Positive Column Plasma Research Articles

Chemical constitution of coatings deposited remotely by activation of hexamethyldisiloxane in positive column plasma of glow discharge in argon flow

The present work deals with the chemical constitution of coatings deposited by plasma activation of hexamethyldisiloxane in positive column plasma of a low-pressure DC glow discharge in an argon flow (mass flow rate 230 mg/min). X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) are used to analyze deposit chemistry. The substance is complex and consists of many constitutional units forming branched and cross-linked irregular macromolecules. Chemical constitution depends on both discharge current (10–60 mA) and monomer flow rate (1–10 mg/min). Using the specific energy (SE) and Arrhenius-like approach, the threshold energy for the plasma polymerization of HMDSO has been estimated to be 12 ± 1 eV. Two regimes of the plasma chemical system have been identified. At SE near the threshold energy (from 5 to 40 eV/molecule), polymethylsiloxane-like coatings are deposited and variation of SE practically does not lead to changes in chemical constitution, but significantly affects the mass yield of deposit. At specific energies much higher than threshold energy, polymethylhydroxysiloxane-like coatings are formed; the coating structure is strongly SE-dependent, while the mass yield of the deposit does not change.

Finding relative negative ions concentration in dc glow discharges during undergraduate laboratory work

In this paper, we present a laboratory work implementing a simple method for finding the relative concentration of negative ions in the plasma of a contracted positive column of DC glow discharge. With this method, university students can study processes in gas discharge plasmas in technological gases during their undergraduate laboratory work. The method is based on measuring the positive column radius after the glow discharge transition from a diffuse to a contracted mode. The basics of the physical processes in electronegative plasma and conditions of the positive column contraction are discussed in the paper as well as the practical implementation of the proposed technique. We describe our experimental setup and conditions, and present the experimental data illustrating the method capabilities. Values of the relative concentration of negative ions found with the offered method reasonably agree with other authors’ data obtained by widely accepted diagnostic techniques.

Operating modes in a low-pressure glow discharge with hollow cathode

The paper deals with investigations of the discharge in hydrogen in conditions close to those for the trigger units of pseudospark switches. The following modes of discharge sustaining are classified: the Townsend discharge, the suppressed glow discharge with hollow cathode, the intermediate mode of the gradual transition from the suppressed discharge to the ordinary glow discharge, and the ordinary discharge in which the negative glow plasma occupies the whole depth of the cathode cavity. As applied to the ordinary discharge, the theoretical model that offers a possibility to estimate the negative glow parameters is developed. In the current range 1.6–10 mA the electron densities in the negative glow region are 1.7–8.7 108 cm−3. The interpretation for the mechanism of the current passage to the anode is also proposed based on the model. In particular, the explanation is given for the fact that the region of the positive column plasma near the flat anode appears at a low discharge current.

A self-consistent hybrid model of kinetic striations in low-current argon discharges

A self-consistent hybrid model of standing and moving striations was developed for low-current DC discharges in noble gases. We introduced the concept of surface diffusion in phase space (r, u) (where u denotes the electron kinetic energy) described by a tensor diffusion in the nonlocal Fokker–Planck kinetic equation for electrons in the collisional plasma. Electrons diffuse along surfaces of constant total energy ɛ = u − eφ(r) between energy jumps in inelastic collisions with atoms. Numerical solutions of the 1d1u kinetic equation for electrons were obtained by two methods and coupled to ion transport and Poisson solver. We studied the dynamics of striation formation in Townsend and glow discharges in argon gas at low discharge currents using a two-level excitation-ionization model and a ‘full-chemistry’ model, which includes stepwise and Penning ionization. Standing striations appeared in Townsend and glow discharges at low currents, and moving striations were obtained for the discharge currents exceeding a critical value. These waves originate at the anode and propagate towards the cathode. We have seen two types of moving striations with the two-level and full-chemistry models, which resemble the s and p striations previously observed in the experiments. Simulations indicate that processes in the anode region could control moving striations in the positive column plasma. The developed model helps clarify the nature of standing and moving striations in DC discharges of noble gases at low discharge currents and low gas pressures.

Positive column dynamics of a low-current atmospheric pressure discharge in flowing argon

We present the results of studies of a cold-plasma atmospheric pressure discharge in argon flow, in both DC and pulse-periodic modes. The burning voltage and the discharge current were 200 V and 15 mA in continuous mode; in pulse-periodic mode the voltage was 220 V and the current 15 mA for a pulse repetition rate of 100 kHz. Special attention was given to the positive column dynamics during the discharge stages of initiation, burning, and extinction. The main method of investigation employed optical diagnostics of the plasma in the visible range with high time resolution. The positive discharge column is shown to be a constricted current channel in the form of an elongated loop. In continuous mode, the length of the positive column, from anode attachment to inflection point outside the discharge system, is several millimeters. In pulsed mode, the positive column is an unsteady plasma formation, the length of which increases at about 4 m s−1, and reaches the same dimension over a time of order 1 ms. We find that the positive column plasma includes hydroxyl molecules OH, nitrogen N2, and atomic oxygen O I, along with atoms of the working gas argon. Observation of the time evolution of the optical emission from the plasma-forming gas shows that the plasma glow of the positive column is mainly determined by argon Ar I radiation, with maximum intensity at the trailing edge of the current pulse.

Low-pressure discharge with hollow cathode and hollow anode in a trigger unit of pseudospark switch

This paper deals with the investigations of the auxiliary glow discharge in the trigger unit of the pseudospark switch. The trigger unit communicates with the grounded electrode of the main high-voltage gap due to the aperture in the flat part of one of the trigger electrodes. Then, the so-called parasitic current closes to the main electrode via this aperture. This paper is mainly concentrated on the problem of the formation of the parasitic current for different regimes of the auxiliary discharge burning and for different designs of the trigger system. At a low current, the auxiliary discharge burns in the suppressed regime, and an increase in the current results in the stepwise transition to the ordinary glow discharge. The conditions of sustaining the auxiliary discharge influence on the value of the parasitic current. In the experiments, the cavity of the trigger electrode can play a role of the hollow cathode or the hollow anode, so that the negative glow plasma or the positive column plasma is generated in this hollow electrode. It is demonstrated that for both cases, the parasitic current is determined by the ion flow from the plasma boundary.

Calculation of nonlocal EDF using a one-dimensional Boltzmann equation solver

A method for the calculation of the nonlocal electron distribution function (EDF) with programs commonly used for solving the one-dimensional Boltzmann kinetic equation (e.g., COMSOL Multiphysics) was proposed and implemented. The capabilities of the proposed method were illustrated using the example of the positive column plasma in argon. Significant differences between the local and nonlocal EDFs were observed, especially at the plasma periphery. This can result in significant differences in the electron transport coefficients and reaction rate constants for electron-induced processes. The proposed approach increases the precision of the numerical modeling of gas discharge devices and equipment.

Influence of electron–electron collisions on the formation of a nonlocal EDF

The influence of electron–electron collisions on the formation of a nonlocal electron distribution function (EDF) and other characteristics of the electron gas are analyzed. Correct expressions for the coefficients in the integral of electron–electron collisions are obtained that are suitable for substitution into the kinetic Boltzmann equation averaged over the volume. This method of EDF calculation is implemented in COMSOL Multiphysics software. The example of positive column plasma in argon shows that accounting for electron–electron collisions leads both to differences between nonlocal and local EDFs and to dependence on the ionization degree.

Modeling of glow discharge in a gas flow

The discharge plasma of positive column in electronegative gas flow has been described by two-dimensional integro-differential system of equations written in the approximation of “narrow channel”. An efficient algorithm of solving this system of equations is suggested. In this work an implicit method is used of solution with selection gradient of the electric field along the channel. The simulation of discharge characteristics was conducted under various boundary conditions at the inlet of the discharge area and with various full discharge currents.

Influence of resonance radiation transfer on ionization balance in a positive column plasma

A method of self-consistent solution of charged particles balance equation, which is described by a differential equation of ambipolar diffusion, and an equation of resonance atom balance, which is described by an integral equation of radiation transfer, is proposed. The method is related to a replacement of an integral operator and a differential operator by a system of linear algebraic equations. The difference between a precise solution and a solution in the approximation of the effective resonance transition probability is shown. The influence of highest diffusion and radiation modes becomes apparent during transition to a contracted state.

Coaxial (tubular) glow discharge in electronegative gases

The positive-column plasma of a low- and medium-pressure electronegative glow discharge initiated in the gap between two coaxial cylindrical tubes has been considered (the current is directed along the tube axis). It is assumed that the gas mixture contains halogens, and ion diffusion is not negligibly weak. It is found that the coaxial discharge is characterized by plasma separation into three coaxial regions with different compositions in the direction transverse to the current. It has been shown that the ionization and excitation frequencies of atoms are higher than in the purely cylindrical case, even for a small (0.05–0.15) ratio of the radii of the inner and outer walls. An asymptotic analysis of the continuity equations yields analytic expressions that make it possible to rapidly and easily estimate the geometrical parameters of the spatial distributions of charge particle concentrations, as well as energy parameters of the plasma for the radii ratio that exceed 0.3. The conditions for the applicability of analytic relations and their accuracy are established from a comparison of the results of analytic and numerical calculations.

Influence of dust particles on ionization and excitation in neon dc discharge

The influence of dust particles on the concentration of metastable neon atoms and ionization was investigated using the developed drift/diffusion model for plasma of positive column of glow discharge. The detailed de-excitation in neon was considered. In addition to usual plasma losses in dusty plasmas, the quenching of metastable atoms on dust particle surface was considered. The strong influence of dust particles on the ionization rate and concentration of metastable neon atoms in a positive column of glow discharge is shown to result from the change in the longitudinal electric field strength.

The physical nature of the phenomenon of positive column plasma constriction in low-pressure noble gas direct current discharges

The essence of the positive-column plasma constriction for static (the diffusion mode) and dynamic ionization equilibrium (the stratificated and constricted modes) is analyzed. Two physical parameters, namely, the effective ionization rate of gas atoms and the ambipolar diffusion coefficient of electrons and ions, determine the transverse distribution of discharge species and affect the current states of plasma. Transverse constriction of the positive column takes place as the gas ionization level (discharge current) and pressure increase. The stratified mode (including the constricted one) is observed between the two adjacent types of self-sustained discharge phases when they coexist together at the same time or in the same place as a coherent binary mixture. In the case, a occurrence of the discharge phase with more high electron density presently involve a great decrease in the cross-section of the current channel for d.c. discharges. Additional physical factors, such as cataphoresis and electrophoresis phenomena and spatial gas density inhomogeneity correlated with a circulatory flow in d.c. discharges, are mainly responsible for the current hysteresis and partially constricted discharge.

High-Current Stages in a Low-Pressure Glow Discharge With Hollow Cathode

This paper presents an interpretation of the dense and superdense glow discharge stages in pseudospark switch geometry. The discharge is treated as a self-organizing system that is able to rearrange itself to provide the current requested by external electric circuit. The principal discharge regions in the glow stages are the hollow-cathode plasma, the positive column plasma, and the double electric layer that separates these plasma regions. A model that allows some quantitative estimates when applied to the hollow-cathode plasma is developed, in which a generalized secondary emission coefficient that considers an external emission current is introduced. The abrupt transition from dense glow stage to superdense glow stage occurs because of microexplosions at the cathode surface and appearing the metal vapor plasma. In terms of the model, this means an abrupt increase in the secondary emission coefficient. The comparison with the experiment is made for discharges in hydrogen and xenon at a current up to several kiloamperes and at a current rise time from several microseconds to hundreds of nanoseconds. The physical reasons for the current quenching effect that manifests itself at a decreased gas pressure are also discussed.

A new viewpoint on the ambipolar diffusion Schottky theory

A modern modification of the Schottky theory is proposed. It enables overcoming some of the difficulties and contradictions of the old theory and extends its capabilities in the analysis of radial distributions of ionized species. This allows us to consider the distributions of positive column plasma in noble gas d.c. discharges within a proper universal framework. The radial distributions of plasma species are basically similar in their nature and are independent of the character and features of plasma reactions.

Analysis of cylindrical Langmuir probe using experiment and different theories

Cylindrical probe data have been analyzed using different theories in order to determine some plasma parameters (electron temperature and electron and ion densities). Langmuir probe data are obtained in a cylindrical DC glow discharge in the positive column plasma at argon gas pressures varied from 0.5 to 6 Torr and at constant discharge current equal to 10 mA. The electron density has calculated from the elec� tron current at the space potential and from Orbital Motion Limited (OML) collisionless theory. Ion density has obtained from the OML analysis of the ion saturation currents. In addition, the electron temperature has measured by three different methods using probe and electrons currents. The electron temperature Te, plasma density ne, and space potential Vs, have been obtained from the measured single cylindrical probe I- V characteristic curves. The radial distribution of the electron temperature and plasma density along the glow discharge are measured and discussed. Using the collisionless theories by Langmuir cylindrical probe and up to several Torr argon gas pressures the differences between the values of electron temperature and electron and ion densities stay within reasonable error limits.

Plasma non-equilibrium of the DC normal glow discharges in atmospheric pressure atomic and molecular gases

Electrical and spectroscopic results of comprehensive investigations of atmospheric pressure glow discharges in helium, argon, nitrogen and air in a large current range are presented. Our attention in these investigations is mainly focused on the characterization of positive column plasmas at discharge gap of 10 mm. It was shown that helium, nitrogen and air positive columns transform from diffuse to constricted mode when discharge current increases. In argon discharge, it is constricted in the whole range of investigated currents. Reduced electric field, gas temperature, average electron energy and electron concentration were determined. The positive column plasmas of all the atmospheric pressure glow discharges studied are non-equilibrium and weakly ionized. Plasma non-equilibrium degree in atomic gases of helium and argon is essentially higher than in molecular nitrogen and air. For all gases, plasma non-equilibrium degree in a positive column decreases with discharge current increase.

Low-Current “Gliding Arc” in an Air Flow

This paper describes the result of the investigation of a gas discharge in a flow of air with electrode geometry typical for the so-called gliding arc. The feature characteristic of the experimental conditions is a rather low level of an average discharge current (of about 0.2 A). The discharge is initiated due to a spark breakdown in the narrow part of the gap. After that, the spark discharge is transformed into a kind of a glow discharge. At the subsequent stage, the plasma column travels in the gap under the effect of a gas flow, and the current is sustained in the regime of a normal glow discharge. The typical cathode-voltage-drop value in this regime is estimated to be about 300 V, an average electric field in the positive column plasma is (700-800) V/cm, and the neutral-particle temperature in the negative glow region is T ≈ 1100 K.

Nonlocal Behavior of Electron Fluxes and Excitation Rates for “Local” EEDF in Moderate and High Pressures DC Positive Column Plasmas

Kinetic simulations of a direct-current positive-column Ar plasma revealed that electron fluxes are sensitive to energy dependence of an elastic collision cross section. Paradoxical behavior of the electron flux in a coordinate-energy phase space is presented. The direction of the electron flux at the “elastic” energy region (5-10 eV) in whole volume, except the region near the wall, turned out to be oppositely directed to that conventionally assumed.

The positive-column plasma in low-pressure noble gas d.c. discharge as an integral plasma-field system

A one-dimension model for positive-column plasma is analyzed. In the framework of this model, a complete, self-consistent set of equations for the plasma column is proposed and justified. Basic prerequisites for the model and the equations used in it are discussed at length to clarify the mathematics and physics that underlie the proposed generalized description of plasma states. A study of the equations has unveiled the existence of two structurally stable types of steady states and three integrals of motion in the plasma system. The first type of states corresponds to spatially homogeneous plasma, and the second type, to the self-forming plasma structure with striations. Analysis of spatio-temporal plasma structures (spatially homogeneous and stratified stationary plasma states) and their attendant phenomena is given in detail. It is shown that the equations offer a more penetrating insight into the physical states and properties of positive-column plasma in dc-driven gas discharges, and into the various phenomena proceeding in the discharge system. Such a behavior is intimately related to the influence which the electric field has on the rate of ionization reactions. The theoretical results are compared to experimental data and can be used for to place the great body of experimental data in their proper framework. The modern fluid bifurcation model proposed to describe the properties of non-isothermic positive-column plasma in dc-driven low-pressure noble-gas discharges proved to be rather realistic, capable of adequately reproducing the basic properties of real field-plasma systems.