The experimental results of complex investigations of the high-current self-sustained glow discharge in helium at atmospheric lressure are presented. The studied discharge was produced in the current range 005 — 15 A. The electrical discharge power had been reached up to 3.5 kW. The different diagnostic methods are used: an electric probe, a tlurmopair, a laser, emission spectroscopy. The electric field strength, gas temperature, electron density and metastable helium atoms density are mapped. The obtained results are discussed with attraction of the existing theory of the cathode fall region taking into account the volumetric heat-evolution.
High-pressure discharges are generated using Micro-Structured Electrode (MSE) arrays with electrode distances between 50 and 250 µm. These arrays consist either of a planar electrode system bonded to a dielectric substrate or of a matrix of holes perforated in a composite sheet made out of two metallic foils separated by an insulator. Stable discharges were produced for pressures ranging from 10 to 1000 mbar using DC and radio frequency excitation. The optical appearance is that of a homogeneous glow discharge. Spectral investigations have revealed the presence of a large number density of metastable atoms and highly energetic electrons, while the discharge has a non-thermal character. The non-equilibrium high -pressure plasma provided by the MSE arrays is suitable for surface activation, thin film deposition and reduction of pollutants.
Electrical and optical study on streamers and spark formation in a steady-state positive pin-plane corona in N2 and ambient air
It is common, diffusive glow regime of steady-state positive corona is followed by streamer regime with increase in overage corona current. The coefficient of ionization multiplication of electron avalanche over interelectrode gap in glow corona is less than that is necessary for the avalanch-to-streamer transition. Under such conditions, a local development of ionization instability nearby the sharpened electrode plays crucial role in initiation of mild streamers in glow corona. Indeed using a fast frame camera we have revealed on electrode the occurrence of small high-density current spots, which provoked the mild streamer in glow corona. A transient behavior of these streamers followed by spark was examined in detail for a positive pin-plane corona in pure nitrogen and ambient air at atmospheric pressure. It was observed that scenarios for the streamers-to-spark transition in air and N2 differed essentially from each other.
Excitation pulse form and power and some other discharge parameters influence on barrier discharge KrC1 excilamp efficiency was studied in present paper. It is shown that at barrier KrC1 excilamp excitation by short (2÷2,5 µs) unipolar or bipolar voltage pulses the efficiency is higher than by sine pulses excitation. Presence of filaments occurs to be a necessary condition to obtain high efficiency since in that case a demanded level of excitation specific power is being achieved. Radiation pulse delay relatively to excitation in the conditions of homogeneous discharge probably demonstrates low efficiency of KrCl* molecules formation at a low level of excitation power.
CO2 decomposition in a non-self-sustained discharge with a controlled electronic component of plasma
Carbon dioxide decomposition in a non-self-sustained discharge was studied by the methods of diode laser spectroscopy and mass-spectrometry. It was show that the effective control of the mechanism of a plamachemical reaction is possible by varying the parameter E/N. This allowed us to reduce the energy cost of dissociation CO2 by more than on an order of magnitude as compared to the dissociation process in a self-sustained glow discharge.
The plasma focus discharge is discussed as a source of intensive x-ray radiation. This radiation is observed in two different regimes of compression —the "filamentary" and the "hot point" types. A typical parameters of radiating pinch are presented and the role of the energetic electron beam on the spectral image is discussed. The presented results of 2D image spectrometry demonstrate detailed space structures of radiating plasma. All the experiments were performed on the installations "TULIP" (FIAN, Moscow) and PF-1000 (IPPiLM, Warsaw).
Time-resolved emission spectroscopy and its applications to the study of pulsed nanosecond high-voltage discharges
Spectroscopy of pulsed nonequilibrium plasma allows to control behaviour of active particles in situ, that is with subnanosecond temporal and high spatial resolution. Present work demonstrates advantages of emission optical spectroscopy as applied to kinetic description of pulse-periodic discharges at high overvoltages in the pressure range 0.1-30 Torr and at atmospheric pressure. Features in common and distinction in experimental observation of such discharges are discussed. Measurements of velocity of the discharge propagation, reduced electric field behaviour and number density of excited particles are represented for a wide range of experimental conditions.
The production and loss kinetics of CF2 and CF radicals in the CF4 glow discharge were studied with using the time— resolved LIF technique. The effective rate constants of the CF4 dissociation into the chanhels with CF2 and CF production were determined in a wide range of reduced electric field: 80-250 Td. It is shown that besides of the CF4 dissociation by direct electron impact there is also another source conditioned by fluorocarbon plasma polymerization processes. The detailed analysis of growth and decay dynamics of radical concentrations at the discharge modulation has allowed us to separate different channels of the radical production and consequently to determine the constants of CF4 dissociation rate by electron impact KCF2 KCF and A comparison of received KCF2 and KCF with calculations made on the base of solving the Boltzman equation by Monte-Carlo technique with using the available data on electron scattering cross-sections on CF4 molecules, has in part allowed us to renormalize the partial dissociation cross-sections near the threshold and to build up the model cross-sections of neutral dissociation of CF4 by electron impact It is shown, that the polymerization processes substantially contribute (particularly at low values of E/N) to production of radicals, as in a plasma volume, and on a surface of the discharge tube. The analysis of the obtained data on characteristic frequencies (times) of these processes has allowed us to study mechanisms of fluorocarbon's polymerization in CF4 plasma in conditions of high relative concentration of fluorine atoms and low ion energy. So it is shown, that with increasing pressure, when the concentration of polymer fragments CxFy in bulk of plasma is comparable with concentration of simple fluorocarbon radicals CFx (x=1-3), the dissociation of these fragments by electronic impact and the reactions of unsaturated fluorocarbon fragments CnF2m+1 as in volume, and on a surface of a fluorocarbon film, covering discharge tube walls become the main production channels of CF2 and CF. Since with lowering pressure a major channel of CF2 and CF destruction is connected with their heterogeneous losses on tube walls, the surface loss probabilities of CF2 and CF radicals in conditions of the fluorinated fluorocarbon film surface are determined. It is shown, that the most probable mechanism of heterogeneous loss of CF2 and CF under these conditions is the surface recombination of chemisorbed fluorine atoms Fch and physadsorbed radicals CFxPh (x=1-2) through the formation of an activated surface complex. The analysis of the experimental data with using this approach has allowed us to estimate the approximate values of activation energies for the formation of the surface complexes Fch. CF2ph and Fh. CFPh - 750 ± 70 K and 1030 ± 100 K respectively.
Comparison of measured and predicted dynamics of excited states of nitrogen under discharge and post-discharge conditions
Dynamics of light emission for the N2(A 3?+u), N2(B 3?g) and N2(C3?u) electronic excited states was studied spectroscopically in a long pulse glow discharge in pure nitrogen and in afterglow at pressure 50 Torn An appropriate mathematical processing was made to derive population of excited states from these measurements A rather complete kinetic model was developed for conditions ofthe experiments. Results ofcomparison are analyzed.
In this work we discuss some physical aspects of surface wave driven discharges operating in pure nitrogen N2 and in N2—Ar mixtures, as sources of active species. The developed theoretical models are based on a self-consistent treatment of particle kinetics, wave electrodynamics, gas thermal balance and plasma-wall interactions. By means of optical emission and absorption spectroscopy the population density of the species of interest has been detected. Important problems concerning inhomogeneous wave—to—plasma power transfer, mechanisms of N2 dissociation, creation of ions and metastable particles are discussed in the terms ofthe obtained theoretical and experimental results.
Local nonintrusive diagnostics of electron components of plasma glow discharge in nitrogen by CARS spectroscopy
The possibility is considered of restoring the electron energy distribution function (EEDF) in the interval of electron energies 0 < e < 5 eV from measurements of translational and vibrational temperatures by the coherent anti-Stokes Raman scattering (CARS) method in a positive column of a glow discharge in molecular nitrogen at 20
Measurement of quenching coefficients and development of calibration methods for quantitative spectroscopy of plasmas at elevated pressures
Measurements of collisional de-excitation (quenching) coefficients required for the interpretation of emission and fluorescence spectroscopic measurements are reported. Particular attention is turned on argon transitions which are of interest for actinometric determinations of atomic ground state populations and on fluorescence lines originating from excited atoms and noble gases in connection with two-photon excitation (TALIF) of atomic radicals. A novel method is described which allows to infer quenching coefficients for collisions with molecular hydrogen of noble gas states in the energy range up to 24 eV. The excitation is performed in these experiments by collisions of energetic electrons in the sheath of an RF excited hydrogen plasma during the field reversal phase which lasts about 10 ns. We describe in addition a calibration method -including quenching effectsf o-r the determination by TALIF of absolute atomic radical densities of hydrogen, nitrogen and oxygen using two-photon resonances in noble gases close by the resonances of the species mentioned. The paper closes with first ideas on a novel technique to bypass quenching effects in TALIF by introducing an additional, controllable loss by photoionization that will allow quenching-free determination of absolute atomic densities with prevalent nanosecond laser systems in situations where collisional de-excitation dominates over spontaneous emission.
Non-linear optical techniques as Coherent Anti-Stokes Raman Scattering (CARS), Two-Photon-Allowed-Laser-Induced Fluorescence (TALIF) and Laser-Induced Photodetachment (LIP) are very successful tools to diagnose process plasmas. They mainly deliver detailed information concerning the population densities of excited states of molecular, atomic and ionic species. Therefore, they are very helpful to appraise the deviations from the equilibrium state, to elucidate the significance of processes and to validate the results of plasma modeling. The diagnostic methods mentioned are utilized to investigate several basic and applied problems in the medium and high pressure range as cw CO2 gas laser plasmas, cw and pulsed microwave discharges for exhaust gas purification, and microwave generated plasmas as test bed for non-linear laser-aided experiments.
Applying a recently developed ns Two-Colour Resonant Four-Wave Mixing (TC-RFWM) technique, a number of resonance lines forbidden by conventional three-level schemes are observed. A theoretical frequency-domain picture is developed with the help of exact (off-diagonal) relaxation matrices and general four-level schemes. The newly observed extra resonances are induced by collisional transfer of rotational energy. The role of the population, orientation and alignment gratings that are formed in both electronic states coupled by laser fields is elucidated. Relaxation models using the fast-collision approximation are elaborated for Hund's cases a and b giving quantitative arguments for the proposed interpretation. The potential of the TC-RFWM technique to study state-to-state transfer rates is emphasized.
Determination of line shift coefficients and temperature in extreme media by femtosecond time-resolved four-wave-mixing spectroscopy
Time resolved nonresonant CARS (Coherent Anti-Stokes Raman Spectroscopy) is applied to probe combustion relevant species with fs time resolution under extreme conditions. The experimental transients on the prototype molecule H2 show detailed beating structures, reflecting the rotational dynamics of all thermally populated ground state J-levels with high fidelity. Modelling of the experimental data yields temperatures and pressures with high accuracy and resolution. The good agreement with values determined by frequency-domain experiments demonstrates the suitability of this method for the determination of J-dependent line-shifting coefficients in gaseous mixtures at high pressures and temperatures, where frequency-domain CARS spectra are difficult to model.
Cross-correlation spectroscopy in investigations of filamentary gas discharges at atmospheric pressure
The technique of spatially resolved cross-correlation spectroscopy is used to carry out diagnostic measurements of the barrier discharge in air at atmospheric pressure. Quantitative estimates for electric field strength E(x,t) and for relative electron density ne(x,t)/nemax are derived from the experimentally determined spatio-temporal distributions of the luminosity for the spectral bands of the 0-0 transitions of the 2nd positive system of N2 (? = 337.2 nm) and the 1st negative system of N2+ (? = 391.5 nm). Experimental results related to the investigations of coronas and pulsed spark discharges are briefly reviewed.
A tuneable diode laser with external cavity in Littrow-configuration was used to probe the 2P6— is5 transition (823,163 nm) and the 2p5 — 1s4 transition (828,012 nm) of xenon in plasmas at elevated pressure, produced by a dielectric barrier discharge and a miniaturized hollow cathode discharge. These discharges where investigated in pure xenon atmosphere and partly in xenon-helium mixtures. After the description of the method a number of examples illustrating the use of diode lasers in plasma diagnostics by means of the absorption technique is presented. From these absorption measurements the optical depth of that above named xenon transitions correlating to the particle number density of the resonant and metastable level were calculated. By observing the afterglow phase in pulsed driven discharges, decay rates can be determined which are specific for various elementary processes. Comparing the obtained decay rates with those from literature fixes the correlated processes. Results of the time behavior of the optical depth show that the late time afterglow is governed by three body collision with neutral xenon atoms. From the spectrally-resolved absorption line-shapes particle number densities of the metastable level and their temperature were determined in a small sized hollow cathode discharge.
Intracavity Laser Spectroscopy (ICLS) is a promising method for studying weak absorption spectra in gases. Due to its high sensitivity (up to 10-11 cm-1) and fast detection of weak absorption lines it can be successfully applied to study of non-stationary processes in plasma. Recent progress in this field makes possible operation of the method in near-IR range of spectrum 1.6-2.5 µm, where various strong vibrational transitions reside. Search for new active media for ICLS capable of operating in wider spectral range is in progress now.
The investigations into the Intracavity laser spectroscopy of excited molecules in low temperature plasma performed at the Institute of Atmospheric Optics SB RAS are presented. Small size of the absorption layer allowed different ways of the molecule excitation: in electrical discharge, in plasma of laser spark. Investigation were performed using ICL-spectrometers in the visible and near IR regions with spectral resolution of 0.02 cm-1 and threshold absorption sensitivity of 10-8 cm-1 that corresponds to use 10000 m absorption path in classical spectrometry. The intracavity laser spectrometer with an additional channel for recording emission spectra is presented. The spectrometer is capable of recording weak absorption spectra of stable molecules and radicals in plasma with high threshold sensitivity, as well as monitoring the molecular composition of a gas using visible and UV emission spectra with temporal resolution of 25ns. Absorption spectra of CO2 and C2 were recorded in plasma. New vibration-rotation bands induced by transitions to highly excited states are assigned; spectroscopic constants of upper states were determined.
The present state of Stark plasma spectroscopy is discussed. The possibilities of measurements of electric field and related plasma parameters are considered. A brief theory of linear and nonlinear Stark effect is outlined. The specific features of Stark effect — shifts, splitting and related line broadening, forbidden transitions — are described for the electric fields of differing nature — correlated external static and alternative fields, plasma turbulence fields, microfields generated by charged particles. Techniques based on classical emission spectroscopy, and novel linear and nonlinear laser techniques are considered. It is shown that the application of recently developed coherent laser spectroscopic methods radically improves the possibilities of electric fields measurements for the high pressure range.
Laser Thomson scattering diagnostics of nonequilibrium high-pressure plasmas as applied to excimer laser pumping and plasma display
Laser Thomson scattering (LTS) has been applied to measure electron density and temperature, or more generally electron energy distribution function (eedf), of non-equilibrium high pressure plasmas used for excimer laser pumping and plasma display panel. These quantities, with very high spatial and temporal resolutions and without plasma perturbation, are not obtainable by other means.
Hollow cathode discharge devices with hole dimensions in the range from 0. 1 —0.5 mm (microhollow cathode discharges or MHCDs) can be operated at high pressure (up to and exceeding atmospheric pressure). MHCDs are known to be efficient sources of non-coherent ultraviolet (UV) and vacuum ultraviolet (VUV) radiation when operated in rare gases, rare gas — halide mixtures, and gas mixtures containing rare gases and trace amounts of gases such as H2, 02, and N2. Highest internal efficiencies in direct current MHCD excimer sources of close to 10% were obtained in xenon at a pressure of 400 Ton. By applying nanosecond electrical pulses to the dc discharge the efficiency could be increased to approximately 20%. The radiative emittance which for dc discharges in xenon was measured as 1 .4 W/cm2 could be increased to over 15 W/cm2 through pulsed operation. In addition to rare gas and rare-gas halide excimer emission, intense, monochromatic atomic line emissions have been reported from high-pressure MHCD plasmas in pure rare gases and in rare gases admixed with trace amounts (less than 1 %) of H2, O2, and N2. . The atomic line emission is the result of a near-resonant energy transfer process involving the excimers and the diatomic molecules. For instance, Ne2* excimers in the bound 3?u state have enough energy to dissociate H2 and excite one of the H atoms to the n —2 state. The subsequent decay of the excited H atom results in the emission ofthe 121.6 nm H Lyman-? line. We discuss the results of dc and time-resolved emission spectroscopy in the UV and VUV to elucidate the microscopic mechanisms of the rare gas excimer formation and emission processes, the properties of the MHCD plasma, and microscopic details of the near-resonant energy transfer processes that lead to the emission of the intense atomic line radiation in the range 100 — 1 30 nm.
In high pressure (p?0.2 bar) silent discharges in He/N2 mixtures we measured by high-resolution optical emission spectroscopy rotational temperatures of the first negative and the second positive systems of nitrogen. Both temperatures differ appreciably. This can be attributed to different types of reactions by species having high potential energy (ions, metastables): fast reactions (Penning ionization and charge exchange by He metastables and ions) occur inside the micro discharge channels or in its immediate neighborhood. Slow reactions (excitation transfer from metastable nitrogen molecules) take place at a much later stage in the cold ambient gas. The high temperature thus indicates the temperature of the microdischarges. This temperature is influenced by the gas dynamic expansion of the plasma channels created by the microdischarges. Experimental results we have fitted to gas dynamic model calculations. From these fits we could deduce parameters like the channel radii. Further we studied the radiation emitted from NO and OH. These species have low rotational temperatures indicating excitation in slow processes and the temperature ofthe ambient gas.
As a diagnosis tool of the atmospheric pressure non-thermal plasma, some Laser-Induced Fluorescence (LIF) techniques are described. The observed radicals are OH and NO in the plasma region generated by the electric pulse discharge or DC corona. Time variations of 2 dimensional distribution of OH or NO were observed just after the pulse excitation. Precise OH or NO concentration changes with time were also recorded related with some contamination and other parameters. Ground-state OH density is maximum at 30 or 50 microseconds after the submicrosecond pulse discharge in arc mode but the OH density is largest just after the pulse discharge if the plasma is the non-thermal plasma. NO decomposition occurs just in the streamer, and decomposing area increases from that streamer area with time, if NO concentration is low.
Spectroscopic determination of gas temperature in high-pressure nonthermal equilibrium microwave discharge in hydrogen
Results of theoretical analysis and experimental determinations of the gas temperature and plasma parameters in a microwave discharge not at thermal equilibrium are presented. The investigation is concerned with high-pressure hydrogen under conditions such that radiation is emitted both by atomic and molecular components of the plasma and primarily governed by interaction of the excited atoms and molecules with heavy particles. Because this takes place the approximate models which are successfully applied to analysis data from spectroscopic diagnostics cannot be used to describe the radiative properties of the discharge. One of the main objectives of research consists of making headway in spectral methods of gas-temperature determination in the high-pressure range by invoking for interpretation of experimental data calculations performed in the framework of the kinetic scheme of thermal non-equilibrium discharge in hydrogen as well as general physical reasoning relating to the role of interactions between heavy particles in nonequilibrium plasma radiation. The following measurement techniques have been used: spectral measurements of Balmerseries radiation; spectral measurement of gas temperature Tg by recording the Doppler broadening of the H?; and spectral measurements of Tg by recording the Fulcher band. Measurements ofthe gas temperature agree satisfactorily under the assumption that collisions of excited particles with heavy components of a gas-discharge medium are of considerable importance. Conclusions regarding the ion composition and degree ofdissociation of hydrogen molecules can be drawn.
We have studied the gas heating and dissociation processes in the direct current (DC) discharge CVD reactor with the closed gas circuit where the working gas mixture is used repeatedly by circulation through the discharge area. Experiments were carried out in mixtures of CO with hydrogen at overall gas pressure of 20 kPa and gas flow up to 40 m/s. The discharge plasma was analyzed by actinometry and optical emission spectroscopy. Twodimensional model of DC discharge CVD reactor has been used to study reactive gas flow, electrodynamics and surface processes. The 2D calculations show that the most important radical production reactions are the CO and H2 dissociation due to electron impact. Calculated results are compared with the experimental ones.
The spectroscopic parameters of transversal RF discharge in gas mixtures CO2 +N2 + He are investigated. It has been observed that the width of the shields in a capacity RF discharge in such mixtures is almost independent on the frequency of RF field; it sufficiently differs from the theoretical predictions. The spatial distribution of the neutral gas temperature in various gas mixtures has been measured and discussed. The influence of the small additives of water vapour on luminescence intensity and its spatial distribution is also discussed.
This paper is devoted to the results of diagnostic research of pulsed microwave discharge at atmospheric pressure. The discharge is used for plasma catalytic processes of hydrocarbon conversion to accelerate chemical reactions. The results of spectroscopic measurements of time dependencies of electric field magnitude, charged particles densities and gas temperature in the discharge plasma are presented. It is shown that the discharge evolution is well described by the «microwave streamer» mechanism followed by heating of the «streamer» and abrupt growth of its plasma ionization degree. Relying on the obtained data it is shown that the chemical process is accelerated in weakly ionized peripheral areas ofthe discharge channels.
Laser-induced fluorescence (LIF) has been proved to be a useful diagnostic method for in-situ observation of the discharge-induced plasma-chemistry processes responsible for NO (NO + NO2) decomposition occurring in nonthermal plasma reactors. In this paper a method and results ofthe LIF measuring ofthe two-dimensional distribution of the ground-state NO molecule density inside a DC positive streamer corona reactor during NO removal from a flue gas simulator [NO (200 ppm)/air] are presented. Either a needle-to-plate or nozzle-to-plate electrode system, having an electrode gap of 30 mm was used for generating the steamer corona in the reactor. For the LIF monitoring of the ground-state NO molecules, NO molecules were excited with a laser line of a wavelength of 226 nm. The LIF monitoring ofNO molecules was carried out under the steady-state DC corona discharge condition. The obtained results showed that the corona discharge-induced removal of NO molecules occurred not only in the vicinity of the plasma region formed by the corona streamers and the downstream region ofthe reactor but also in the upstream region of the reactor. This information is important for optimizing the non-thermal plasma reactors used for NOx abatement.
Densities of N, O and H atoms have been determined by NO titration in Ar-N2 and Ar-O2 and by a Ni catalytic probe in Ar-02 and Ar-H2 flowing DC and microwave post-discharges. For similar discharge parameters : tube diameter of 6 mm, gas flow rate of about 1N1 min-1 and pressure of 2-3 Torr, it is found higher densities of N and 0 atoms in the microwave post-discharge than in the D.C. one. About the same values of 0 atom density are obtained by NO titration and by the Ni catalytic probe. Spatial distribution of the atomic species was found homogeneous in the post discharge reactor by the use of a three dimensional hydrodynamic and kinetic model. Absolute densities of H atoms have been obtained in pure H2 post-discharge by the Ni probe.
Laser induced plasma spectroscopy (LIPS) is nowadays recognized as a fast and accurate technique for elemental analysis of unknown samples. Analytical implications of the non-thermal equilibrium plasma generated at high pressure and electron temperature in 8000-15000K range, after the interaction of a sharply focused laser beam with a metals surface are investigated. On the basis of recent laboratory experiments, an attempt is made to defme the effects of non-equilibrium on the species population densities and on their spectroscopic emissions, which affect the achieved analytical accuracy. Experimental data on a steel sample are presented and discussed in combination with some theoretical modelling. The laser induced plasma is formed at the sample surface in air at atmospheric pressure, so that it is mainly constituted of light elements from air and evaporated atoms and ions from the sample surface. Species considered for modelling are metals such as iron and chromium with well known spectroscopic parameters, which are of special interest for analytical applications such as their characterization of ancient metal alloys, soil analysis and marine sediments analysis.