Investigation of the electron energy distribution function in hollow-cathode glow discharges in nitrogen and oxygen

The mechanism for the formation of the inverse electron distribution function is proposed and realized experimentally in a nitrogen plasma of a hollow-cathode glow discharge. It is shown theoretically and experimentally that, for a broad range of the parameters of an N₂ discharge, it is possible to form a significant dip in the profile of the electron distribution function in the energy range ε=2–4 eV and, accordingly, to produce the inverse distribution with df(ε)/d?>0. The formation of a dip is associated with both the vibrational excitation of N₂ molecules and the characteristic features of a hollow-cathode glow discharge. In such a discharge, the applied voltage drops preferentially across a narrow cathode sheath. In the main discharge region, the electric field E is weak (E<0.1 V/cm at a pressure of about p～0.1 torr) and does not heat the discharge plasma. The gas is ionized and the ionization-produced electrons are heated by a beam of fast electrons (with an energy of about 400 eV) emitted from the cathode. A high-energy electron beam plays an important role in the formation of a dip in the profile of the electron distribution function in the energy range in which the cross section for the vibrational excitation of nitrogen molecules is maximum. A plasma with an inverted electron distribution function can be used to create a population inversion in which more impurity molecules and atoms will exist in electronically excited states.     

Nonlocal nature of the electron energy spectrum in a glow discharge in pure O2: I. Nonlocal character of the electron distribution function

Abstrac A study is made of the nonlocal nature of the electron energy distribution function in the positive column of a glow discharge in a tube filled with pure oxygen. The distribution function and the axial (E _z) and radial (E _r) electric fields as functions of radius are measured using an array of mobile probes. The experimentally obtained spatial profiles of the distribution function are used to test the applicability of the two-term approximation to the distribution function of the electrons with a nonlocal energy spectrum. The distribution function in a specified electric field E=E _z +E _r (where E _z ⊥ E _r) is calculated by solving the coordinate-dependent Boltzmann equation in the two-term approximation and by directly integrating the equations of electron motion using the Monte Carlo method. A comparison between the experimental data and the results of simulations carried out for a broad parameter range shows that, in the case of a highly nonlocal electron energy spectrum, the two-term approximation makes it possible to calculate the electron distribution function with a fairly good accuracy, in which case, however, in imposing the boundary conditions, the electron losses at the plasma surface should be treated in the kinetic approximation. It is shown that using the reflection coefficient of the plasma surface for electrons instead of the loss cone in space makes it possible to accurately calculate the electron energy distribution function over the entire parameter range under consideration, including the transient region in which the electron-energy relaxation length is comparable to the characteristic plasma dimension. __________ Translated from Fizika Plazmy, Vol. 26, No. 11, 2000, pp. 1038–1045. Original Russian Text Copyright ? 2000 by Ivanov, Klopovskii, Lopaev, Rakhimov, Rakhimova.     

Effect of dust size distribution on ion-acoustic solitons in dusty plasmas with different dust grains

Theoretical studies are carried out for ion acoustic solitons in multicomponent nonuniform plasma considering the dust size distribution. The Korteweg?de Vries equation for ion acoustic solitons is given by using the reductive perturbation technique. Two special dust size distributions are considered. The dependences of the width and amplitude of solitons on dust size parameters are shown. It is found that the properties of a solitary wave depend on the shape of the size distribution function of dust grains.     

Charging of dust grains in a nonequilibrium plasma of a stratified glow discharge

A theoretical model is presented that describes the charging of dust grains in the positive plasma column of a stratified glow dc discharge in argon. A one-dimensional self-consistent model is used to obtain axial profiles of the electric field, as well as the electron energy distribution function along the axis of the discharge tube. Radial profiles of the electric field are determined in the ambipolar diffusion approximation. It is assumed that, in the radial direction, the electron distribution function depends only on the total electron energy. Two-dimensional distributions of the discharge plasma parameters are calculated and used to determine the potential and charge of a test dust grain at a certain point within the discharge and the electrostatic forces acting on it. It is shown that the grain charge distribution depends strongly on the nonequilibrium electron distribution function and on the nonuniform distribution of the electric field in a stratified glow discharge. A discussion is presented on the suspension of dust grains, the separation of grains by size in the discharge striations, and a possible mechanism for the onset of vortex dust motion at the edge of a dust cloud.     

Study of the process of dust grain discharging in the afterglow of an RF discharge

The process of decay of dust structures formed of polydisperse grains injected into an RF discharge is investigated. The dust grain velocities after switching-off of the discharge are measured. The number density, dimensions, and residual charges of dust grains are estimated from the balance of forces acting on the grains after discharge is switched off.     

One-way electron discharge subsequent to the photochemical charge separation in Photosystem I

Subsequent to the photochemical charge separation in Photosystem I, three fates are possible: (a) recombination of the photooxidized P700⁺ and photoreduced P430⁻; (b) a cyclic electron flow involving P700⁺, P430⁻ and another electron carrier present in its oxidized and reduced forms; and (c) a non-cyclic electron flow involving one electron donor reacting with P700⁺ and another electron acceptor reacting with P430⁻. This note deals with a fourth fate which is brought about only when an autooxidizable secondary electron acceptor is present but the secondary electron donor is either absent or blocked. In this case, only P430⁻ reverts to the uncharged state in the dark by discharging its electron; P700⁺ remains oxidized and reverts to the uncharged state only extremely slowly.     

Formation of an anisotropic electron velocity distribution function and production of multicharged ions in the micropinch discharge plasma

Analysis of the results of polarimetric measurements of X-ray line radiation of multicharged ions in a Z-pinch discharge indicates that the formation of an anisotropic electron velocity distribution in the neck of the current channel and the generation of highly charged ions are separated in time. The generation of a fast electron beam in the longitudinal ohmic electric field in the stage of plasma compression in the neck results in the polarization of X-ray bremsstrahlung continuum. In the stage of expansion of the hot dense micropinch plasma, the radial electric field prevails, due to which X-ray line radiation of multicharged ions becomes linearly polarized.     

Effect of the nonlocal nature of the electron energy spectrum on the dissociation of oxygen molecules in a discharge

The effect of the nonlocal nature of the electron distribution function on the dissociation rate of oxygen molecules in a dc glow discharge is studied. The concentration of oxygen atoms and the probability of their loss at the discharge tube wall are measured as functions of the discharge parameters by means of the timeresolved actinometric method involving argon atoms. An analysis of the measurement data in terms of both a discharge model in which the effect of the nonlocal nature of the electron energy spectrum is taken into account and a model in which this effect is ignored makes it possible to thoroughly investigate the balance of oxygen atoms in the discharge. The production rate of O(³P) atoms and their concentration in the plasma are calculated with allowance for the nonlocal nature of the electron energy distribution function. The calculated values agree well with the experimental data and differ substantially from those obtained using a spatially homogeneous distribution function.     

Positive column of a glow discharge in neon with charged dust grains (a review)

The effect of charged micron-size dust grains (microparticles) on the electric parameters of the positive column of a low-pressure dc glow discharge in neon has been studied experimentally and numerically. Numerical analysis is carried out in the diffusion-drift approximation with allowance for the interaction of dust grains with metastable neon atoms. In a discharge with a dust grain cloud, the longitudinal electric field increases. As the number density of dust grains in an axisymmetric cylindrical dust cloud rises, the growth of the electric field saturates. It is shown that the contribution of metastable atoms to ionization is higher in a discharge with dust grains, in spite of the quenching of metastable atoms on dust grains. The processes of charging of dust grains and the dust cloud are considered. As the number density of dust grains rises, their charge decreases, while the space charge of the dust cloud increases. The results obtained can be used in plasma technologies involving microparticles.     

Screening of the dust grain charge in a nonequilibrium plasma with two positive ion species

The screening of the grain charge in a nonequilibrium plasma is studied with allowance for electron and ion fluxes to the grain surface and the bulk processes of production and loss of charged particles in an argon plasma. The objective of the paper is to investigate how the conversion of monatomic Ar⁺ ions into diatomic Ar₂⁺ ions influences the screening of the dust grain charge in a plasma produced by an external gas ionization source. It is found that the conversion of positive ions leads to the onset of a second ion species in the plasma and, as a consequence, to a three-exponential screening of the grain charge; moreover, in a certain range of plasma parameters, all three screening constants are of the same order of magnitude. Analytical results are compared with the data of numerical simulations carried out based on the drift-diffusion approximation.     

Concerning the procedure of recovering the electron energy distribution function from the soft X-ray spectrum

In the course of experiments on electron-cyclotron resonance heating of plasma in the L-2M stellarator, it was found that, at high specific heating powers, the SXR spectrum was distorted in the entire photon energy range under study (from 1.5 to 15 keV). To determine the character of this distortion, a procedure was proposed for recovering the electron energy distribution function (EEDF) from soft X-ray (SXR) measurements. The SXR spectra were analyzed for different values of the specific heating power. Numerical calculations show that the edge plasma contributes insignificantly to the chord-averaged SXR spectrum. A conclusion is drawn on the applicability and accuracy of the procedure for recovering the EEDF at high specific heating powers.     

Mechanisms of dust grain charging in plasma with allowance for electron emission processes

The process of dust grain charging is described with allowance for secondary, ion-induced, photoelectric, and thermal electron emission from the grain surface. The roughness of the grain surface is taken into account. An intermediate charging regime involving ion–atom collisions and electron ionization in the perturbed plasma region is analyzed using the moment equations and Poisson’s equation. A calculation method is proposed that allows one to take into account the influence of all the above effects and determine the radius of the plasma region perturbed by the dust grain.     

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.     

Adenylate energy charge and hemoglobin function in developing mouse embryos

1. Adenylate energy charge (EC) was measured in mouse embryos and showed a significant rise from 0.79 at day 11 of gestation to 0.87 at day 15 (P less than 0.05). 2. The low value of EC between days 11 and 13 corresponds with the predominance of the embryonic hemoglobin fraction E-I which has high affinity and poor cooperativity. 3. The highest value of EC corresponds with the predominance of adult hemoglobin which has lower affinity and cooperative oxygen binding. 4. Weight-specific oxygen uptake rates of isolated embryos decreases with weight gain and it is suggested that the energy charge is a manifestation of metabolic regulatory processes which sustain embryonic growth.     

Investigating the shape and size distribution of dust in mechanized longwall method in Tabas coal mine

When the average size of coal dust particles is reduced, risk of explosion as well as probability of emergence of numerous diseases is increased. To find the rate of negative effects in mechanized longwall mining face, the amount of spread coal dust and the scattering manner of particles in each part of the face and tail gate are investigated. Accordingly, 19 samples were taken from the face and tail gate of Tabas mechanized coal mine. Then, using the Zetasizer device and TEM (Transmission electron microscopy), the samples were analyzed and their shapes were determined, respectively. Results showed that shape of the particles was almost spherical and their size followed Rosin–Rammler distribution in the tail gate and along the longwall. In the face, since there was no opportunity for the deposition of coarse particles, some disturbances were observed in their distribution. At the beginning of the tail gate and close to the face, as the height increased, coarser particles were observed, which deposited after passing through the tunnel and getting away from the face. Considering these results, the aggregation and sequestration regions of dust could be found and the ventilation system can be modified. Also, more suitable solutions can be adopted for dust control or more advanced equipment can be proposed for miners in terms of confronting these particles in their working environment.     

Estimation of the electron density and radiative energy losses in a calcium plasma source based on an electron cyclotron resonance discharge

The parameters of a calcium plasma source based on an electron cyclotron resonance (ECR) discharge were calculated. The analysis was performed as applied to an ion cyclotron resonance system designed for separation of calcium isotopes. The plasma electrons in the source were heated by gyrotron microwave radiation in the zone of the inhomogeneous magnetic field. It was assumed that, in such a combined trap, the energy of the extraordinary microwave propagating from the high-field side was initially transferred to a small group of resonance electrons. As a result, two electron components with different transverse temperatures—the hot resonance component and the cold nonresonance component—were created in the plasma. The longitudinal temperatures of both components were assumed to be equal. The entire discharge space was divided into a narrow ECR zone, where resonance electrons acquired transverse energy, and the region of the discharge itself, where the gas was ionized. The transverse energy of resonance electrons was calculated by solving the equations for electron motion in an inhomogeneous magnetic field. Using the law of energy conservation and the balance condition for the number of hot electrons entering the discharge zone and cooled due to ionization and elastic collisions, the density of hot electrons was estimated and the dependence of the longitudinal temperature T _e∥ of the main (cold) electron component on the energy fraction β lost for radiation was obtained.