Development of a linear corona torch discharge

An anomalous kind of the positive corona—a linear corona torch discharge—is investigated. The discharge is nearly steady-state and operates with various electrode configurations, e.g., wire-plane, needle-plane, wire-cylinder, and two or more wires placed in parallel. It is found that the discharge exists in the form of a pulsed corona or an ordinary positive corona, which alternatively change each other under the action of a spark-gap switch.     

The effect of a corona discharge on a lightning attachment

The interaction between the lightning leader and the space charge accumulated near the top of a ground object in the atmospheric electric field is considered using analytical and numerical models developed earlier to describe spark discharges in long laboratory gaps. The specific features of a nonstationary corona discharge that develops in the electric field of a thundercloud and a downward lightning leader are analyzed. Conditions for the development of an upward lightning discharge from a ground object and for the propagation of an upward-connecting leader from the object toward a downward lightning leader (the process determining the point of strike to the ground) are investigated. Possible mechanisms for the interaction of the corona space charge with an upward leader and prospects of using it to control downward lightning discharges are analyzed.     

Evaporation rate enhancement of water with air ions from a corona discharge

Space charges (air ions) produced by a single point-to-plane corona electrode system were used to study the enhancement in the evaporation rates of water at three ion current levels. The maximum evaporation rates of 0.019 and 0.017 g·min^?1 were observed at a 1 cm electrode gap for negative and positive air ions, respectively. The cumulative evaporation rates were linear with time and an ion-enhanced rate was about 4 times greater than the control. The current density distribution measurements agreed fairly well with those predicted from the Warburg law. The principal driving force for the observed evaporation enhancement was an ion drag phenomenon which created vortex motions in water when air ions were subjected to an externally applied electric field. Theoretical considerations from derived relationships in fluid mechanics demonstrate that the mass transfer coefficient is higher for positive than negative ions of the same current strength because of the mobility difference between the charges in the medium.     

Electric characteristics of a surface barrier discharge with a plasma induction electrode

Static and dynamic current-voltage and charge-voltage characteristics of a surface barrier discharge with a plasma induction electrode have been investigated experimentally. The dependences of the discharge current on both the gas pressure in the induction electrode tube and the winding pitch of the corona electrode, as well as of the discharge power efficiency on the applied voltage, have been measured.     

Transition of a multipin negative corona in atmospheric air to a glow discharge

It is commonly accepted that, as the current increases, a diffuse negative corona inevitably goes over to a strongly nonuniform and nonsteady spark discharge. In this paper, a new effect—the transition of a negative corona to a diffuse glow discharge at atmospheric pressure—is studied experimentally and numerically. The evolution of the corona parameters during the transition to the regime of a glow discharge is traced.     

Potato slab dehydration by air ions from corona discharge

Space charge (air ions) produced by single corona electrodes was used to enhance drying rates from fresh slabs of potato. The drying path was traced by a beta-ray gauge which provided both sensitivity and reproducibility to the measurements of drying time. The rate of evaporation was increased 2.2 to 3.0 times when subjected to fluxes of 3.02×10¹² positive ions alone or in combination with 7.31×10¹² negative air ions/cm² per s compared to that from an air-drying control slab. Electric wind caused by an ionic drag force seems to be the principal driving force for the observed enhancement in drying rates.     

Characteristics of a DC discharge with a water cathode in argon

The characteristics of a dc discharge excited between a metal anode and a water cathode in argon were studied experimentally. The dimensions of the positive column and the electric field in it were measured, and the vibrational temperature in the positive column was determined from the N₂ C ³Π_u → B ³Π_g (0–2) emission band. It is shown that the power deposited in the positive column is almost entirely spent on gas heating. The obtained dependence of the reduced electric field on the gas pressure and the ionization frequencies calculated by solving the Boltzmann equation indicate that electrons are lost diffusively, whereas ionization proceeds in a stepwise manner via the lower metastable states of argon atoms.     

Comparison of fungicidal properties of non-thermal plasma produced by corona discharge and dielectric barrier discharge

The inactivation of four micromycete species by action of non-thermal plasma was followed. Two sources of plasma were compared, namely, positive corona discharge and dielectric barrier discharge. The corona discharge appeared as suitable for fungal spore inactivation in water suspension, whereas the barrier discharge inactivated spores on the surface of cultivation agar. Cladosporium sphaerospermum was the most sensitive, being inactivated within 10 min of exposure to plasma, whereas Aspergillus oryzae displayed decrease in viable cell count only, the complete inactivation was not achieved even after 40 min of exposure. Intermediate sensitivity was found for Alternaria sp. and Byssochlamys nivea. The significant delay of growth was observed for all fungi after exposure to sublethal dose of plasma, but we failed to express this effect quantitatively.     

Ozone produced by corona discharge in the presence of water

Significant levels of ozone have been detected in a reaction flask that was designed for studies using negative air ions generated from corona discharge. While diluting the gas in the vessel at the rate of 1.5 liters/min., more than 1.0 ppm O₃ was measured in the vessel when grounded water was present. Ozone levels were much less (15 ppb) in ambient air near the ion generator. Even in the absence of water, O₃ levels were only 7 ppb in the reaction flask. Without the corona discharge apparatus, levels of ozone in the gas phase (with water present) averaged 6 ppb. These results demonstrate that when negative air ions are generated from corona discharge in the presence of water, significant levels of ozone are produced. Therefore, O₃ could be directly responsible for many of the oxidizing effects that have been reportedly due to negative air ions.     

Investigation of the barrier discharge structure near the electrode with a cylindrical cross section

The approximate analytical correlations allowing for the investigation of the effect of different geometrical and physical parameters on the barrier discharge evolution near the electrode with a cylindrical cross section are obtained. The found correlations make it possible to estimate the time of the barrier discharge existence. New essential peculiarities of the barrier discharge evolution are revealed and verified.     

Comparison of the AC barrier corona with DC positive and negative coronas and barrier discharge

Results are presented from experimental studies of ac corona discharges between a point electrode and a dielectric-coated plate in nitrogen, argon, helium, and air in the voltage frequency range f=50 Hz–50 kHz. The characteristic features of this type of discharge are compared with the well-known features of dc positive and negative coronas and a barrier discharge between plane electrodes. It is shown that the presence of a dielectric barrier on the plane electrode significantly changes the electric characteristics and spatial structure of the corona, whereas the main phases of the discharge evolution remain unchanged as the voltage increases. With a point electrode, the breakdown voltage of the barrier corona decreases substantially as compared to the breakdown voltage of a barrier discharge with plane electrodes. This leads to softer conditions for the streamer formation in a barrier corona, which becomes more stable against spark generation.     

Structure of a discharge induced by a coaxial microwave plasmatron with a gas-supply channel in the inner electrode

The structure of a discharge induced by a coaxial microwave plasmatron with a gas-supply channel in the inner electrode of a coaxial waveguide is investigated. A plasmatron with a power of up to 10 W operates at a frequency of 10 GHz. Depending on the operation regime, the discharge takes either a filament or torch form. A plasma filament arises at low flow rates of the working gas (argon) and occurs at the border of the potential core of the gas jet. A torch discharge occurs at high flow rates and has the form of a hollow cone. In both cases, the discharge arises in the potential core of the gas jet and does not spread beyond it. The distribution of the microwave field in the discharge plasma is determined.     

Kinetic processes initiated by a nanosecond high-current discharge in hot air

Results from numerical investigations of kinetic processes initiated by a pulsed nanosecond discharge in hot (T ₀ ≥ 1000 K) air at atmospheric pressure are presented. The calculated results on the dynamics of the electron density, the population of the N₂(B³Π _g ) and N₂(C³Π _u ) states, and the atomic oxygen density in the axial discharge region agree with experiment. The method for determining the gas temperature by measuring the rotational structure of the transitions N₂(C³Π _u , ν) → N₂(B³Π _g , ν′) of the 2⁺ nitrogen system is analyzed. It is shown that, in relatively weak reduced electric fields typical of secondary discharge pulses, the electron impact excitation of the N₂(C³Π _u ) state from the ground state N₂(X¹Σ _g ⁺) can be accompanied by its additional step population from the N₂(B³Π _g ), N₂(a′Σ _u ⁻), and other electronic states. This effect substantially influences the rotational distribution of nitrogen molecules in the N₂(C³Π _u , ν) state; moreover, the temperature determined from this distribution can be substantially higher than the true gas temperature.     

Evolution of the radial structure of a negative corona during its transformation into a glow discharge and a spark

With the proper stabilization of a negative corona, it is possible to increase the threshold current at which the corona discharge in the point-plane gap in air transforms into a spark. Then, in the current range corresponding to the transition region between the corona discharge and the spark, a new type of discharge arises—an atmospheric-pressure diffuse glow discharge. The transformation of the negative corona into a glow discharge and then into a spark is accompanied by the rearrangement of the discharge structure. The experiments show that, as the corona current increases, the radial current profile at the anode shrinks and the glow diameter near the anode increases. The radial profiles of the current and the corona glow during the transition to a glow discharge are measured. The longitudinal structure of the corona is computed using a 1.5-dimensional model that, unlike the other available models, includes gas ionization in the drift region of the corona. The experimental data are used to determine the effective cross section of the current channel at the anode. The radial glow profile near the anode is calculated using the measured current profile and assuming that the field profile is parabolic.     

The persistent microbicidal effect in water exposed to the corona discharge

This article describes and particularly explains a new phenomenon of persistent microbicidal effect of water previously exposed to the low-temperature plasma, which cannot be attributed to the acidification only. The direct microbicidal action of plasma is well documented, being mediated by number of reactive particles with a short lifetime. However, we observed the microbicidal effect also in exposed water stored for a month, where it must be mediated by stable particles. In water and in phosphate-buffered saline, the formation of NO_x and corresponding acids, H₂O₂ and O₃ was confirmed after exposition to the low-temperature plasma generated in air by DC negative glow corona and positive streamer discharge. The time course of acidification, H₂O₂ and O₃ formation were deremined. Except uncertain traces of HCN, SIFT-MS analysis of exposed liquids reveals no additional reactive compounds. The microbicidal effect persists almost unchanged during 4 weeks of storage, although O₃ completely and H₂O₂ almost disappears. Staphylococcus epidermidis and Escherichia coli were inactivated within 10 min of incubation in exposed liquids, Candida albicans needs at least 1 h. The solutions prepared by artificial mixing of reactive compounds mimic the action of exposed water, but in lesser extent. The acid milieu is the main cause of the microbicidal effect, but the possibility of still unidentified additional compound remains open.     

Simulations of a longitudinal glow discharge in a hot air flow at atmospheric pressure

A one-dimensional axisymmetric time-dependent model of a discharge in a gas flow is developed. The model includes a fairly complete set of plasmochemical reactions and describes the heating and gasdynamic expansion of a plasma channel in air. The processes governing the distribution of nitrogen molecules in the N₂(C ³Π_u, v) state over vibrational levels are considered. The parameters of a longitudinal glow discharge in a hot (T ₀ = 1500?3000 K) air flow at atmospheric pressure are calculated. It is found that gas preheating considerably influences the parameters of the discharge channel. The results of calculations are compared with the experimental data.     

Microwave electrode discharge in nitrogen: Structure and characteristics of the electrode region

The parameters of the electrode region of an electrode microwave discharge in nitrogen are studied by emission spectroscopy. The radial and axial distributions of the intensities of the bands of the second (N₂(C ³Π _u → B ³Π _g )) and first (N₂(B ³Π _g → A ³Σ _u ⁺ )) positive systems of molecular nitrogen and the first negative system of nitrogen ions (N ₂ ⁺ (B ²Σ _u ⁺ → X ²Σ _g ⁺ )), the radial profiles of the electric field E and the electron density N _e , and the absolute populations of the vibrational levels v _C = 0–4 of the C ³Π _u excited state of N₂ and the vibrational level v _Bi = 0 of the B ²Σ _u ⁺ excited state of a molecular nitrogen ion are determined. The population temperature of the first vibrational level T _V of the ground electronic state X ¹Σ _g ⁺ of N₂ and the excitation temperature T _C of the C ³Π _u state in the electrode region of the discharge are measured. The radius of the spherical region and the spatially integrated plasma emission spectra are studied as functions of the incident microwave power and gas pressure. A method for determining the electron density and the microwave field strength from the plasma emission characteristics is described in detail.     

Gas temperature in the plasma of a low-pressure electrode microwave discharge in hydrogen

The gas temperature in an electrode microwave discharge in hydrogen at pressures of 1–8 torr and input powers of 20–90 W is determined from the relative intensities of the rotational lines of the electronically excited molecules of the Fulcher α system of molecular hydrogen. It is found that the gas temperature in the discharge is no higher than 800 K over the entire range of the experimental conditions under study. For this reason, plasma resonance cannot be regarded as a factor determining the physical processes in the discharge over the entire pressure range. Since the discharge unit is a nonuniform gas-dynamic system (the gas is fed through a small hole into a chamber of limited size), there is a possibility of generating vortex flows that intensively mix the gas. This results in a uniform distribution of the gas temperature throughout the entire volume of the spherical plasma structure produced in the experiment.     

Spectroscopic measurements and numerical simulations of the electrode plasma of an electrode microwave discharge in hydrogen

The structure of an electrode microwave discharge in hydrogen at pressures of 1–8 torr and incident powers of 20–100 W is studied using optical spectroscopy. A two-dimensional computer code is developed for self-consistently simulating a self-sustained steady-sate electrode microwave discharge ignited at the end of the inner conductor of a coaxial line. The model is based on simultaneously solving time-dependent Maxwell’s equations, the balance equations for charged particles, and a homogeneous Boltzmann equation. The numerical results referring to the electrode region of the discharge are in fair agreement with the experimental data. This confirms the early suggestion (inferred from experimental data) of the combined “self-sustained-non-self-sustained” character of the electrode discharge. It is shown that the self-sustained discharge domain is located in the electrode region of the discharge.     

Low-current medium-pressure RF discharge with electron photoemission in the electrode sheath and penetration of the sheath electrons into the discharge column

A model is developed for simulating a low-current moderate-pressure RF discharge with allowance for such characteristic discharge properties as the existence of two sheaths near both electrodes throughout the RF field period; the formation of an electron cloud at the sheath boundary that periodically fills the sheath and leaves it, depending on the phase of the applied RF voltage; the production by the sheath electrons of metastable gas particles that interact with the cloud electrons during subsequent field periods, followed by the excitation of metastable states to the emitting levels; the formation of a sheath in a low-current RF discharge due to the overlap of the secondary electron avalanches triggered by electron photoemission from the electrode surface; and the conditions under which the sheath electrons penetrate into the positive column and accumulate there, which makes, thereby making a low-current RF discharge similar to a non-self-sustained discharge. The parameters of the sheath in a low-current RF discharge are determined by the conditions under which the electron photoemission current from the electrode surface in the sheath is self-sustaining and, like the parameters of the positive discharge column, depend on the sort of gas, the gas pressure, the frequency of the applied RF field, and the interelectrode distance. The results of calculating the parameters of the sheath and column of a low-current RF discharge for nitrogen and helium at different pressures, as well as for different field frequencies and interelectrode distances, are presented and are compared with the experimental data.