Simulations of a surface glow discharge in a supersonic gas flow in the presence of external ionization

Results of two-dimensional hydrodynamic simulations of a surface glow discharge operating at pressures of 0.2–0.5 Torr in a nitrogen flow propagating with a velocity of 1000 m/s in the presence of external ionization are presented. The effect of the external ionization rate on discharge operation is analyzed. The current-voltage characteristics of the discharge are calculated for different intensities of external ionization in both the presence and absence of secondary electron emission from the cathode. The discharge structure and plasma parameters in the vicinity of the loaded electrode are considered. It is shown that, when the discharge operates at the expense of secondary emission from the cathode, the discharge current and cathode sheath configuration are insensitive to external ionization. It is also demonstrated that, even at a high rate of external ionization, the discharge operates due to secondary emission from the cathode.     

Pulsed mode of a negative corona in nitrogen: I. Experiment

The pulsed mode of a negative corona discharge in air has long been known; however, in electropositive gases, this mode has not been previously observed. This paper presents the results from a systematic study of a newly discovered pulsed mode of a negative corona in nitrogen over a wide range of experimental parameters. The conditions under which the pulsed mode is realized are described in detail. The dynamic characteristics of current pulses are determined. The shapes and parameters of current pulses in nitrogen and air are compared.     

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.     

Phenomenology of a high-current negative point-to-plane corona in nitrogen

Results are presented from experimental studies of a steady-state negative point-to-plane corona in atmospheric-pressure nitrogen at anomalously high (milliampere) currents. The evolution of the corona glow with increasing discharge current is traced up to the corona conversion into a spark. It was found that the structure and current-voltage characteristic of the corona change significantly when the working gas is slowly blown trough the discharge cell. It is shown that, starting from a current of I≥0.5 mA, a negative corona in a nitrogen flow acts as a streamer corona, whereas there are no streamers in the absence of gas blowing. In contrast to the quadratic dependence of the current versus voltage in a low-current corona, the current-voltage characteristic of a high-current corona is linear. The time evolution of the radial profile of the current density at the anode is studied under different experimental conditions.     

Drift model of the cathode region of a glow discharge

A one-dimensional drift model of the cathode region of a glow discharge with allowance for both electron-impact ionization and charged particle loss is proposed. An exact solution to the model equations is obtained for the case of similar power-law dependences of the ion and electron drift velocities on the electric field strength. It is shown that, even in the drift approximation, a relatively wide transition layer in which the ion-to-electron current ratio approaches a constant value typical of the positive column of a glow discharge should occur between the thin space-charge sheath and the quasineutral plasma, the voltage drop across the space-charge sheath being comparable to that across the transition layer. The calculated parameters of the normal and anomalous glow discharges are in good agreement with available experimental data.     

Expansion of the cathode spot and generation of shock waves in the plasma of a volume discharge in atmospheric-pressure helium

The expansion of the cathode spot and the generation of shock waves during the formation and development of a pulsed volume discharge in atmospheric-pressure helium were studied by analyzing the emission spectra of the cathode plasma and the spatiotemporal behavior of the plasma glow. The transition of a diffuse volume discharge in a centimeter-long gap into a high-current diffuse mode when the gas pressure increased from 1 to 5 atm and the applied voltage rose from the statistical breakdown voltage to a 100% overvoltage was investigated. Analytical expressions for the radius of the cathode spot and its expansion velocity obtained in the framework of a spherically symmetric model agree satisfactorily with the experimental data.     

Numerical simulations of Trichel pulses in a negative corona in air

Numerical simulations of a negative corona in air demonstrate that the experimentally observed regime of self-oscillations, known as Trichel pulses, is well described by a three-dimensional axisymmetric model that is based on the standard transport equations and in which only the ion-induced secondary electron emission at the cathode is taken into account. The quantitative difference between the measured and calculated values of the mean current and the pulse repetition rate most likely stems from the insufficiently large dimensions of the computation region and from the fact that the point shape adopted in simulations somewhat inexactly conforms to that used in experiments. It was found that the transverse discharge structure near the cathode radically changes during the pulse. Specifically, as the current grows, a cathode sheath forms at the discharge axis and expands over the cathode surface. When the current falls off, the cathode sheath is rapidly destroyed; as a result, the characteristic field structure is well defined only near the discharge axis and becomes virtually indistinguishable as the current decreases by an order of magnitude.     

Study of the plasma parameters in a high-current pulsed magnetron sputtering system

Results are presented from experimental studies of the current-voltage characteristics and spatial and temporal parameters of the plasma in a high-current pulsed magnetron sputtering system with a 10-cm-diameter plane disk cathode. It is shown that the plasma density in such a system is three orders of magnitude higher than that in conventional dc magnetron discharges and reaches 10¹³ cm⁻³ at a distance of 250 mm from the cathode at a peak discharge current of 500 A. The plasma propagates from the cathode region at a velocity of 1 cm/μs in the axial direction and 0.25 cm/μs in the radial direction. Optical emission spectroscopy shows that the degree of plasma ionization increases severalfold with increasing discharge current, mainly at the expense of the sputtered material.     

Spatiotemporal evolution of the current and the integral and spectral emission characteristics of a negative corona in nitrogen during its transformation into a spark

Results are presented from experimental studies of the conversion of a steady-state negative corona into a spark. It is found that a spark in a negative corona in nitrogen and air is formed in the absence of fast primary streamers. It is shown that, in atmospheric-pressure nitrogen, the conversion of a corona into a spark begins with the propagation of a plasma channel (secondary streamer) from the point electrode (cathode) to the plane electrode (anode). In contrast, the plasma channel in air originates near the plane electrode and then propagates towards the point electrode. The propagation velocity of the secondary streamer is very low, V=10³–10⁴ cm/s. Two possible scenarios of the formation of the spark channel in a negative corona in nitrogen are described on the basis of the concept of a contracted volume glow discharge. Results are presented from time-resolved spectral measurements of plasma emission from different regions of the corona during its transformation into a spark.     

Experimental study of a low-pressure glow discharge in air in large-diameter discharge tubes: I. conditions for the normal regime of a glow discharge

The initiation and characteristics of a low-pressure glow discharge in air in large-diameter discharge tubes are studied. A deviation from the Paschen law is observed: the breakdown curves U _dc(pL) shift toward the higher values of U _dc and pL as the interelectrode distance L increases. It is shown that the normal regime of a glow discharge is accompanied by gas ionization in the anode sheath. This takes place only for pL values lying to the right of the inflection point in the breakdown curve. The cathode-sheath characteristics in the normal and abnormal regimes of an air discharge for a duralumin cathode are determined. The axial profiles of the ion density, electron temperature, and plasma potential, as well as the anode voltage drop, are measured at various air pressures.     

Abnormal and subnormal regimes of a spherical glow discharge in the drift-diffusion approximation

A spherical glow discharge with a pointlike anode is considered in a self-consistent drift-diffusion approximation. The model includes the time-dependent continuity equations for ions and electrons in the drift-diffusion approximation and Poisson’s equation for the radial electric field. In finding steady-state distributions, Ohm’s law is used to relate the discharge voltage and discharge current. Steady-state distributions of the plasma parameters across the discharge gap, current-voltage characteristics, and cathode characteristics for an abnormal spherical discharge in molecular nitrogen are obtained. In a subnormal glow-discharge regime, oscillations in the conduction current, potential, and other discharge parameters are revealed. Similar regimes are also observed in conventional discharges in tubes.     

Dynamics of the ion energy spectrum in EUV-induced hydrogen plasma

The dynamics of the ion energy spectrum in low-pressure (10–100 Pa) hydrogen plasma induced by extreme ultraviolet (EUV) pulses in the wavelength range of 10–20 nm was studied experimentally. The plasma was generated under cathode irradiation due to both direct gas ionization by EUV photons and impact ionization by high-energy secondary electrons. The dynamics of the spectra of ions incident on the cathode was measured using a time-resolved retarding field energy analyzer. It is shown that the ion spectrum dynamics is completely determined by the time evolution of the cathode sheath. At low gas pressures (<20 Pa), the ion spectrum at early moments after the EUV pulse has a peaked shape, typical of a collisionless plasma sheath, and is mainly determined by the cathode voltage. As the pressure increases, the peak broadens and low energy ions appear in the spectrum due to ion collisions in the cathode sheath. An increase in the role of collisions with decreasing plasma density is also observed in the time evolution of ion spectra.     

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.     

Simulation of oscillatory processes in a beam-plasma system with a virtual cathode in gas-filled interaction space

Physical processes occurring in an intense electron beam with a virtual cathode in an interaction space filled with neutral gas are studied in a two-dimensional model. A mathematical model is proposed for investigating complicated self-consistent processes of neutral gas ionization by the beam electrons and the dynamics of an electron beam and heavy positive ions in the common space charge field with allowance for the two-dimensional motion of charged particles. Three characteristic dynamic regimes of the system are revealed: complete suppression of oscillations of the virtual cathode as a result of neutralizing its space charge by positive ions; the pulsed generation regime, in which the ions dynamics repeatedly suppresses and restores the virtual cathode oscillations; and the continuous generation regime with an anomalously high level of noisy oscillations.     

Specific features of radiation from a negative air corona operating in the Trichel-pulse mode

Experimental studies of spatiotemporal characteristics of radiation from a negative corona operating in the Trichel-pulse mode in the point-to-sphere electrode geometry have revealed two emission zones. In addition to the well-known glow near the point electrode, there is also an anode glow, whose intensity depends substantially on the shape of the anode. It is found that the anode glow is delayed with respect to the beginning of the Trichel pulse by a time that depends on the gap length and gap voltage. The emission spectrum of the anode glow in the wavelength range 300–400 nm is identified as the spectrum of the second positive system of nitrogen (the C ³Π _u -B ³Π _g transition).     

Excitation of RF Oscillations in a Discharge with Negative Differential Conductivity

The excitation of oscillations in a discharge with negative differential conductivity is studied experimentally. The possibility is demonstrated of amplifying oscillations in the cathode dark space at frequencies close to the electron plasma frequency of the positive-column plasma. The phase velocities of waves at these frequencies are determined. When the waves pass from the cathode dark space to the discharge positive column, their phase velocities decrease; the closer the frequency is to the electron plasma frequency, the more pronounced the decrease in the phase velocity. As the intensity of oscillations increases, the discharge becomes non-steady-state. This is confirmed by the time evolution of the current-voltage characteristic. The shape of the current-voltage characteristic, its splitting, and the rate at which it varies depend on the input RF power. The decrease in the cathode dark space indicates that the ionization processes in the discharge are strongly influenced by electron plasma oscillations excited due to the collective interaction of the electron beam formed at the cathode with the discharge plasma. It is these processes that determine the maximum values of both the frequency of the excited oscillations and the power that can be withdrawn from the discharge.     

Shape of the current tube of a negative point-to-plane corona in air

Results are presented from the experimental studies of a negative point-to-plane air corona discharge with controlled geometry. It is shown that, in a certain range of parameters, the steady-state current-voltage characteristic of the corona can be fitted by a parabola. The results obtained are used to determine the time-averaged shape of the current tube of a negative corona.     

Atmospheric-pressure electric discharge as an instrument of chemical activation of water solutions

Results of experimental studies and numerical simulations of physicochemical characteristics of plasmas generated in different types of atmospheric-pressure discharges (pulsed streamer corona, gliding electric arc, dielectric barrier discharge, glow-discharge electrolysis, diaphragmatic discharge, and dc glow discharge) used to initiate various chemical processes in water solutions are analyzed. Typical reactor designs are considered. Data on the power supply characteristics, plasma electron parameters, gas temperatures, and densities of active particles in different types of discharges excited in different gases and their dependences on the external parameters of discharges are presented. The chemical composition of active particles formed in water is described. Possible mechanisms of production and loss of plasma particles are discussed.     

Plasma diagnostics from the dynamic current-voltage haracteristic of an electric probe

A method for measuring the plasma electron density from the dynamic current-voltage probe characteristic is considered. For this purpose, an additional rectangular negative voltage pulse or a high-frequency sinusoidal voltage is applied to a negatively biased probe. Analytic expressions are derived for two cases: when the space charge sheath obeys the 3/2 law and when the voltage varies over a time shorter than the ion transit time through the sheath so that the sheath does not obey the 3/2 law. Dynamic and statistic probe methods for determining the electron density are compared experimentally.