Eigenschaften und Parameterabhängigkeit der Temperaturverteilung und der Charakteristik eines zylindersymmetrischen Stickstoffbogens

The purpose of this work is to examine the properties and dependence upon parameters of the temperature distribution, and to ascertain the characteristic of a stationary, cylindersymmetrical nitrogen arc with negligible convection. Proceeding from the local balance of energy (the Elenbaas-Heller differential equation), the article makes known a process designed to determine the axial field strength in the arc discharge tube explicity with the aid of the boundary conditions of the temperature and the material functions. To this end, the differential equation is converted to a nonlinear integral equation. This equation can then, provided that the material functions are known and the parameters — radius of discharge tube, axis temperature, wall temperature — are established, be solved either numerically or graphically by a method analogous with the Picard's method of successive approximations. The numerical results, parameter limitations, and assumptions concerning material functions, enable the graphic and analytical relationship between temperature distribution and characteristic to be ascertained within a predetermined range of parameters. Thus we are enabled to interpret and theoretically record a few empirical beginnings and laws.     

Ozone production in oxygen by means of F2-laser irradiation at λ=157.6 nm

Ozone is generated in pure oxygen (p5 kPa), synthetic air (p7 kPa) and oxygen-argon mixtures (p3 kPa) by irradiation of these gases with the VUV light of a repetitively pulsed (f _L15 Hz) F₂-laser at =157.6 nm with maximum about 4 mJ/pulse. An absorption photometer measurement operating at 253.7 nm (Hg line) determines the ozone concentration as a function of oxygen and/or additive gas pressure, the repetition frequency of the laser and the wall temperature of the reaction chamber. The temporal development of the ozone concentration as a function of these parameters is calculated by means of rate equations for the species O(³ P), O₂(X ³ _g ^– ), O₃(¹ A ₁), O(¹ D), O₂(a ¹_g), O₂(b ¹ _g ⁺ ) and vibrationally excited O ₃ ^* (¹ A ₁) and the photon distribution. The maximum concentration of O₃ in the sealed-off chamber reaches 1.6% in pure O₂, 4.1% in air and 1.2% in a 1:5 O₂-Ar mixture at 3 kPa. The annihilation of O₃ by the wall and temperature dependent volume processes (300 KT395 K) is studied and the experimental and theoretical results are compared.     

Experimental and Theoretical Studies of a Pulsed Microwave Excited Ar/CF4 Plasma

The present work deals with a pulsed microwave discharge in an Ar/CF ₄ gas mixture under a low pressure (1–10 mbar). The discharge chamber developed has a cylindrical geometry with a coupling window alternatively made of quartz or alumina. The setup allows one to investigate the plasma–wall interactions (here etching of the quartz window) and the ignition process of the pulsed microwave plasma. Microwave pulses with a duration of 50–200 s and repetition rate between 1 and 10 kHz are typical for the experiments. The space-time behavior of the fluorine number density in the discharge has been investigated experimentally by optical actinometry. The discharge kinetics is modeled using electron-transport parameters and rate coefficients derived from solutions of the Boltzmann equation. Together with the solution of the continuity and electron balance equations and the rate equations describing the production of CF _x (x=2, 3, 4) radicals and F atoms, a good agreement between experimental and theoretical data can be achieved.     

Temperature measurements in CO2-laser-induced pyrolysis flames for SiC and ternary SiC/C/B powder synthesis by means of CARS

The hot luminescent reaction zones of CO₂-laser-induced pyrolysis flames using SiH₄/C₂H₂ gas mixtures with different silane to acetylene ratios and with and without diborane additives were investigated by means of H₂ Q-branch CARS spectroscopy, leading to spatial temperature profiles in gas flow direction. In the case of B₂H₆ additive to the stoichiometric SiH₄/C₂H₂ mixture a high temperature plateau ( 800–1000 K) of the reactant gas volume develops already several millimetres before reaching the CO₂-laser focus line. This precursor preheating zone could be explained by the catalytic effect of boron atoms or boron-containing intermediate species in the flame. A similar behaviour for acetylene-rich flames operating at half laser power was not observed.     

Diagnostics of an RF Plasma Flash Evaporation Process Using the Monochromatic Imaging Technique

The high densities and high gas temperature of rf plasmas at pressures near 1 atm are favorable for the development of plasma sources capable of evaporating solid precursors in the plasma zone. In the cooler region downstream of the plasma, the evaporated material condenses to nanoparticles and/or coatings. The complete evaporation of precursors injected into a thermal plasma depends on plasma and precursor parameters and is studied in this paper. Since many parameters contribute to the evaporation, fast experimental techniques are necessary to carry out a systematic study of the evaporation process. The monochromatic imaging technique applied in this work uses an intensified CCD camera with optical filters for the detection of characteristic plasma emission lines. The high spatial and temporal resolution of this technique results in a detailed picture of plasma emission and particle evaporation for different process parameters. These results are compared to model calculations for particle evaporation.     

Coherent Anti-Stokes Raman Scattering Applied to Hydrocarbons in a Microwave Excited Process Plasma

Coherent anti-Stokes Raman scattering (CARS) carried out using the BOXCARS technique is applied to quantify the density and temperature of hydrogen, methane, and methyl in a process plasma generated by a surfatron in the pressure range from 30 to 3000 Pa. Below approximately 500 Pa all species and their temperatures have a flat distribution dominated by diffusive processes. At higher pressure the plasma localizes near the inner surface of the surfatron tube. A strong depletion of the methane ground state density is observed, where the gas temperature remains at 400 K on a moderate level. On the other side the methyl density is high at the position of high methane depletion. The measured methyl density is a factor of 200 lower than the methane density, in contrast to the factor 10–20 often reported in the literature. The measured data can be interpreted by proper modeling by means of the Boltzmann equation of electrons, balancing of chemical reactions and diffusive processes. The calculated methane and methyl densities agree with the measured ones only if the electron temperature is assumed to be near 1 eV and thus much lower than predicted in previous papers.     

Investigation of a Xenon-plasma produced in cascaded arcs

This investigation extends earlier measurements and calculations performed on cascaded arcs burning in the noble-gases He, Ne, Ar and Kr to a Xenon discharge. Starting from intensity and line width measurements of some Xe lines, temperature and electron density profiles as well as transition probabilities are determined.E-I characteristics evaluated in conjunction with intensity profiles and temperature measurements lead to accurate results for the electrical conductivity. The pressure inside the arc chamber is variable. In a broad region of parameters LTE is established.     

Holographic Interferometry of Arc Discharges Displaced by Magnetic Fields

The first part of the paper gives a survey of some theoretical aspects of the behavior of a free burning arc in transverse cross-flow and magnetic field. After a short discussion of the variety of forces acting on the arc, the low current case is investigated, in which the external magnetic field is assumed to change aperiodically with time. In the second part, an arc experiment is described which allows the examination of the theoretical aspects of the arc motion. As a diagnostic method, a holographic technique is applied which visualizes the position of the arc as function of time. The calculated and measured displacements of the arc agree within the error limits.     

Resonance enhanced coherent anti-stokes raman scattering and laser induced fluorescence applied to CH radicals: a comparative study

Resonance enhanced CARS and LIF have been applied to the CH radicals in a microwave excited Ar/H/CH plasma ( Pa, kW). Both techniques yield similar nonthermal rotational population distributions of CH(X) in its vibrational ground state (), which can be described by two rotational temperatures, K being in the order of the gas temperature for rotational states with , and a considerably higher for the higher rotational states. This result is in good agreement with previous resonance CARS and LIF measurements in similar plasmas. With resonance CARS additional measurements on CH in the state could be performed yielding a vibrational temperature of 2440 K, the total CH density was about m. The detection limits of both techniques are determined, in our case about CH radicals per quantum state in the detection volume, and their advantages and disadvantages are discussed. Received: 29 April 1996 / Accepted: 19 June 1996