Dark phase effect in the evolution of the positive column of a glow discharge in nitrogen

The characteristics of the initial stage of the formation of the positive column of a glow discharge in nitrogen at reduced pressures are studied experimentally and numerically. A dip in the plasma emission intensity in the initial stage of the discharge (the so-called “dark phase”) is observed experimentally at the positive polarity of the high-voltage electrode (the cathode is grounded). The dark phase is preceded by an ionization wave (IW). When the anode is grounded, neither an IW nor a dip in the discharge emission intensity are observed. A theoretical model capable of describing the discharge development under the actual experimental conditions is constructed. It is shown that the dark phase effect may be caused by the high electron density (above the steady-state one) produced in the gas during the passage of the IW across the discharge gap. This mechanism of the dark phase formation differs from the mechanism proposed earlier to explain a similar effect in noble gases. Additional experiments carried out with pure argon, helium, and helium with a nitrogen admixture have shown that, in the case of a grounded cathode, gas breakdown is also accompanied by the passage of an IW, whereas in the case of a grounded anode, no IW is observed; however, the dark phase is present in both cases. It is shown using computer simulations that, in nitrogen (in contrast to noble gases), the mechanism resulting in the dark phase effect does not operate in the absence of an IW.     

New mechanism for the influence of Xe on the concentration of CO<Subscript>2</Subscript> molecules in self-sustained CO-laser discharges

The dependence of the CO₂ concentration on the discharge conditions and the mixture composition in a CO laser is studied experimentally. The experimental data are compared with the calculated results. A scheme of the reactions that govern the concentration of CO₂ molecules under the experimental conditions in question is constructed. It is shown that, in a gas-discharge plasma, an admixture of Xe in a mixture containing CO molecules gives rise to a new mechanism for the dissociation of CO₂ molecules by metastable xenon atoms. Under conditions close to the operating conditions of sealed-off CO lasers, the dissociation of CO₂ molecules in collisions with metastable. Xe(³P₂) atoms becomes the dominant dissociation mechanism in a He: CO mixture because it proceeds at a fast rate. This explains the observed decrease in the CO₂ concentration in a xenon-containing He: CO mixture.     

Influence of a Nitrogen Admixture on the Value and Radial Profile of the Metastable Argon Atom Density in a DC Glow Discharge in Argon

Plasma Physics Reports - Results of measurements of the value and radial profile of the density of Ar(3P2) metastables in a dc discharge in pure argon and Ar + 0.1%N2 and Ar + 1%N2 mixtures are...     

Theoretical investigation of the bistability effect in non-self-sustained discharges in Kr and Ar

The electron energy distribution function and the related plasma parameters in non-self-sustained discharges in Kr and Ar are studied theoretically. The investigations are carried out by numerically solving the corresponding Boltzmann equation for the electron energy distribution function with allowance for electron-electron collisions. The electron energy distribution and electron density are calculated self-consistently as functions of the intensity q of the source of secondary electrons and the magnitude of the reduced electric field E/N. The main goal of the investigations was to determine the conditions under which the plasma exhibits bistable parameters. Calculations show that, for discharges in Kr, there is a certain range of q and E/N values in which the Boltzmann equation has two different stable solutions. For an Ar plasma, such a bistability effect was not found: over the parameter range under consideration, the Boltzmann equation has a unique solution. Various plasma parameters (such as the effective electron temperature, electron drift velocity, and electron current density) are calculated for different discharge conditions, including those corresponding to the bistability effect.     

Experimental and theoretical study of the near IR emission of xenon excited by a fast electron beam

Emission of xenon excited by a 120-keV electron beam at gas pressures of 100, 200, 500, and 760 Torr nm was studied experimentally and theoretically. More than 30 spectral lines were identified in the wavelength range of 750–1000 nm. A self-consistent kinetic model is developed to calculate the emission intensity of xenon atoms in the near IR range. The model includes balance equations for the number densities of electrons, ions and excimer molecules; equations for the populations of electron levels; and the Boltzmann equation for the low-energy part of the electron energy distribution function with a source of slow electrons. Excitation and ionization rates of xenon by the beam electrons and the energy spectrum of slow electrons are calculated by the Monte Carlo method. It is shown that, under these conditions, the main mechanism of xenon atom excitation is dissociative recombination of Xe₃ ⁺ ions.     

Effect of the shape of the electron energy distribution function on the dust grain charge and its screening in glow discharge plasmas

The dust grain charge in the plasma of a glow discharge in noble gases and nitrogen is calculated in the orbit motion limited model for reduced fields in the range of E/N = 1–20 Td. The calculations were performed using the electron energy distribution functions (EEDFs) obtained by solving the Boltzmann equation numerically with allowance for elastic and inelastic electron scattering and analytically with allowance for only elastic scattering and (for nitrogen) excitation of rotational levels, as well as using a Maxwellian EEDF. In the latter case, either the characteristic electron energy or mean electron energy multiplied by two thirds was used as the electron temperature. It is shown that the calculations with the use of a Maxwellian EEDF yield larger values of the grain charge as compared to those calculated with EEDFs obtained by solving the Boltzmann equation. The range of E/N values is determined in which analytical expressions for the EEDF obtained with allowance for elastic scattering and excitation of rotational levels are applicable to calculating the grain charge. The effect of the EEDF shape on the screening of the dust grain charge in plasma is investigated. The Debye screening length in case of a Maxwellian EEDF is shown to be shorter than that obtained with EEDFs calculated by numerically solving the Boltzmann equation.     

Memory effect in the ignition of a low-pressure glow discharge in nitrogen in a long discharge tube

Results are presented from experimental studies of breakdown and the initial stage of a discharge in a long tube (with an interelectrode distance of 40 cm and a diameter of 2.8 cm) at a pressure of ～1 Torr and pulse discharge current of ～10 mA. Breakdown was produced by positive voltage pulses with a linearly growing leading edge with a steepness of dU/dt ～ 10⁶–10⁸ V/s. The time interval between pulses was varied from τ = 0.5 ms to 1 s, the pulse duration being 10 ms. The work was aimed at studying the memory effect of the discharge gap, namely, the influence of the previous pulse on the breakdown characteristics of the next one. In the experiments, the breakdown voltage was measured at different values of dU/dt and τ. It was found that the memory effect was absent at τ ～ 1 s. At the same time, an increase in the breakdown voltage with increasing dU/dt was observed. In the range of τ ≈ 50–200 ms, the breakdown voltage also did not depend on τ, but the memory effect took place. The memory effect in this case consisted in that the breakdown voltage decreased with increasing dU/dt, so that, at dU/dt ～ 10⁷ V/s, the breakdown voltage was two times lower than in the case of τ ～ 1 s. For τ ～ 1–10 ms, the memory effect manifested itself in that the breakdown voltage depended on τ: it could either decrease (the “normal” effect) or increase (the “anomalous” effect) with increasing τ. Breakdown of the discharge gap was preceded by the propagation of an ionization wave, except for the case of small τ values in the domain of existence of the anomalous effect. Estimates allowing one to qualitatively explain the experimental results are made.     

Development of a self-consistent model of dust grain charging at elevated pressures using the method of moments

A model of dust grain charging is constructed using the method of moments. The dust grain charging process in a weakly ionized helium plasma produced by a 100-keV electron beam at atmospheric pressure is studied theoretically. In simulations, the beam current density was varied from 1 to 10⁶ μA/cm². It is shown that, in a He plasma, dust grains of radius 5 μm and larger perturb the electron temperature only slightly, although the reduced electric field near the grain reaches 8 Td, the beam current density being 10⁶ μA/cm². It is found that, at distances from the grain that are up to several tens or hundreds of times larger than its radius, the electron and ion densities are lower than their equilibrium values. Conditions are determined under which the charging process may be described by a model with constant electron transport coefficients. The dust grain charge is shown to be weakly affected by secondary electron emission. In a beam-produced helium plasma, the dust grain potential calculated in the drift-diffusion model is shown to be close to that calculated in the orbit motion limited model. It is found that, in the vicinity of a body perturbing the plasma, there may be no quasineutral plasma presheath with an ambipolar diffusion of charged particles. The conditions for the onset of this presheath in a beam-produced plasma are determined. __________ Translated from Fizika Plazmy, Vol. 29, No. 3, 2003, pp. 214–226. Original Russian Text Copyright ? 2003 by Filippov, Dyatko, Pal’, Starostin.     

Partially constricted glow discharge in an argon-nitrogen mixture

The characteristics of a glow discharge in Ar:N₂ mixtures with a low content of nitrogen (0.02–1%) are studied experimentally. Most studies were performed at pressures higher than 10 Torr, at which the discharge constriction goes by a jump and the hysteresis effect is well pronounced. It is found that the time during which the discharge switches from the diffuse to the constricted mode (and back) can reach ～1 s. The transition between these modes begins with the development of a constriction at one end of the positive column. Then, the constricted part of the discharge extends toward the other end until it occupies the entire column. The reverse transition occurs in a similar way. By varying the parameters of the electric circuit during the transition, the constriction front can be stopped to form a steady-state partially constricted discharge. It is shown that this type of discharge corresponds to points lying inside the hysteresis loop of the conventional I-V characteristic measured without affecting the discharge during a transition. A comparative analysis of the discharge characteristics in Ar:N₂ mixtures and in pure argon is performed.     

Electron temperature in nitrogen afterglow: Dependence of theoretical results on the adopted set of cross sections and on the type of molecular distribution over vibrational levels

The electron energy distribution function in nitrogen afterglow is simulated using different available sets of cross sections for electron scattering by vibrationally excited molecules. The calculations are performed for two types of molecular distribution over vibrational levels, namely, for the Boltzmann and Treanor-Gordiets distributions. It is shown that the calculated values of the electron temperature in nitrogen afterglow depend strongly on the set of cross sections used and on the type of molecular distribution over vibrational levels. The validity of comparison between theoretical and experimental results is discussed.