Scaling laws for the spatial distributions of the plasma parameters in the positive column of a dc oxygen discharge

Comprehensive self-consistent simulations of the positive column plasma of a dc oxygen discharge are performed with the help of commercial CFDRC software (), which enables one to carry out computations in an arbitrary 3D geometry using fluid equations for heavy components and a kinetic equation for electrons. The main scaling laws for the spatial distributions of charged particles are determined. These scaling laws are found to be quite different in the parameter ranges that are dominated by different physical processes. At low pressures, both the electrons and negative ions in the inner discharge region obey a Boltzmann distribution; as a result, a flat profile of the electron density and a parabolic profile of the ion density are established there. In the ion balance, transport processes prevail, so that ion heating in an electric field dramatically affects the spatial distribution of the charged particles. At elevated pressures, the volume processes prevail in the balance of negative ions and the profiles of the charged particle densities in the inner region turn out to be similar to each other.     

Crystallization of a dusty plasma in the positive column of a glow discharge

The formation of macroscopic ordered structures in the standing striations of a stationary glow discharge in Ne is observed. A Coulomb quasicrystal is formed by spherical glass particles with diameters of 50–63 μm and charge Z _p~7·10⁵ e. The interparticle distance is approximately 300 μm. This corresponds to a nonideality parameter Γ~5·10⁴, which leads to crystallization in the Yukawa model. The factors leading to the formation of a quasicrystal in the striations are discussed. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 2, 86–91 (25 July 1996)     

Fluid model of the positive column in argon‐oxygen direct current glow discharge

The positive column of argon‐oxygen direct current (DC) glow discharge was investigated using a fluid model at pressures of around 410 Pa and discharge currents from 10 to 40 mA. The model was used to study the influence of an oxygen admixture on the properties of argon discharge. The simulated intensity of the electric field strength was compared with measured values at 5% and 100% of oxygen concentration. The one‐dimensional model in a cylindrical tube is based on the drift‐diffusion approximation of particle flux and the mean‐electron‐energy approximation is used to describe the electron interaction. The model takes into account the radial profile of particle concentration, the neutral gas kinetic temperature profile, and interactions with the wall in the cylindrical glass discharge tube. It was shown that 2% of the oxygen admixture causes a significant increase in longitudinal electric field strength and gas heating.     

Simulation of shock-wave propagation in the argon plasma of a positive column discharge

The computer simulation of shock-wave propagation in the argon plasma of positive column discharge was performed. A one-dimensional model of the gas-discharge plasma is used, which comprises the continuity equations for the electron and ion plasma components and the equation of electrostatics with allowance for initial and boundary conditions. The distribution of plasma parameters in the shock wave was obtained; the effect of its intensity was evaluated. The simulation results were compared with experimental data.     

Self-consistent hybrid model for a stratified positive column of a low pressure glow discharge

The stratification of a positive column of a low-pressure glow discharge in inert gases has been studied with the help of a self-consistent hybrid model. The model is based on the solution of a nonlocal kinetic Boltzmann equation for the electron distribution function, a nonstationary drift-diffusion equation for the ions, and the Poisson equation for the electric field. Spatial electron and ion density distributions and the electric field distribution in the positive column were obtained. The converged solution of the model gives a self-consistent resonant strata length L and the value and the form of the modulated plasma parameters. An unexpected surprising result was obtained: for a given potential drop in the positive column of a low-pressure glow discharge, a self-consistent spatially modulated striation-like electric field does not lead to the resonant increasing of the ionization frequencies in the discharge as compared with a constant electric field with the same potential drop. Usually, it was assumed that, in spatially modulated field distributions, all the parameters in a striated plasma will be more pronounced and have a resonant form. The text was submitted by the authors in English.     

Radial distribution of plasma density in the positive column of thehigh-current stationary glow discharge

A model is presented which describes the radial distributions of the plasma density, electric potential, current density, and magnetic field in the positive column of a stationary glow discharge not in contact with the longitudinal walls of the discharge chamber. In this model, the compression of the positive column is provided by the azimuthal magnetic field created by the discharge current. The value of, the discharge current is obtained for the case where charged-particle diffusion is balanced by the actions of the radial electric field and the magnetic pinch effect. The radial distributions of plasma parameters are also calculated for the case of high-current glow discharges where charged-particle diffusion can be ignored     

Nonlocal phenomena in the positive column of a medium-pressure glow discharge

Commercial CFDRC software () is used to self-consistently simulate the plasma of the positive column (PC) of a medium-pressure dc discharge in argon. The software allows simulations in an arbitrary 3D geometry by using Poisson’s equation for the electric potential and fluid equations for the heavy components and by solving a nonlocal kinetic equation for electrons. It is shown that, in calculating the electron distribution function, the local approximation is almost always inapplicable not only at relatively low pressures (pR<1 cm Torr), but also at relatively high pressures (pR<10 cm Torr), i.e., under the real conditions of a diffuse PC usually met in practice. The use of the local approximation in solving the kinetic equation for electrons leads to significant errors in determining the main parameters of the PC. A paradoxical effect has been revealed: the peaks of the profiles of the excitation rates shift from the discharge axis toward the periphery as the pressure increases from low to medium values (1 cm Torr<pR<10 cm Torr). It is shown that the effect is related to the nonlocal character of the electron distribution function.     

Theoretical foundation of a proposed new method of the optical diagnostics of positive column plasma in glow discharge

An integral equation has been constructed which describes the image of cylindrically symmetric, longitudinally homogeneous transparent systems radiating in the optical region. As a base, the construction of the three-dimensional impulse response of a point light source was used. If an experiment is arranged so that it can be described by such an equation, radial variations of atomic concentration in different levels may be determined (e.g., in the positive column of the glow discharge).     

Radial extraction of ions from a plasma column obtained by reflex discharge

We describe here a system extracting ions perpendicularly to the magnetic field which confines the plasma. This system uses an efficient magnetic screen and a lens focusing the ions onto a quadrupole mass spectrograph. We give results in H₂-plasma.     

Dynamics of the plasma column of a capillary discharge soft X-raylaser

A sequence of time-resolved soft X-ray pinhole images shows the rapid plasma compression in a fast high-power capillary discharge. The compressed plasma column has a length-to-diameter ratio >500 and a good axial uniformity, characteristics that have allowed for the first demonstration of large soft X-ray amplification in a discharge-created plasma     

Diagnosis of electron temperature in Ar/O2 mixed gas and destruction of toluene/benzene by positive dc discharge plasma

The electron temperature (T_e) of positive dc corona plasma in Ar/O₂ atmosphere was diagnosed, and plasma decomposition of toluene/benzene was studied in a razor–plate reactor. Experimental results revealed that T_e would increase with corona current until it reached a peak value, and then decrease; the volume fraction of Ar (φ_Ar) in Ar/O₂ mixed gas also influenced T_e, the higher φ_Ar, the lower T_e. Though the decomposed volume fraction of toluene/benzene was positively related to the input power, the decomposition efficiency did not monotonically increase with the specific energy density. The highest energy yield reached 3.8 g-toluene/kWh and 2.4 g-benzene/kWh, respectively.     

Influence of ambipolar diffusion on the positive column constriction in a cryogenic discharge

An earlier theory concerning the positive column constriction is briefly summarized. This theory explains the constriction as a thermal effect: due to the heating the gas becomes rarefied near the tube axis and this results in a sharp increase of molecular ion production in the central part of the tube. The production is balanced by the dissociative (volume) recombination and the radial diffusion. The relevant equations are solved numerically for the wall temperature kept at –196 °C corresponding to the cooling of the walls by the liquid nitrogen. On example of this cryogenic discharge the influence of the ambipolar wall diffusion is examined in detail.The authors greatfully acknowledge the stimulating discussions on the subject with Dr.Karl Wojaczek.     

Nonlocal phenomena in the positive column of a glow discharge in molecular gases

The influence of nonlocality of the electron energy distribution function on the parameters of a positive column of a molecular gas discharge at a mean pressure from the range 1 ≤ pR ≤ 10 cm Torr is studied. Under these conditions, the electron energy relaxation length is small compared with the characteristic size of the plasma region; that is, calculation can be carried out in the local approximation. For fast electrons at the periphery of the discharge, where the ambipolar field exceeds the longitudinal one, the local approximation may introduce significant errors into the basic parameters of the positive column. The nonlocality of the electron energy distribution function is found to increase excitation constants from the center of the discharge to its periphery.