Temporally resolved laser diagnostic measurements of atomic hydrogen concentrations in RF plasma discharges

Ground-state atomic hydrogen produced in radio-frequency plasma discharges (20 KHz-5 MHz) has been detectedin situ using two-photon absorption laser-induced fluorescence (TALIF). Atomic ground-state concentration measurements have demonstrated excellent spatial resolution in the interelectrode gap of a planar discharge configuration with 10 nsec temporal resolution at all phases of the RF driving voltage waveform. Concentrations were measured in gas mixtures of helium and hydrogen down to 2×10¹³ H atoms/cm³.     

Dependence of the self-bias on the plasma parameters and on the electrode areas in RF plasma reactors

From the current-voltage characteristics for the collisionless sheath the dependence of the self-bias on the plasma parameters (electron temperature, ratio of electron temperatures and electron densities at the two electrodes), on the applied external voltage, and on the ratio of the electrode areas is investigated. Sinusoidal (is well as periodic rectangular and triangular time dependences of the voltage are considered. The integral equation for the self bias voltage is solved numerically. For large external RF voltages in comparison to the floating potential, simple analytical formulas result for the dependence of the bias voltage on the plasma parameters and the dimensions of the electrodes, which can be useful in practice.     

Modeling of RF plasma torch with a metallic tube inserted for reactant injection

Flow, temperature, and electromagnetic (EM) fields in a radio-frequency thermal plasma torch designed for the preparation of superconducting powders or films have been analysed by using a new two-dimensional modeling approach with the electric field intensity as the fundamental EM field variable. The insertion of a stainless steel injection tube into the torch leads to large induction currents in this tube. Although such large induction currents cause pronounced changes of the EM fields near the injection tube, flow and temperature fields are little affected. There exists only one large toroidal vortex in the upper part of the present torch, while the maximum temperature appears at an off-axis location within the coil region.     

Diagnostics and decomposition mechanism in radio-frequency discharges of fluorocarbons utilized for plasma etching or polymerization

Optical emission (180–800 nm) and mass spectroscopy have been used to study the CF₄, CF₄+O₂, C₂F₆, C₂F₆+H₂, CF₃Cl, and C₂F₄ decomposition in radio-frequency discharges. The analysis of the stable and unstable discharge products has allowed the suggestion of decomposition channels for the various gases and to classify the fluorinated gases according to their predominant etching or polymerizing characteristics on the basis of the active species present in the plasma. A new broad emission continuum centered at =290 nm (FWHM=66 nm) has also been identified and it has been tentatively assigned to CF⁺ ₂.     

Microwave and RF surface wave sustained discharges as plasma sources for plasma chemistry and plasma processing

The surface wave produced plasma belongs to a class of RF and microwave induced plasmas. It results from the propagation of an electromagnetic wave which uses the plasma column it sustains and the plasma tube as its sole propagating media. This type of plasma offers several advantages compared to the positive column plasma of dc discharges or to other RF and microwave produced plasmas. Surface wave plasmas require no internal electrodes, and they can be applied over an extremely broad range of wave frequencies (27 MHz to 10 GHz demonstrated) and gas pressures (about 10^–4 Torr to a few times the atmospheric pressures). Using the surface wave plasma technique, a large variety of plasma column diameters have been created (0.5–150 mm demonstrated) and no limitation on plasma column length (column up to 6 m long demonstrated) has been found. The surface wave produced plasma is used in elemental analysis and to sustain emission in lasing media. This article is intended as a guide for potential users of surface wave plasmas in the field of plasma processing and plasma chemistry.     

Cathode erosion phenomena in a transferred-arc plasma reactor

Phenomena occurring on file surface of a thoriated tungsten cathode operating in a transferred-arc reactor were investigated. The effects of cathode geometry (pointed-tip vs. flat-tip) and plasma gas composition (argon vs. helium) on the rate and mechanisms of cathode erosion were studied experimentally by examining the morphology of the surface before and after runs of prespecified duration, up to one hour in length. For flat-tip cathodes in argon, the major characteristic was the migration of thoria and its concentration at segregated sites. Both geometries in helium operated at much higher temperatures, around the boiling point of tungsten, giving rise to extensive vaporization of cathode material, followed by apparent redeposition of the ionized species carried by file ionic current, in characteristic ringlike sites on the surface. Erosion rates were low and similar in magnitude, except for pointed-tip cathodes operated in argon, where the formation of a large molten sphere of tungsten and its subsequent release gave rise to a higher rate of erosion.     

射频等离子体热解废轮胎的研究

Disposal of the huge piles of used tires is increasingly becoming a problem all over the world. Incineration may utilize the energy content of waste tires, but it is associated with the generation of SO2, NOx and other hazardous emissions. Pyrolysis is an alternative disposal method with the possibility for recovery of valuable products from used tires. Recently, waste processing technology based on plasma processes has received much attention due to a number of advantages such as high treatment rate, small space volume, etc. In this paper, the pyrolysis of waste tire using a capacitively coupled radio-frequency （RF） plasma reactor was investigated. The lab-scale RF plasma reactor was operated with RF powers between 1 600 W and 2 000 W, pressures between 3 000 Pa and 8 000 Pa （ absolute pressure） and temperatures between 1 200 K and 1 800 K. In the tire powder pyrolysis experiments, two product streams are obtained： combustible gas and char. The physical properties （surface area, porosity, particle morphology） as well as chemical properties （elemental composition, heating value and surface functional groups） of char were examined to exploit the potential applications of the char. The results indicate that the RF plasma pyrolysis would be a useful technology for waste disposal.     

The use of electrostatic probes for plasma diagnostics—A review

The use of electrostatic, or Langmuir, probes for plasma diagnostics is reviewed. The emphasis is on experimental implementation and current techniques, and particular attention is paid to sources of error in theoretical interpretation as well as to experimental problems that can occur in complex, reactive plasmas.     

Analysis of glass and glass melts during the vitrification process of fly and bottom ashes by laser-induced plasma spectroscopy. Part I: Normalization and plasma diagnostics

For laser-induced plasma spectroscopy (LIPS) analysis of the main components (Si, Al, and Ca) in glasses utilized for vitrification of ashes from waste incineration, a normalization procedure for line ratios is presented. Even in homogeneous glass samples, considerable pulse-to-pulse variations of the plasma electronic excitation temperature and electron density were observed because of changes in the material–laser interaction. A normalization procedure is outlined using Saha–Boltzmann equilibrium relationships to include the electronic excitation temperature and density in the calibration model. As a result of the normalization, the variation of the line ratios is reduced and linear calibrations for LIPS intensity ratios versus concentration ratios are achieved. For samples with high aluminum concentrations, the analysis was hampered by self-reversal effects.     

In-flight particle diagnostics in induction plasma processing

A laser Doppler anemometer combined with a particle-emission spectrometer, are used for the study of the induction plasma spraying process. For this, the effects of chamber pressure, spray distance and torch nozzle design on the particle surface temperature and velocity as well as the fraction of hot particles included in the stream of processed material, were investigated. A comparison between the velocity measurements by laser Doppler anemometry (LDA) and by the particle time-of-flight technique is presented in order to emphasize the deference between the velocity of the hot particles, and that of the total particle population, cold and hot. The influence of the individual particle mass on particle entrainment in the plasma jet from the ambient atmosphere in the vacuum chamber is discussed.     

Modeling of a counterflow plasma reactor

Modeling of a counterflow plasma reactor is presented, using liquid injection for the synthesis of fine particles. An experimental reactor has been developed in this laboratory, and feasibility has been demonstrated for synthesizing advanced ceramic powders. The flow field calculations show two major recirculating regions which are of importance for increasing the particles' residence time inside of the reactor. In addition, the temperature within these recirculation zones remains relatively uniform. For simulation, water droplet trajectories have been calculated for droplets produced by an injection probe. It is shown that the droplets in a size range below 50 m in diameter will follow the streamlines and evaporate completely within a short traveling distance. This finding suggests that this reactor configuration provides a favorable environment for the synthesis of fine particles using liquid precursors.     

An overall mechanism for the deposition of plasma polymers from methane in a low-pressure argon plasma jet

An overall mechanism for plasma polymer deposition from a methane-seeded argon plasma jet was established from experimental measurements and a simplified model of reaction kinetics within the plasma jet. Total mass deposition rates were obtained at various substrate positions and methane flow rates. Methane consumption was estimated from residual gas analysis. The influence of substrate coolant temperature on deposition rate was evaluated. The model was based on particle densities, jet temperature, and jet velocity data published previously, and reaction rate constants from the literature were used. No adjustable parameters were employed in this model. Experimental results for total deposition rate and methane consumption were in good agreement with model predictions. The overall deposition mechanism consists of three steps: Penning ionization of methane by excited argon neutrals, followed by dissociative recombination of CN _x ⁺ to yield CH, followed by incorporation of CH into the growing film upon impact. Contributions of species other than CH to the total deposition rate are minor, and adsorption is not a prerequisite for incorporation into the growing film.     

Particle densities and non-equilibrium in a low-pressure argon plasma jet

Plasma diagnostic techniques have been employed to determine particle densities and temperatures in a low-pressure argon plasma jet generated by a cascade arc. These measurements allow characterization of the extent to which the plasma jet deviates from thermodynamic equilibrium and provide a basis for predicting how reactive gases will interact with the excited and ionized species in the plasma jet. It was found that the distribution of atomic states in the plasma jet is not adequately described by either local thermodynamic equilibrium (LTE) or partial local thermodynamic equilibrium (pLTE), and the jet was optically thick for 3p4s transitions across the jet radius. Excited argon neutrals outnumber ions by a large ratio, and dominate subsequent dissociation/excitation phenomena. The rate of methane destruction in the plasma jet shows that estimates for particle densities, temperature, and jet velocity are self-consistent.     

Doppler-broadened line shapes of atomic hydrogen in a parallel-plate radio frequency discharge

Doppler-shifted atomic hydrogen emission (Balmer ) is observed from a low-frequency rf discharge through molecular hydrogen by collecting the light through a hole in one of the electrodes. Doppler shifts as large as 0.7 nm, corresponding to an energy of 540 eV or 85% of the peak applied voltage, are observed when ions are accelerated by the sheath electric field. The mechanisms for hot atom production are discussed in terms of both gas-phase and surface ion-impact phenomena. Hot atoms are produced via gas-phase ion-atom and ion-molecule collisions, as a result of simultaneous neutralization and reflection of ions at the electrode surface, and/or by sputtering of adsorbed hydrogen. As much as 30% of the atomic hydrogen emission is substantially Doppler shifted, indicating that most of the atomic emission in the sheaths is actually produced by ion impact and not by electron impact.     

Critical comparison of emission spectroscopic determination of dissociation in hydrogen RF discharges

The degree of dissociation of molecular hydrogen in a RF parallel plate plasma reactor is analyzed by three dif erent emission spectroscopic methods. These schemes are based on the comparison of line intensities of atomic and molecular hydrogen and of argon which is admixed as a tracer gas (actinometry). The different schemes yield similar results in the parameter ranges where they are applicable. The basic assumptions, the available cross sections, and the conditions and limitations of applicability of the different schemes are compared and discussed.     

Comparison of microwave and lower-frequency discharges for plasma polymerization

A plasma sustained by surface waves (SW) has been used to study the deposition rate R of hydrocarbon and fluorocarbon plasma polymer films as a function of excitation frequency f=/2 in the range 12–400 MHz. The SW technique allows one to vary only f while keeping constant all other parameters known to influence R, for example, power density in the plasma P. A plot of R/P at a total pressure of 200 m Torr (27 Pa) displays two plateaus, that at f<30 MHz being about five times lower than that at f100 MHz. This is attributed to the fact that electron energy distribution functions differ fundamentally at radio- (f50 MHz) and microwave (f100 MHz) frequencies, for the gas pressure range considered.     

Lasers for plasma diagnostics,time resolved measurement and molecular fluorometry in analytical science

In chemical measurement and characterization, lasers are playing a unique role in improving sensitivity, enhancing spatial and spectral resolution, and enabling time resolution on the fastest time scales that are chemically significant. Furthermore, lasers have permitted entirely new classes of measurements to be undertaken that would not be possible without the high radiant power, directionality, and coherence of a laser beam. In this paper, a number of these capabilities are illustrated with examples from the authors' laboratory. Prominent among these examples is the use of a high-power pulsed laser for producing scattering and fluorescence from species in an inductively coupled plasma (ICP). With the appropriate laser and photometric equipment, such measurements enable the determination of time-resolved and spatially resolved values for electron concentration, electron energy distribution, gas-kinetic temperature, and the concentrations of important sample and intrinsic species that the plasma contains. Another example shows how either a continuous wave (CW) or repetitively pulsed laser can be coupled with relatively simple electronic instrumentation to permit measurements to be obtained on a sub-nanosecond time scale. Interestingly, a recent development might obviate the need for a sophisticated laser in such schemes. Lastly, a relatively simple experimental configuration can be used to determine as few as 10⁶ molecules in a real sample. In this arrangement, a single aliquot of the sample is dispensed in a volume as small as 6 nL. This aliquot, in droplet from, then constitutes the sample cell itself. As the droplet falls through the focused laser beam, its contents can be determined with extraordinarily high sensitivity. Methods to improve even this detection capability will be outlined.     

Laser-based characterization of a flame-assisted plasma

A well-characterized flame-assisted plasma was developed to understand the role of flow nonuniformities and plasma/wall interactions in plasma devices for use in validation of laser-based Doppler shift spectroscopic methods. A hydrogen/oxygen capillary diffusion flame burner was used as a plasma source, with barium seeded into the reactants to provide a source of ions and electrons. For analysis the plasma was assumed to be a stationary, partially ionized, collision dominated, thermal plasma consisting of barium ions, electrons, and neutrals between two parallel-plate electrodes. The plasma was examined in terms of the continuum equations for ions and electrons, together with Poisson's equation to predict spatial profiles of electron and positive ion density and potential as functions of applied potential. First an analytic solution based on constant plasma properties and negligible difusion was introduced. The model was then extended by including effects of diffusion and variable plasma properties. Experimentally, current/voltage characteristics of the plasma were measured conventionally, relative ion concentration and temperature were measured with laser-induced fluorescence, and local potential distribution was measured using an electrostatic probe. The diffusionless theory predicted well the bulk behavior of the plasma, but not the correct spatial distributions of ion concentration and potential. The extended model produced a more satisfactory fit to the data. At conditions of 1.4 equivalence ratio, 70 torn pressure, 300 ppm seed concentration, and 100–400 V applied potentials, electric fields of the order of 10², 10³ V/cm were observed near the powered electrode, and of few tens of V/cm in the hulk of tire plasma. The field strength in the sheath ensures the operation of the Doppler shift diagnostics, once the recommendations tor LIF signal detectability are fulfilled.     

Electron temperature and radiative attachment continua in enclosed inductively coupled plasma in argon and chlorine

Inductively coupled plasma (ICP) discharges in chlorine, argon, and their mixtures sustained inside a spherical quartz container at atmospheric pressure have been investigated. Continua of radiative attachment of a free electron to a chlorine atom in ICP are elucidated and are utilized to derive a spatial profile of electron temperature. A qualitative picture of elementary processes in enclosed ICP is given. Differences between electron and gas temperatures are discussed. Electron temperature is maximum in a periphery layer near the induction coil that is chosen for impurity determination. Concentrations of carbon and metal impurities are estimated.     

Optical emission spectroscopy diagnostics of inductively-driven plasmas in argon gas at low pressures

An optical emission spectroscopy method for determination of electron temperature, electron density and gas temperature is developed and applied for diagnostics of inductively-driven argon discharges in a cylindrical geometry. The discharges are maintained at frequency 27 MHz, applied power varied in the limits P = (90 – 160) W and gas pressure in the range p = (1.1 – 117.3) Pa. The method combines measurements of emission spectral line intensities and profile broadenings with a collisional-radiative model of argon plasma at low pressure. The model is employed for investigation of the plasma kinetics governing the population densities of 3p⁵4s and 3p⁵4p argon configuration levels, treated separately. In the numerical calculations the electron density and electron temperature are varied whereas the values of the third plasma parameter — the gas temperature — are involved as obtained data from the experiments. Comparison of the experimental results of the line-intensity ratios with those calculated by the model yields the values of the electron density and temperature. The dependence of the electron temperature, electron density and gas temperature on the discharge conditions is obtained and discussed in the study.