma

Characteristics and Parameters of Plasma of a High-Voltage Nanosecond Discharge in Argon at Atmospheric Pressure with an Ectonic Mechanism of Copper Vapor Introduction into Plasma

Kinetics of Low-Temperature Plasma-Assisted Ignition of Ethanol-Gasoline Surrogate under Gasoline Engine like Conditions

ABSTRACT The plasma-assisted reaction kinetic mechanism for high-voltage nanosecond discharge at high pressure was developed for lean and stoichiometric ethanol-gasoline surrogate/O2/N2/AR mixture. The numerical results show that plasma-assisted ignition of ethanol-gasoline surrogate is mainly associated with the electron impact dissociation of O2, nC7H16, iC8H18 and C2H5OH molecules and excited O2* produced from electron impact during discharge phase. The generation of H and O atoms, radicals and O2* facilitates plasma-assisted ignition (PAI). The formation of C2H5OH+, C8H18 + and O− increases ignition delay time. PAI improves the ignition stability of lean mixture mainly by increasing the reaction rate in reaction iC8H18+ HO2⇋C8H17+ H2O2. The reaction CH3+ HO2⇋CH4+ O2 promotes auto-ignition. But it delays the plasma-assisted ignition of stoichiometric mixture while it facilitates plasma-assisted ignition in lean mixture. The ignition delay time in lean-burn conditions can be shortened only by a proper timing of low-temperature plasma discharge at low temperature.

High-Voltage Nanosecond Discharge as a Means of Fast Energy Switching

The formation of a nanosecond discharge with the use of a Hamamatsu streak-camera and with simultaneously wideband (10 GHz) measurement of voltage and displacement current caused by a streamer in one pulse has been studied. Nanosecond voltage pulses of various amplitudes (16, 20, and 27 kV) were applied across a point-to-plane gap (8.5 mm) filled with air at various pressures (13, 25, 50, 100, and 200 kPa). It was found that the voltage across the gap drops as soon as a streamer appears in the vicinity of the pointed electrode. At the same time, a pre-breakdown current begins to flow. The magnitude of the pre-breakdown current, as well as the voltage drop, is determined by the rate of formation of dense plasma and, accordingly, by the rate of redistribution of the electric field in the gap. The streamer velocity determines the rise time and amplitude of the current. The higher the streamer velocity, the shorter the rise time and the higher the amplitude of the pre-breakdown current. The propagation of a backward and third ionization waves was observed both with the streak camera and by measuring the displacement current. As they propagate, the discharge current increases to its amplitude value.

Time behavior of an electron beam current pulse in the axial and peripheral zones of an anode in vacuum and gas-filled diodes

The study of the time behavior of a current pulse of an electron beam generated during a high-voltage nanosecond discharge in gas-filled and vacuum diodes has been carried out. As follows from the experimental results, in both cases, the distribution of the beam current density in the plane of a grounded anode is non-uniform. The highest beam current density is recorded in the axial part of the anode. It was established that in the case of a gas-filled diode, ~ 2 ns after the onset of the beam current pulse, its shape in the axial anode zone changes relative to that in the peripheral one. It is assumed that the most probable reason for this is the effect of compensation of the charge of the beam electrons by the positive charge of ions arising in the ionization process in the paraxial zone.

Experimental study of atmospheric pressure single-pulse nanosecond discharge in pin-to-pin configuration

We present an experimental study of nanosecond high-voltage discharges in a pin-to-pin electrode configuration at atmospheric conditions operating in a single-pulse mode (no memory effects). Discharge parameters were measured using microwave Rayleigh scattering, laser Rayleigh scattering, optical emission spectroscopy enhanced with a nanosecond probing pulse, and fast photography. Spark and corona discharge regimes were studied for electrode gap sizes of 2–10 mm and a discharge pulse duration of 90 ns. The spark regime was observed for gaps <6 mm using discharge pulse energies of 0.6–1 mJ per mm of the gap length. Higher electron number densities, total electron number per gap length, discharge currents, and gas temperatures were observed for smaller electrode gaps and larger pulse energies, reaching maximal values of about 7.5 × 1015 cm−3, 3.5 × 1011 electrons/mm, 22 A, and 4000 K (at 10 μs after the discharge), respectively, for a 2 mm gap and 1 mJ/mm discharge pulse energy. An initial breakdown was followed by a secondary breakdown occurring about 30–70 ns later and was associated with ignition of a cathode spot and transition to a cathodic arc. A majority of the discharge pulse energy was deposited into the gas before the secondary breakdown (85%–89%). The electron number density after the ns-discharge pulse decayed with a characteristic time scale of 150 ns governed by dissociative recombination and electron attachment to oxygen mechanisms. For the corona regime, substantially lower pulse energies (∼0.1 mJ/mm), peak conduction current (1–2 A), electron numbers (3–5 × 1010 electrons per mm), and gas temperatures (360 K) were observed.

The Effect of Gas Heating on the Decay of Plasma with Hydrated Ions after a High-Voltage Nanosecond Discharge

The Effect of Gas Heating on the Decay of Plasma with Hydrated Ions after a High-Voltage Nanosecond Discharge

Main modes of runaway electron generation during a breakdown of high-pressure gases in an inhomogeneous electric field

Using our recently proposed method, we have identified three modes of runaway electron generation. This method is based on the recording of a dynamic displacement current using a current collector. It makes it possible to unambiguously interpret the dynamics of ionization processes occurring at the stage of formation of a high-voltage nanosecond discharge in a strongly inhomogeneous electric field and dense gaseous media. Simultaneous and separate recording by the current collector of a runaway electron beam current and the dynamic displacement current as well as the recording of discharge characteristics and high-speed framing of plasma glow at the breakdown stage made it possible to interpret the runaway electron generation modes in terms of the rate of ionization processes. It has been shown how the reduced electric field strength, the gas kind, the design and material of a high-voltage cathode with a small radius of curvature, the amplitude and rise time of a voltage pulse as well as the delay in the onset of the explosive emission process at the cathode affect the implementation of one or another generation mode.

The dynamics of a nanosecond gas discharge development with an extended slot cathode in argon

The article presents the results of an experimental study and numerical modelling for the formation and development dynamics of a high-voltage transverse nanosecond discharge generated by a slot cathode in an argon medium at a pressure range of 1–10 Torr. Numerical modelling was carried out under similar experimental conditions for the processes of formation and propagation of ionisation waves, electron density distribution, excited atom and average electron energy in the discharge gap, including the cavity inside the cathode. At a pressure of p = 1 Torr, a classical version of a high-voltage discharge is demonstrated to take place with no penetration of the plasma into the cathode cavity and no observed hollow cathode effect. An increase in gas pressure to 5 Torr leads to a penetration of plasma into the cathode cavity with the formation of a cathodic potential drop (CPD) region. Electrons emitted from the side surfaces of the cavity pass through the CPD region without collisions, oscillate inside the cathode cavity; the hollow cathode effect is fully manifested. At р = 10 Torr, the modelling results qualitatively coincide with the results at р = 5 Torr; in this case, however, hardly any accelerated electrons are observed in the gap between the electrodes, due to their energetic relaxation both inside the cathode cavity and when exiting from it. In both cases, the plasma structure formed at the exit of the cathode cavity involves a concentration of charged particles an order of magnitude higher than that in the rest of the gap, leading to a self-limiting discharge current effect. The results of the numerical modelling are in good agreement with experimental data.

Plasma decay in hydrocarbons and hydrocarbon- and H2O-containing mixtures excited by high-voltage nanosecond discharge at elevated gas temperatures

Plasma decay after a high-voltage nanosecond discharge was experimentally and numerically studied in pure hydrocarbons (C2H6 and C3H8), and H2O:N2 and C3H8:O2 mixtures for pressures in the range 2–4 Torr and gas temperatures from 300 to 600 K. In a stoichiometric C3H8:O2 mixture, plasma decay was also studied in a repetitively pulsed discharge for varing numbers of discharge pulses (varying degrees of fuel oxidation). The rate of plasma decay was determined from the temporal evolution of electron density measured using the microwave interferometer. It was observed that gas heating to 600 K leads to a decrease in the rate of plasma decay in all cases. The effect of heating on plasma decay was most profound in the H2O:N2 mixture (after a single discharge pulse) and in the C3H8:O2 mixture for high degrees of fuel oxidation. A kinetic scheme was developed to numerically simulate the plasma decay in hydrocarbons and combustible mixtures. Numerical analysis showed that, under the conditions studied, plasma decay was controlled by dissociative electron recombination with simple molecular and cluster ions. Gas heating led to a decrease in the rate of the electron-ion recombination and the rate of conversion of molecular ions to cluster ions. As a result, the gas temperature increase caused a decrease in the fraction of cluster ions for which the recombination coefficients are an order of magnitude higher than the recombination coefficients for molecular ions. The influence of gas heating on the decrease of the amount of cluster ions was more important when the ion composition was dominated by hydrated H3O+(H2O)k ions. The rates of the formation of these ions are extremely sensitive to any variations in gas temperature. As a result, in agreement with our observations, gas heating led to an anomalous decrease in the rates of plasma decay in the H2O:N2 mixture, as well as in the C3H8:O2 mixture when H2O molecules were produced due to fuel oxidation.

The effect of electron heating on hydrocarbon plasma decay after high-voltage nanosecond discharge

An experimental study of plasma decay in the afterglow of a high-voltage nanosecond discharge in ethane:argon mixtures was performed under conditions when the mean electron energy significantly exceeded the mean energy of neutral particles at room temperature and pressures from 2 to 10 Torr. Electrons were heated during the plasma decay (i) by retarding electron thermalization through a strong dilution of the hydrocarbon with argon and (ii) by the application of an additional external electric field. Using the microwave interferometer, the temporal evolution of the electron density during plasma decay was studied in the range from 5 × 1010 to 3 × 1012 cm−3. The analysis of the experimental data allowed us to obtain the dependence of the effective recombination coefficients of electrons with hydrocarbon ions on the electron temperature. A simplified kinetic scheme was proposed for the description of electron and ion reactions during the plasma decay for our experimental conditions. Calculations on the basis of this scheme showed that the plasma decay is controlled by the dissociative recombination of molecular and cluster hydrocarbon ions.

Cumulation of a High-Current Electron Beam During a Nanosecond High-Voltage Discharge in a Low-Pressure Diode

The results of an experimental investigation of the effect of cumulation of a beam of runaway electrons formed in a high-voltage nanosecond discharge at a reduced air pressure are presented. The optimal conditions of this effect in a discharge gap in a tubular cathode – grounded planar anode geometry were achieved at an air pressure of ≈5 Pa and an interelectrode gap of 2.75 mm. An electron-beam current pulse is recorded with a high time resolution (up to about 80 ps) behind the flat foil anode. It is found out that due to this effect a through hole is formed in a 20 μm-thick aluminum foil after 2–3 discharge pulses. The results obtained suggest that the electron energy in the second part of the beam current pulse is lower than that in its first part.

Shock-tube study of dimethyl ether ignition by high-voltage nanosecond discharge

A shock tube with a discharge cell was used to experimentally analyze the kinetics of dimethil ether (DME) ignition in DME:O2:Ar and DME:O2:Ar:He mixtures at temperatures from 1075 to 1860 K. We measured ignition delay time in lean and stoichiometric mixtures after a reflected shock wave. Measurements were made in the mixtures activated by a high-voltage nanosecond discharge and without discharge plasma. The dominant mechanisms of DME ignition were studied on the basis of a numerical simulation of the discharge and ignition stages. We calculated the densities of chemically active species generated in the discharge and used these data to model ignition. The calculated ignition delay times agreed reasonably with the measured data for autoignition and plasma-assisted ignition. The limiting processes during the discharge and ignition were shown using sensitivity analysis. It was demonstrated that the partial replacement of Ar with He in the mixtures studied allowed the same values of plasma-assisted ignition delay times at much lower gas temperatures being opposite to ignition without plasma where the delay time was not sensitive to the replacement of Ar with He. Calculations showed that this is explained by an increased specific discharge energy spent on the excitation of DME, O2 and Ar, whereas the excitation of He was negligible. It followed from the calculations that electron-impact ionization of DME molecules is of great importance for the production of active species in the discharge phase and consequently for ignition with plasma. The electron-impact ionization cross section of DME is poorly known and its variation influences the calculated ignition delay time. In addition, at reduced gas temperatures, the effect of plasma nonuniformity is important for shock-tube studies.

Development of high-voltage nanosecond discharge in strongly non-uniform gas

The results of the experimental and numerical study of a high-voltage nanosecond discharge developing through a shock wave in air and combustible mixtures were presented for various neutral density jumps across the wave. The discharge uniformity was investigated using ICCD camera images in a shock tube. The results demonstrated that the discharge plasma became more uniform when the ionization wave moved from a high-density region to a low-density region. The radiation intensity emitted by the plasma in the low-density region was higher than that emitted by the plasma in the high-density plasma. The results of a zero-dimensional simulation in the case of quasi-uniform plasma agree well with the measured ratios of the intensities radiated by the plasmas before and after the shock front. A two-dimensional simulation of the development of a steamer intersecting the shock wave showed that the streamer characteristics change drastically during the transition from the high-density region to the low-density region. Streamer penetration into the low-density region was accompanied by the formation of primary and secondary ionization waves.

Processes controlling properties of high-voltage nanosecond discharge plasma in combustible mixtures

We present the results of the experimental study of high-voltage nanosecond repetitively pulsed discharge and of its afterglow in ethane, dimethyl ether and their mixtures with oxygen. The measurements were made for room temperature and pressures from 2 to 6 Torr. The measured specific deposited energy and mean discharge current varied with the number of voltage pulses monotonously in ethane and dimethyl ether and nonmonotonously in the hydrocarbon:oxygen mixtures. A microwave interferometer was used for time-resolved electron density measurements in the discharge afterglow. The effective recombination coefficients were obtained from the analysis of the measured data. These coefficients varied with the number of voltage pulses monotonously in ethane and peaked in pure dimethyl ether and its mixture with oxygen. Possible mechanisms of the nonmonotonous behavior of the discharge characteristics were discussed.

Recombination of electrons with water cluster ions in the afterglow of a high-voltage nanosecond discharge

The results of the experimental and numerical study of high-voltage nanosecond discharge afterglow in H2O:N2 and H2O:O2 mixtures are presented for room temperature and at pressures from 2 to 5 Torr. Time-resolved electron density during the plasma decay was measured with a microwave interferometer for initial electron densities in the range between 1 × 1012 and 2 × 1012 cm−3. Calculations showed that the plasma decay was controlled by recombination of thermalized electrons with H3O+(H2O)n ions for n from 0 to 4. Agreement between calculated and measured electron density histories was obtained only when using the recombination coefficients measured in the pulsed plasma afterglow experiments. The electron densities calculated using the data from the storage ring experiments were consistently greater than the values measured in this work for all conditions. It was concluded that the measurements of recombination coefficients for H3O+(H2O)n ions in the pulsed plasma afterglow were more appropriate for simulating the properties of high-density plasmas with high fractions of H2O, O2 and N2, such as discharge plasmas in water vapor and in humid air instead of the measurements in the storage ring experiments.

Application of the High-Voltage Nanosecond Discharge for the Control of Proliferation of Fibroblast Cells on the Surface of Fluorine-Carbon Polymer Implants

The possibility of modifying the surface of film implants of porous fluoroplastic by treating them with a low-temperature plasma of a high-voltage discharge with a nanosecond duration was investigated. The specimens were films for reconstructive surgery (FRS) and films for the replacement of bone defects (FRBD) from polytetrafluoroethylene (PTFE). The duration of the plasma treatment of the specimens ranged from 1 to 10 s. The surface quality of the films was estimated by their roughness and hydrophilic behavior using an atomic force microscope. It has been found that the plasma treatment significantly increases the surface roughness, which is associated with a change in its structure. The degree of the hydrophilicity of the surface of the FRS and FRBD films was determined by the roll-off angle of a water droplet and depended on the initial state of the surface (the smooth or corrugated side of the specimen), the gas discharge gap width, and the duration of the plasma treatment. A correlation between the roll-off angle of a water droplet and the proliferative capacity of the cells that were inoculated was found: the larger the roll-off angle, the higher the density of the cells observed on the surface. It has been found that the plasma treatment hardly affects the mechanical strength of the films and ensures the sterility of the films for 28 days after the plasma treatment for 1 s.

Inhibition of plasma-assisted ignition in hydrogen–oxygen mixtures by hydrocarbons

The effect of C2H4 and C2H6 addition on the ignition of a stoichiometric hydrogen:oxygen mixture was experimentally analyzed using a shock tube with a discharge cell. Ignition delay time was measured behind a reflected shock wave after a high-voltage nanosecond discharge and in its absence using mixtures with small percentages of hydrocarbons. The obtained results were compared with ignition delay times in a mixture without hydrocarbon addition. It was shown that C2H4 and C2H6 inhibit the hydrogen–oxygen reaction and that the inhibition effect is much more profound when the mixtures are ignited after a high-voltage nanosecond discharge. A numerical simulation of the discharge and ignition phases was used to show the main mechanisms that control the inhibition of hydrogen:oxygen ignition by hydrocarbons. The densities of atoms, radicals, excited particles and charged particles produced in the discharge plasma were calculated and used as input parameters for ignition modeling. Reasonable agreement was obtained between the calculated and measured plasma-assisted ignition delay times in the mixtures under consideration. The analysis of primary reaction pathways for hydrocarbon species was used to demonstrate the inhibition mechanisms of ignition in hydrogen:oxygen mixtures excited by non-equilibrium discharge plasma.

Burning velocity measurement of lean methane-air flames in a new nanosecond DBD microplasma burner platform

This paper presents the initial characterization of a new burner design to study the effect of non-thermal plasma discharge on combustion characteristics at atmospheric pressure. The burner allows stabilizing an inverted cone flame in a mixture flowing through a perforated plate designed as a microplasma reactor. The design principle of the microplasma reactor is based on the dielectric barrier discharge scheme which helps to generate a stable non-thermal plasma discharge driven by nanosecond high-voltage pulses in the burner holes. The consumed power and pulse energy have been calculated from simultaneously measurements of current and voltage of the electrical pulses. Time-resolved measurements of direct emission spectra for nitrogen second positive system N2(C-B) have been done to determine the rotational and vibrational temperatures of the plasma discharge. By fitting the spectra with SPECAIR simulation data, it was found that the rotational and vibrational temperatures are 480 K and 3700 K, respectively, for the discharge in methane-air mixture with an equivalence ratio of 0.5 at atmospheric pressure. The influence of a high-voltage (5 kV) pulsed nanosecond discharge on the laminar burning velocity of methane-air flame has been investigated over a range of equivalence ratios (0.55–0.75). The laminar burning velocity was calculated by the conical flame area method which has been validated by other published data. CH* chemiluminescence image analysis has been applied to accurately determine the flame area. The results show an increase of the burning velocity of about 100% in very lean (Φ=0.55) flames as a result of the plasma discharge effect.

Plasma Decay in the Afterglow of High-Voltage Nanosecond Discharges in Unsaturated and Oxygenated Hydrocarbons

Results of experimental and theoretical study of plasma decay in the afterglow of high-voltage nanosecond discharges in gaseous ethylene and dimethyl ether at room temperature and pressures from 2 to 20 Torr are presented. Using a microwave interferometer, the time behavior of the electron density in the range from 2 × 1010 to 3 × 1012 cm–3 during plasma decay is investigated. By processing the experimental data, the effective coefficients of electron–ion recombination as functions of the gas pressure are obtained. It is found that these coefficients substantially exceed the recombination coefficients of simple hydrocarbon ions. This distinction, as well as the increase in the effective recombination coefficient with pressure, is explained by the formation of cluster ions in three-body collisions, which recombine with electrons more efficiently than simple molecular ions. The coefficients of three-body conversion of simple molecular ions into cluster ions in the plasmas of ethylene and dimethyl ether, as well as the coefficients of recombination of electrons with cluster ions in these gases, are determined by analyzing the experimental data.