UV Radiation of Low Pressure XeCl* and KrCl* Glow Discharges

The positive column of low pressure DC glow discharges in Xe/Cl₂ and Kr/Cl₂ gas mixtures has been investigated with respect to its UV radiation power and radiative efficiency for a wide range of parameters (total gas pressure: 1–30 mbar, current 10–30 mA, partial pressure of Cl₂: 1–33%). Also the radial distribution of the particle number densities of XeCl* (B) and KrCl* (B) has been determined by absolutely calibrated emission spectroscopy. Optimum UV (190–350 nm) output and efficiency has been found in gas mixtures with a relative Cl₂ partial pressure of 2–3% and total gas pressure of 12–18 mbar. At these parameters, the UV radiation power per column length is greater than 40 Watt/m with a radiative efficiency of 15–18%. These discharges could be used for UV induced photochemical processes.     

Isotopic thermal diffusion factors of the noble gases

A new set of isotopic thermal diffusion factors for Ar, Kr and Xe is reported. The data have been obtained from thermal diffusion column measurements in a column previously calibrated with Ne gas, and complete the α_T data for the binary mixtures of noble gases given previously. The results lead to the conclusion that the thermal diffusion properties of the noble gases and their mixtures are well described by the law of corresponding states first proposed by Kestin, Ro and Wakeham. It is also concluded that the calibration technique used makes the thermal diffusion column a valuable instrument for the determination of the thermal diffusion factors of isotopic and non-isotopic dilute gas mixtures.     

Lokale Diagnostik einer Argon-Niederdruckentladung mit Hilfe der resonanten Laserstreuung

We report on the experimental investigations on a laser-like low pressure argon gas discharge. The discharge parameters varied in the range from 0,05 to 1 Torr for the gas pressure and from 10 to 30 A for the discharge current. Local electron densities and temperatures have been measured with an adjustable spherical probe. Spatially resolved measurements of the temperatures and drift velocities of the Ar⁺-ions in the radial and axial direction of the positive column have been carried out by the resonant laser light scattering (fluorescence technique). The radial density profiles of the excited Ar⁺-ions have been determined in two different ways and have been compared with the calculations based on a simple collisional radiative model.     

Längsfeldstärke,Elektronentemperatur, mittlere Neutralgasdichte,Ionenstromdichte auf die Wand und mittlere Entladungsstromdichte in der Freifallsäule bei hohem Ionisationsgrad

On the basis of a foregoing paper new theoretical results for the positive column at low pressure and strong ionization, especially for discharges in noble gas ion lasers, are given. The mean velocity v_n0 of the neutral atoms reemitted from the wall is taken into account. The electric conductivity is calculated for an argon plasma. The formulas connecting the electron temperature, the mean neutral gas density, and the electric field strength are derived. The electron temperature, the axial electric field intensity, the degree of ionization, the axial electron drift velocity, the ion flux to the wall, and the force density causing the main part of gas pumping along the column are calculated as functions of the product of the mean current density and the tube radius, and of v_n0 for argon. The axial drift velocity of the electrons is still smaller than the mean thermal electron velocity for high discharge currents, except at very low gas pressures. In general, the ion flux to the wall is not directly proportional to the discharge current. The factor for the determination of the charged particle density by means of probe measurements at the wall is discussed. The self-magnetic field affects the discharge only at high electron temperature, high degree of ionization, and relatively large tube radius, i.e. at high current density and low gas pressure in not too narrow discharge channels.     

First observation of spin flip in dμ-atoms via formation and back decay of ddμ molecules

We present the results of precision measurements of dμ spin-flip rates in pure D₂, in pure HD gas, and in the non-equilibrium and equilibrium H₂+D₂ gas mixtures. The experiments were performed at PSI in 1994-1996 using the high pressure ionization chamber from Gatchina as a 100% efficient detector of the charged fusion products. The measurements have been carried out in the temperature range 28-300 K, at gas density of 5% of LHD. In pure deuterium, the measured temperature dependence of the spin-flip rate allowed us to separate the two components (resonant and non-resonant) of the spin-flip processes. From the experiments in H/D mixtures we have measured the non-resonant spin-flip rate in collisions of dμ-atoms with HD molecules. Comparison with the theory revealed considerable disagreement in the case of non-resonant spin-flip in collisions of the dμ-atoms with D₂ molecules. This revised version was published online in August 2006 with corrections to the Cover Date.     

Refractive index and mass density of quench condensed argon-xenon films

The refractive index and mass density of Ar _x –Xe_100–x films, which are condensed on a silicon substrate held at liquid helium temperature, are determined by ellipsometric measurements in combination, with a quartz microbalance. The mass density is typically about 10% less than the corresponding bulk density. This is consistent with the existence of a porous film structure. No annealing effect, except for pure Xe, can be observed until the Ar evaporates from the layer. Films condensed with a high initial Ar concentration remain in a highly porous sponge structure after the Ar atoms have left the film with a mass density of less than 50% of the Xe bulk density.     

Longitudinal rf discharge in Xe/Cl<Subscript>2</Subscript> mixtures

Results are presented from the studies of the electrical and emission characteristics of the low-temperature plasma of a longitudinal rf (f₀=1.76 MHz) discharge in Xe/Cl₂ mixtures at pressures of 100–800 Pa. The discharge was ignited in a cylindrical quartz tube with an inner diameter of 1.4 cm and interelectrode distance of 3.0 cm. The discharge emission within the spectral range of 190–670 nm is studied. The dynamics of the discharge current and discharge emission at different pressures and compositions of a Xe/Cl₂ mixture are investigated. It is shown that a discharge in a Xe/Cl₂ mixture acts as a wideband excimer-halogen lamp with a cylindrical output aperture emitting in the spectral range of 220–320 nm. The broad plasma emission spectrum is formed due to the overlap of the XeCl(D, B-X; B, C-A) bands that are broadened at low working-gas pressures. The composition of the working mixture is optimized to achieve the maximum power of the wideband UV plasma emission. Longitudinal rf discharges in low-pressure Xe/Cl₂ mixtures are of interest for developing small-size wideband (Δλ=220–450 nm) cylindrical-aperture lamps, whose efficiency can, on average, exceed the efficiency of conventional hydrogen lamps by more than one order of magnitude.     

Intensity of Ar spectral lines in an Ar-O2 glow discharge

Measurements of relative intensities of spectral lines and of plasma parameters have been carried out in an Ar-O₂ d.c. glow discharge under the following conditions: gas pressures of 1, 2 and 3 torr and partial pressures for O₂ of 0, 0.01, 0.02, 0.05 and 0.1 torr. Many of the Ar line intensities decreased when O₂ was added, while some were enhanced. Enhancement of the 6677A and 7504A lines has been most notable. The number density of metastable Ar atoms is decreased when O₂ is added. Upon addition of O₂, the electron temperature is decreased and the electron density increased, except at a gas pressure of 1 torr; the electric field strength in the positive column is also increased.     

An experimental investigation of the pressure and concentration dependence of muonic Coulomb capture and cascade in gases

Muonic Coulomb capture ratios and x-ray intensity patterns in four binary mixtures of the gases N₂, Ne, Ar, Kr, and Xe have been measured at three atomic ratios. An influence of the concentration has been established. The Lyman series intensity patterns of pure N₂, Ne, Ar, Kr, and Xe were measured at pressures between 0.4 and 51 bar and found to depend on the pressure. Possible explanations are discussed.     

Dry etching of bulk single-crystal ZnO in CH4/H2-based plasma chemistries

The effect of inert gas additive (He, Ar, Xe) to CH₄/H₂ discharges for dry etching of single crystal ZnO was examined. The etch rates were higher with Ar or Xe addition, compared to He but in all cases the CH₄/H₂-based mixtures showed little or no enhancement over pure physical sputtering under the same conditions. The etched surface morphologies were smooth, independent of the inert gas additive species and the Zn/O ratio in the near-surface region decreases as the mass number of the additive species increases, suggesting preferential sputtering of O. The plasma etching improved the band-edge photoluminescence intensity from the ZnO for the range of ion energies used here (290-355 eV), due possibly to removal of surface contamination layer.     

Excitation of iron atoms in a plasma of excimer emitters under transverse-discharge pumping

The optical characteristics of a transverse-discharge plasma initiated in He/Xe(Kr)/HCl(CF₂Cl₂) mixtures were studied. The mixtures contained a small amount of iron vapor due to metal cathode erosion. The iron atoms were shown to be excited by the spontaneous emission of KrCl (λ=22 nm) and XeCl (λ=308 nm) molecules in a nanosecond transverse discharge.     

An investigation of flame zones and burning velocities of laminar unconfined methane-oxygen premixed flames

Numerical and experimental investigations of unconfined methane-oxygen laminar premixed flames are presented. In a lab-scale burner, premixed flame experiments have been conducted using pure methane and pure oxygen mixtures having different equivalence ratios. Digital photographs of the flames have been captured and the radial temperature profiles at different axial locations have been measured using a thermocouple. Numerical simulations have been carried out with a C2 chemical mechanism having 25 species and 121 reactions and with an optically thin radiation sub-model. The numerical results are validated against the experimental and numerical results for methane-air premixed flames reported in literature. Further, the numerical results are validated against the results from the present methane-oxygen flame experiments. Visible regions in digital flame photographs have been compared with OH isopleths predicted by the numerical model. Parametric studies have been carried out for a range of equivalence ratios, varying from 0.24 to 1.55. The contours of OH, temperature and mass fractions of product species such as CO, CO₂ and H₂O, are presented and discussed for various cases. By using the net methane consumption rate, an estimate of the laminar flame speed has been obtained as a function of equivalence ratio.     

Study of the synthesis and Cl2 gas-sensing properties of nanometer HNO3-doped SnO2

We focused on the effects of the inorganic acid HNO₃ on the gas-sensing properties of nanometer SnO₂ and prepared the powders via a dissolution-pyrolysis method. Furthermore, the powders were characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM), field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectra (EDS). Several aspects were surveyed, including the calcining temperature, concentration of nitric acid and the working temperature. The results showed that the gas response of 3 wt% HNO₃-doped SnO₂ powders (calcined at 500 °C) to 10 ppm Cl₂ reached 316.5, at the working temperature 175 °C. Compared with pure SnO₂, appropriate HNO₃ could increase the gas sensitivity to Cl₂ gas more significantly.     

Radiale Änderungen der Energieverteilungsfunktion der Elektronen im Plasma der positiven Säule elektrischer Entladungen

In the low-current positive column measurements of the electron energy distribution function show considerable changes in radial direction. At any point of the positive column of the Ne-discharge under investigation it is possible to approximate the energy distribution function by the Druyvesteyn-standard form. For the case of standard energy distribution functions a theory of positive column is developed which takes into account radial changes of electron temperature. Formulae for the calculation of radial behaviour of electron temperature, space potential, and radial electric field strength are given and compared with experimental results.     

Spectral characteristics of a barrier discharge in cadmium dihalide vapor-inert gas mixtures

Spectral characteristics of a barrier-discharge plasma produced in atmospheric-pressure mixtures of cadmium diiodide and cadmium dibromide vapors with neon, krypton, and xenon at a repetition rate of sine voltage pulses up to 130 kHz are studied. The emission from the discharge is studied within the spectral range 200–900 nm with a resolution of 0.05 nm. Emission of exciplex molecules CdI(B → X) and CdBr(C, B → X), and cadmium and inert gas atoms is revealed, as well as emission of exciplex molecules XeI(B → X, A) and XeBr(B → X, A) in xenon-containing mixtures. The emission of xenon halides prevails in the spectra at a mixture temperature up to 200°C. With a further temperature increase, the emission of cadmium halides becomes dominating. It is ascertained that the most intense emissions of CdI(B → X) and CdI₂/CdBr₂/Xe/Kr and CdBr(B → X) molecules are observed, respectively, in CdI₂/CdBr₂/Xe/Kr and CdI₂/CdBr₂/Xe mixtures. The cadmium dihalide-inert gas mixtures are of interest for the use as a radiating gas in a multiwavelength and broadband excilamp emitting in the UV and visible spectral ranges.     

Effective ionization coefficients and electron drift velocities in gasmixtures of CF3I with N2 and CO2 obtained from Boltzmannequation analysis

The electron swarm parameters including the density-normalized effective ionization coefficients (α-η)/N and the electron drift velocities V_e are calculated for the gas mixture of CF₃I with N₂ and CO₂ by solving the Boltzmann equation in the condition of steady-state Townsend (SST) experiment. The overall density-reduced electric field strength is from 100 Td to 1000 Td (1 Td=10^-17 V·cm²), while the CF₃I content k in the gas mixture can be varied over the range from 0% to 100%. From the variation of (α-η)/N with the CF₃I mixture ratio k, the limiting field strength (E/N)_lim for each CF₃I concentration is derived. It is found that for the mixtures with 70% CF₃I, the values of (E/N)_lim are essentially the same as that for pure SF₆. Additionally, the global warming potential (GWP) and the liquefaction temperature of the gas mixtures are also taken into account to evaluate the possibility of applying in the gas insulation of power equipment.     

A Miniaturized XeCl Dielectric Barrier Discharge as a Source of Short Lived,Fast Decaying UV Radiation

A dielectric barrier discharge (DBD) of coaxial geometry has been examined as source of intensive, short‐lived UV radiation. A binary gas mixture consisting of 98% Xe + 2% Cl₂ and a ternary composition of 96% Ne + 6% Xe + 0.2% HCl were investigated in the pressure range between 10 and 750 mbar. The discharge was excited by unipolar high voltage square pulses with amplitudes of 3 to 7 kV at a repetition rate of 100 Hz. UV radiation intensity is increased by combining 5 square pulses with a period of 1 μs each in one train (burst). Radiation decay on three orders of magnitude within 10 μs was obtained for both gas mixtures in burst mode. Maximum energy of the light pulse was the same for both gas mixtures and estimated as 40 nJ. It is shown that the optimum gas pressure for highest radiation intensity depends not only on gas composition but also on applied voltage and number of pulses in the train. A variation of the spectral band shape of XeCl emission during the pulse is detected (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Xe2Cl fluorescence and absorption in self-sustained discharge XeCl lasers

Fluorescence at 490 nm from the triatomic excimer Xe₂Cl^* has been investigated to determine the 308 nm absorption due to this species in an x-ray preionized, self-sustained gas discharge XeCl laser. The dependence of Xe₂Cl^* density on laser intensity (at 308 nm), buffer gas and Xe and HCl partial pressures has been determined for discharges with a peak electrical power deposition of 2.5 GWl^–1. Xe₂Cl^* absorption is estimated to reach 0.6% cm^–1 under non-lasing conditions but decreases to a non-saturable 0.2% cm^–1 for intracavity laser intensity>1 MW cm^–2. XeCl^* and Xe₂Cl^* fluorescence intensities were found to be a similar for both helium and neon buffer gases but laser output was a factor of two greater with a neon buffer.     

The double M-effect induced by noble gases activated with negative ions

The M-effect (monochromatization-effect) is a powerful tool which can give high intensity monochromatic spectra with a certain wavelength depending on the type of used gas mixtures to generate plasma state. The effect consists in the emission of a single spectral line of plasmas ignited in certain gas mixtures. The main condition to obtain the M effect is the presence of an electropositive and an electronegative gas mixture. For example, in the case of Ne+H₂ monochrome radiation was obtained, the wavelength of Ne being 585.3 nm (1s²–2p⁵). In this paper we prove the general character of this effect, i.e. if the optical emission spectra reduced to nearly one line can be observed also in other gas mixture discharges, for example in the case of one electronegative gas and two electropositive gases. Different other mixtures, as Xe+Ne+H₂ and Xe+Ar+H₂ have been studied. In all these cases, the M-effect appeared without doubt.     

Pulverized coal swirl flames in oxy-fuel conditions: Effects of oxidizer O2 concentration on flow field and local gas composition

In an experimental study the effects of varied oxygen concentrations in the oxidizer gas on resulting flow fields, combustion products and general behavior of pulverized coal swirl flames under oxy-fuel conditions have been investigated. Experiments were carried out in a small scale down-fired cylindrical combustion chamber equipped with an annular swirl burner. Studied flames had a constant power output of 40 kW_th and O₂/CO₂ oxidizer gas mixtures with O₂ concentrations ranging from 23 to 33 vol%. Detailed two-dimensional flow field measurements are obtained from laser Doppler anemometry (LDA). Velocity profiles (Mean and RMS) have been obtained for all conditions investigated and serve as basis for identification of flow field characteristics. Velocity RMS values are provided as supplementary material. To complement flow field measurements, in-flame gas composition measurements were also conducted using a sampling probe combined with infrared gas absorption analysis via Fourier-transform infrared (FTIR) spectrometry. The results obtained show increased velocities, particularly along the main vortex for flames with increased oxygen contents, while lower velocities are found to occur inside the recirculation regions. The opposite occurs with lower O₂ concentrations, showing significantly reduced velocities in the main vortex, but stronger recirculation than the high oxygen counterparts. This effect is attributed to a modification of the swirl level introduced by the expansion of product gases. Measured NO and CO in-flame concentrations showed significant variations under different O₂ concentrations in the oxidizer.