Positive Column Of Discharge Research Articles

Low-current gliding DC discharge in high-speed flows

Abstract A low-current gliding discharge (current range 1–5 A) in high-speed air flows of 100–250 m s−1 was experimentally studied. A high-voltage direct current source with a maximum voltage of 4.5 kV was used to create the discharge. The average electron concentration n e ∼ 1014 cm−3 and the plasma ionization degree were determined by measuring the Stark broadening of the hydrogen H β line (λ Hβ = 486.1 nm). The estimates of the electric field (E ∼ 100 V cm −1 ÷ 600 V cm−1) in the discharge positive column were found using time-synchronized high-speed video recordings and oscillograms. The gas rotational temperature T g = 7000–9500 K and the vibrational temperature T v = 7000–11 000 K were estimated using optical emission spectroscopy. Time-resolved spectroscopy is used to investigate the effective plasma channel spatial regions from which the N, NH, N2 +, O and OH molecules radiate. The difference of the obtained radii indicates the presence of a radial temperature gradient and inhomogeneous plasma composition in the discharge cross section. The possibility of using of gliding discharge to ignite hydrocarbon-air mixtures in the ramjet engines combustors has been experimentally demonstrated.

Oxygen atom and ozone kinetics in the afterglow of a pulse-modulated DC discharge in pure O2: an experimental and modelling study of surface mechanisms and ozone vibrational kinetics

The chemical kinetics of oxygen atoms and ozone molecules were investigated in a fully-modulated DC discharge in pure oxygen gas in a borosilicate glass tube, using cavity ringdown spectroscopy (CRDS) of the optically forbidden O(3P2)→O(1D2) absorption at 630 nm. Measurements were made over a range of tube temperatures (10 °C and 50 °C) gas pressures (0.5–4 Torr) and discharge current (10–40 mA). The discharge current was square-wave modulated (on for 0.2 s and off for 1 s), allowing the build-up to steady-state and the decay in the afterglow to be studied. This paper focusses on the afterglow period. The O atom density decays non-exponentially in the afterglow, indicating a surface loss probability dependent on incident active particle fluxes. The oxygen atom absorption peak lies on a time-varying absorption continuum due (in the afterglow) to the Chappuis bands of ozone. The ozone density passes through a maximum a few 100 ms into the afterglow, then decays slowly. An existing time-resolved self-consistent 1D radial model of O2 positive column discharges was modified to interpret the new results. The ozone behaviour in the afterglow can only be modelled by the inclusion of: (1) surface production of O3 from the reaction of O2 molecules with adsorbed O atoms, (2) reactions of vibrationally-excited ozone with O atoms and with O2(a1Δg) molecules, and (3) surface loss of ozone with a probability of around 10−5.

Specificity of the electron energy distribution function in a low-pressure nitrogen plasma

In this work, self-consistent simulations of a glow discharge positive column in low-pressure nitrogen plasma are presented. The electron energy distribution function (EEDF) was calculated by solving the complete (i.e. nonlocal) Boltzmann kinetic equation for electrons in both the energy and coordinate (space) variables. The accuracy and range of applicability of the two limiting cases are considered when the kinetic equation is reduced to a dependence on only one variable: kinetic energy in the local approximation or total energy in the so-called totally nonlocal (Holstein–Tsendin) approximation. It is shown that the Holstein–Tsendin approximation, when electron spatial transport dominates the electron energy relaxation (i.e. the ratio between the typical plasma inhomogeneity characteristic length and the electron energy relaxation length ), can be used at low nitrogen pressures (when the parameter pR ⩽ 0.05 Torr). With a further increase in pressure, there is a large intermediate region where the EEDF is nonlocal in the sense that the full kinetic equation must be solved. So, even at pR ≈ 0.5 Torr, the EEDF in nitrogen plasma is nonlocal. Therefore, the use of a local approximation to find the EEDF, when all terms with spatial gradients and an ambipolar field can be discarded in the kinetic equation, requires careful verification. Particular attention should be paid to the periphery of the discharge volume, where the self-consistent ambipolar field inevitably exceeds the external electric field that heats the electrons and produces plasma.

Time-resolved fourier transform infrared emission spectroscopy of NH radical in the X3Σ− ground state

The NH radical is an extremely important specie in nitrogen chemical reaction networks, in the interstellar medium and atmospheric chemistry. Time resolved Fourier transform spectroscopy technique in the frequency range 10–13 µm has been applied for the measurement of a pure rotational spectrum of the NH free radical (NH) in the ground X3Σ− electronic state. Twelve high N (26–29) triplet-resolved pure rotation lines of NH were experimentally observed in the laboratory and compared with satellite ACE (Atmospheric Chemistry Experiment) solar data. In addition, discharge-generated vibration-rotation NH radical bands in the spectral range 1923–3571 cm−1 have been measured with a microsecond time resolution and spectral resolution of 0.02 cm−1. The spectra of the NH radical have been studied in two experimental arrangements. Firstly, in a pulsed positive column discharge of pure hydrogen-nitrogen mixture and secondly, in a discharge of nitrogen-ammonia mixture in the presence of argon buffer gas. Both production methods are described and compared. The population analysis of the experimental spectra was performed via modelling using the accurate MoLLIST line lists for NH. It was shown that laboratory data can be well reproduced by use of a mixture of the local-thermal-equilibrium (LTE) and non-LTE models corresponding to high temperatures (up to 8000 K and up to 6000 K rotational).

Quenching of O2(b1Σg + ) by O(3P) atoms. Effect of gas temperature

We present a detailed study of the density and kinetics of O2(b1Σg +) in steady-state and partially-modulated DC positive column discharges in pure O2 for gas pressures of 0.3–10 Torr and 10–40 mA current. The time-resolved density of O2(b1Σg +) was determined by absolutely-calibrated optical emission spectroscopy (OES) of the A-band emission at 762 nm. Additionally, the O2(b1Σg +) density was determined by VUV absorption spectroscopy using the Fourier-transform spectrometer at the DESIRS beamline at Synchrotron Soleil, allowing the absolute calibration of OES to be confirmed. The O(3P) atoms were detected by time-resolved sub-Doppler cavity ringdown spectroscopy (CRDS) using the O(3P2) → O(1D2) transition at 630 nm. The CRDS measurements were synchronized to the discharge modulation allowing the O(3P) dynamics to be observed. As a function of gas pressure the O2(b1Σg +) density passes through a maximum at about 2 Torr. Below this maximum, the O2(b1Σg +) density increases with discharge current, whereas above this maximum it decreases with current. The gas temperature increases with pressure and current, from 300 to 800 K. These observations can only be explained by the existence of fast quenching process of O2(b1Σg +) by O(3P), with a rate that increases strongly with gas temperature, i.e. with a significant energy barrier. The data are interpreted using a 1D self-consistent model of the O2 discharge. The best fit of this model to all experimental data (including the O2(b1Σg +) average density as a function of pressure and current, the radial profiles, and the temporal response to current modulation) is achieved using a rate constant of k Q = 10−10 exp(−3700/T) cm3 s−1.

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.

Spatial distribution and time evolution of metal-containing plasma of a low-current atmospheric pressure discharge with magnesium cathode

The spatial intensity distribution and temporal dynamics of the plasma generated by an atmospheric pressure discharge with magnesium cathode in an argon flow are investigated in a coaxial geometry discharge system. The repetition rate of unipolar pulses was 56 kHz and the pulse duration was 12 μs. The steady-state amplitude of the discharge current was 100 mA at a voltage of about 130 V. Under this operating mode, a local melting of the active cathode surface took place. The evaporated magnesium atoms were captured by the working gas flow and formed a green glow plume around the positive discharge column outside the anode nozzle. The image of the plasma formation was projected onto the entrance slit of the monochromator. The spatial distribution of the radiation intensity and evolution in time of its selected monochromatic components were measured. The radiation spectrum contained groups of ion and magnesium atom lines with wavelengths of 285.21 nm (singlet resonant Mg I); 383.08, 383.36, 383.9 nm (triplet Mg I); 517.3, 517.5, 518.1 nm (triplet Mg I). The results of this work are promising with regard to studying open-type spontaneous radiation sources, as well as the generation of combined gas-metal plasma flows at atmospheric pressure.

Simulating glow discharge positive column parameters in neon

The nonlocal self-consistent model for the low temperature plasma parameters in a glow discharge positive column is presented. The model implies relaxation method for the solving of the nonlocal Boltzmann equation for electron energy distribution function, continuity equation for ions distribution and Poisson equation for radial electric field. Calculations are provided for a cylindrical discharge tube used in the Plasma Kristall-4 experiments for neon as a plasma forming gas for the gas pressures of 30-70 Pa and discharge currents of 0.5-2.5 mA. Axial electric field strength, electron temperature and electron density are calculated as functions of the discharge current and the gas pressure. The results are in good agreement with the data provided by other authors.

Kr I spectra in the 5–14 µm range

Using the high-resolution time-resolved Fourier transform infrared spectroscopy of a plasma created in a positive column discharge cell, we record Kr I spectrum in the 700–7600 cm−1 range. We report 88 new Kr I lines in the 700–2800 cm−1 range previously not accessed by laboratory observations. This allows us to extract the excitation energies of the 6h, and 7h, 7i levels of Kr I, which have not been measured before. We include these newly measured lines into the level optimisation together with the results of other high-resolution measurements to obtain an updated system of Kr I levels which is internally consistent to within 0.0019 cm−1.

Plasma Source for Bipolar Electron-Optical System

The paper presents the experimental study of pulsed plasma source with hollow cathode intended for bipolar electron-optical system with open-boundary plasma. The proposed configuration of the plasma source includes two ring-shaped discharge cells based on plasma thruster technology. The plasma source can operate both in a low-current high-voltage mode and a high-current low-voltage mode. The main attention is paid to the highcurrent mode which is similar to the glow discharge positive column. It is found that along with the discharge in closed electron drift the ignition of additional, non-self-sustained hollow cathode discharge with electron oscillation is possible, thus considerably reducing the voltage of the closed-drift discharge. Current–voltage characteristics of the high-current discharge are measured for these conditions. Using the double probe technique, the local plasma parameters are measured together with their spatial distribution in the discharge region. The maximum plasma density along the plasma source axis is (6.5–6.8)•1012 сm–3.

Effect of gas flow on the form and characteristics of Glow Discharge

The article is devoted to an experimental study of the effect of gas flow on the characteristics of a glow discharge. The design of the discharge chamber made it possible to change the interelectrode distance a = (2–8) cm. The family of current-voltage characteristics was measured in the following ranges of gas pressure P, its flow rate G, discharge current I and voltage U: P = (4. 7 - 26) kPa, G = (0 – 0.05) g / s, I = (20 - 100) mA, U = (1. 5 – 4. 2) kV. The oscillations of the discharge voltage were recorded with an oscilloscope, the current-voltage characteristic of the discharge was recorded by a two-coordinate recorder. The current-voltage characteristics had a negative differential resistance. The influence of the interelectrode distance on the type and quantitative relationships of these characteristics is analyzed. When gas is pumped through the discharge chamber, the current-voltage characteristics do not change qualitatively, but the discharge voltage and the steepness of the characteristics increase compared to a discharge without gas flow. At pressures of the order of P = 26 kPa, the discharge stability deteriorates, and high-frequency oscillations of the discharge voltage are observed in the gas flow (f ∼ 100 kHz). In this case, the discharge voltage decreases by a value of the order of tens of volts, and the glow of the positive column of discharge becomes intermittent. The typical oscillograms of discharge voltage pulsations are given.

An analytical model of glow discharge positive column in axially symmetric channel with variable cross section

A model of glow discharge positive column in gas flow is considered in this paper. It is supposed the discharge is between the permeable mesh electrodes installed in the initial and end sections of axially symmetric channel. In general case the channel width changing along the stream is accounted. The positive column in gas flow is described by integro-differential system of equations with an approximation of constant gas flow density. The border conditions for gas ionization degree and temperature in the initial section of channel are given by any functions. An analytical solution of that system was obtained. The special cases of the boundary conditions are analyzed.

On an estimate of turbulent transport in a magnetized plasma (on the 90th anniversary of the birth of B B Kadomtsev)

In 1964, B B Kadomtsev proposed that the results of a linear plasma instability analysis alone can be used to estimate the coefficients of turbulent transport across the magnetic field in a plasma. In this paper, we discuss examples of experimental support for this proposal that concern, specifically, the properties of a turbulent positive discharge column in a strong magnetic field, the ionization turbulence of a weakly ionized plasma in crossed electric and magnetic fields, and turbulence in a tokamak wall plasma.

Theoretical simulation of hydrogen glow discharge in the positive column of plasma sources

Direct current (DC) glow discharge positive column has played an important role in the field of plasma material surface modification. In this kinetic study of hydrogen glow discharge in the positive column, it is found that electron energy distribution function (EEDF) obviously departs from Maxwell distribution. The EEDF tail moves to higher energy when the ratio between electric field E and density of neutral species N (E/N) increases and the tail moves toward higher energy if hydrogen atom proportion increases at the same E/N. It is also shown that electron temperature augments with the increase of E/N. Additionally, the dissociation ratio percentage of H2 rises with the increase of E/N.

GAS PRODUCTS OF ARGON PLASMA INTERACTION WITH POLYARAMID AND POLY(ETHYLENE TEREPHTHALATE)

Non-equilibrium plasma is widely used for surface modification of polymer materials. Reactions of plasma active species with polymers lead not only to the surface modification, but to the formation of gaseous products, which change the plasma composition and internal plasma parameters. It results in the dependence of surface etching and modification kinetics on the quantity of material been treated (on sample sizes in a reactor). These phenomena are known as so called “loading effect” and have been studied for the treatment of polypropylene and polyimide films, poly(ethylene terephthalate) films and fabrics in oxygen and air plasma. It can be expected that gas products of noble gas plasma action on polymers will change strongly plasma parameters and modification results. In this paper, experimental data are represented on composition of gas products and their evolution rates at the treatment of polyaramid films, fibers and poly(ethylene terephthalate) fabric in low-pressure argon plasma. Poly(ethylene terephthalate) textile fabric (PET) made of monofilament yarns (SAATI, S.p.A., Italy) and polyaramide (PA) films and complex yarns “Rusar®” (Termotex, Russia) were used in experiments. Direct current discharge was excited in a flow of argon (technical grade) in a glass tube reactor with 3 cm inner diameter. Fabric samples were placed as cylinders on the reactor wall in the discharge positive column. Square of PET samples was varied from 18 to 111 cm2. The PA yarns with the total length of 550 or 1020 cm were placed in the reactor on special holder with the length of 20 cm. Total gas pressure in reactor was varied from 30 to 300 Pa at discharge current of 20 – 110 mA. Gas flow rate in the reactor was kept constant and equal to 30 cm´s-1. Gas phase analysis was carried out by the methods of plasma emission spectroscopy and mass-spectrometry. Plasma emission spectra and mass-spectra show evolution of H2, CO and H2O molecules at the treatment of PA and PET in argon plasma. Evolution rates for different gas products and their mole fractions have been obtained as functions of total gas pressure. Increase in polymer sample square has been shown to result in changing the ratios between evolution rates for different gas products. Sum of CO, H2 and H2O mole fractions increases with polymer sample square and decreases with gas pressure. Dissociation of molecular gas products results in the changing plasma active species. Oxygen and hydrogen atom lines and OH emission bands are observed in plasma emission spectra. Alteration of intensity ratio for О(3p3P ® 3s3S0) and Ar (4p3D3 ® 4s3P2) lines (IO/IAr) versus time after discharge starting shows the possible participation of oxygen atoms in heterogeneous reactions with polymers. Evolution of molecular gas products of heterogeneous reactions influences plasma properties and rates of plasma chemical reactions. The data obtained will be used for the further analysis of mechanisms of heterogeneous reactions of plasma active species with polymers.

Modeling of glow discharge in longitudinal gas flow in a plane channel of variable cross section

A two-dimensional model of glow discharge positive column in gas flow has been suggested in this paper. It is considered that the discharge maintained between the permeable mesh electrodes installed in the initial and end sections of the plane channel. In general case the channel width changing along the stream is accounted. The positive column in gas flow is described by integro-differential system of equations with an approximation of constant gas flow density. The border conditions for gas ionization degree and temperature in the initial section of channel are given by any functions. An analytical solution of that system was obtained and special cases of the boundary conditions analyzed.

Direct-current glow discharge in neon at cooling

The paper presents the results of the experimental measurement and the simulation of electric characteristics of dc glow discharge with the hollow cathode in neon, carried out during cooling of the discharge to the temperature of liquid nitrogen. The experiments were performed at pressure P = 18–187 Pa and discharge currents I = 0.01–3.5 mA. The behavior of the border of transition to the normal glow discharge during cooling was studied. Cooling in a subnormal discharge mode at constant current led to a change in the discharge regime. Upon cooling, the boundary of the transition to the normal glow discharge shifted to lower current, the electric field in the positive column of discharge and at the boundary of the normal glow discharge increased, but the reduced electric field decreased throughout the entire range of pressure and the discharge current.

Experimental study of voltage fluctuations in a glow discharge

The article is devoted to the experimental study of voltage pulsations in a glow discharge. The longitudinal discharge was ignited in a stream of air moving in a cylindrical discharge chamber (DC). Copper hollow electrodes were located at a distance of a = (2-8) cm from each other along the axis of the DC. The gas pressure P = (4.7 - 26) kPa and its flow rate G = (0-0.06) g/s were monitored. The discharge current and voltage varied accordingly in the ranges: I = (20-100) mA, U = (1, 5 - 4, 2) kV. The current-voltage characteristics of the discharge were measured. The voltage decreased with increasing current, which is typical for a given type of discharge. The oscillations of the discharge voltage were registered by an oscilloscope; the current-voltage characteristic of the discharge was recorded by a two-coordinate recorder. Three regions of the current-voltage characteristic were detected. The luminous positive discharge column (PC) occupied the entire interelectrode space in the current region I = (100-70) mA. In the current range I = (70-40) mA there is a smooth transition to the discharge without visible glow of the PC with a significant increase in the discharge voltage. With further reduction of the current intensity (the third region), the discharge exists almost at a constant voltage until its quenching. Low-frequency and high-frequency oscillations of the discharge voltage were observed at the boundary of the second and third regions of the current-voltage characteristic.

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.

Mutual influence of higher diffusion and radiation modes on the contraction of the positive column discharge

Fourier analysis of various plasma components in contracted discharge is made by an expansion over diffusion and radiation modes. Resonance atoms transport is traditionally described by an approximation of the effective lifetime by Holstein which considers only a fundamental radiation mode. Proposed method makes it possible to estimate the role of resonance radiation transport quantitatively by comparing the mode spectra. Behavior of resonance atoms successively considered on simple three-level energy models in a linear Shottky theory and in a semi-analytical non-linear diffusion-recombination theory, describing a discharge contraction. Suggested Fourier analysis method has been applied to a detailed model of the DC column contraction in Argon glow discharge. An expansion of different plasma components (electron density, metastable and resonance atoms densities) over the corresponding orthonormal set of diffusion or radiation modes is performed. The comparison of spectra obtained using the traditional Holstein approximation and in case of accurate treatment of resonance radiation transport shows an increase of fundamental diffusion and radiation modes and an effect of higher harmonics suppression in the modes spectra when the resonance radiation transport is described precisely. The role of higher radiation modes in formation of radial profiles of the electron density, metastable and resonance atoms densities as well as current–voltage characteristics is demonstrated by specific examples.