Table of contents

Laser-induced fluorescence (LIF) techniques currently used as an optical diagnostic to study and characterize non-thermal equilibrium plasmas are reviewed. The general mechanisms occurring in one- or two-photon resonant absorption-induced fluorescence when used for the detection of species in their fundamental state are analysed. We emphasize the techniques based on the simultaneous absorption of two photons for the detection of light atoms and molecules and, particularly, two-photon absorption laser-induced fluorescence (TALIF) and derivative techniques, such as resonant enhanced multi-photo-ionization, two-photon absorption laser-induced stimulated emission (TALISE) and photo-fragment translational spectroscopy, in which molecules are photo-dissociated and the ejected atomic fragments are simultaneously detected by TALIF. The kinetics of one- and two-photon absorption are treated in detail including, for the latter, a modelling, which indicates the conditions of emergence of TALISE. The main calibration methods allowing one to obtain the absolute density of species detected by LIF and some examples of applications demonstrating the interest of these diagnostic techniques in plasma processing are presented.

We report on the use of insulating Langmuir-Blodgett (LB) films to improve the quantum efficiency of light emitting devices based on polymeric LB films. The insertion of arachidic acid layers between a poly-(2-methoxy,5-(2'ethylhexyloxy)-p -phenylenevinylene) (MEH-PPV) film and an aluminium cathode was found to decrease the turn-on voltage for electroluminescence and to double the quantum efficiency. In contrast, depositing the fatty acid layer between the polymer and the indium tin oxide anode produced little change in the device efficiency. Dual-layer structures using MEH-PPV as the light emissive layer and poly-(p -(3-hexyl pyridylene)) as an electron transport layer were also tested. Again, an increase in device efficiency could be obtained by placing arachidic acid LB layers immediately beneath the metallic cathode.

The permittivity of different microporous systems, including sintered glass filters, Berea sandstones and reservoir rocks, was measured in the frequency range 10² -10¹⁰Hz. For each system, the natural pore wettability was modified to get two different sets of samples: those strongly oil-wet and those water-wet. The samples were saturated with a hydrocarbon oil and water at different degrees of water content. Dielectric spectra showed a conductivity contribution due to ionic carriers in addition to a Maxwell-Wagner-Sillars relaxation, active up to high frequencies. Because of the dominant role that wettability plays in the electrical response of partially saturated microporous solids, water-wet and oil-wet samples showed remarkable differences in the loss tangent spectra, as well as different times and strengths of the Maxwell-Wagner-Sillars relaxation. The dielectric spectra were analysed by using some standard models for extracting valuable information about the pore wettability. Moreover, the use of impedance measurements for in situdetermination of the wettability of reservoir rocks was evaluated.

Tungsten trioxide films were deposited on (100) SrTiO₃and R-plane (10 2) cut sapphire substrates by dc magnetron sputtering, using a tungsten target in an Ar/O₂sputtering gas mixture at substrate temperatures ranging from 500 to 850 °C. Deposited films were characterized by x-ray diffraction using -2 scans and pole figure analysis. X-ray results showed that films deposited on both types of substrate were epitaxial. The equilibrium phase was monoclinic -WO₃ , confirmed by Raman spectroscopy. Films on both substrates were (001) oriented. This preferred orientation improved as the deposition temperature was reduced. The in-plane orientation relationship of the films with the substrate was obtained from the pole figures.

The spray pyrolysis technique is employed to prepare thin films of SnS on glass substrates and SnS₂on glass as well as fluorine doped tin oxide (FTO) coated glass substrates. The pyrolysis temperatures of SnS and SnS₂are 350 °C and 275 °C, respectively. The films are found to be n-type semiconductors. The SnS thin film is amorphous, whereas the SnS₂shows polycrystalline structure and the calculated average grain size is 115 Å for SnS₂on glass and 324 Å on the FTO substrate. From an analysis of the high absorption region data a direct-allowed transition at 2.44 eV has been observed for SnS₂ . In the optical investigations of SnS layers an non-direct-allowed transition at 1 eV is revealed. The corresponding absorption coefficients near the fundamental absorption edge are ~2 × 10⁴cm^-1and 4.6 × 10³cm^-1for SnS₂and SnS thin films. The room temperature dark conductivity of SnS is of the order of 10^-3-1cm^-1and 10^-4 -10^-3-1cm^-1for SnS₂films. The corresponding photoconductivity values of SnS, SnS₂on glass and SnS₂on FTO are ten, four and two times higher than the dark conductivity.

Monte Carlo simulations were performed to study the effect of local variations of the domain wall coercivity on the rate of domain growth in ultra-thin magnetic films with strong perpendicular anisotropy. The analytic models used in previous studies are refined, compared with the Monte Carlo data and shown to be inadequate to describe realistically the domain growth process. When the field is increased a depinning transition occurs where the domain expands by a sequence of avalanches. If the avalanche size is small, the field dependence of the wall velocity is exponential, as in a thermo-activated process, so the full extent of the depinning transition may not be evident in experimental v (H ) curves. The depinning field H₀ , when the avalanche size diverges, is weakly dependent on the width of the coercivity distribution. When H >H₀ , the phenomenological relation v= v₀ +µ´(H -H₀ ) that is observed experimentally in Au/Co(8 Å)/Au films is shown to result from the temporary obstruction of the domain boundary by relatively hard pinning centres and µ´ is found to be larger than the wall mobility.

The behaviour of a free burning SF₆arc in a two-volume cylindrical chamber, which simulates the initial arcing in a modern high voltage auto-expansion circuit breaker, has been investigated both experimentally and theoretically. Arc voltage and pressure variations at four strategically chosen locations in the chamber were measured for electrode gap lengths of 30, 40 and 50 mm in the peak current range from 5 to 20 kA of a positive half-cycle alternating current at 50 Hz. A simplified model for free burning arcs in an enclosure was developed by radially integrating the governing equations under the assumptions of a uniform radial temperature profile and a variable radial profile of the axial velocity (equation (1)) in the arc. The effects of Lorentz force, Ohmic heating, radiative energy transfer, and turbulence enhanced momentum exchange are taken into account. The computed arc voltage and pressure variation in the chamber agree with the experimental results within the estimated range of uncertainty.

The development of the hollow-cathode effect in a plane-parallel hollow cathode dc argon glow discharge was investigated experimentally and by means of a two-dimensional self-consistent hybrid model, combining the fluid description of positive ions and slow electrons with a particle simulation of fast electrons. In the experiments the discharge was formed between two flat disc copper electrodes (of 3.14 cm diameter and separated by a L= 2 cm gap) serving as cathodes and a metal tube surrounding these electrodes which served as the anode. The electrical characteristics of the discharge and the spatial intensity distribution of selected spectral lines (Ar I 750.3 nm, 811.5 nm and Ar II 476.5 nm) were recorded at current densities 0.1 mA cm^-2j 0.5 mA cm^-2and for gas pressures 0.2 mbarp 1 mbar. While at pressures of ~1 mbar the cathode regions are developed separately for both cathodes, the light intensity distribution measurements demonstrated the gradual merging of the negative glows with decreasing pressure. At pL 0.8 mbar cm, a common negative glow is formed in the discharge. Complementing the experimental observations, the simulations made it possible to determine various discharge characteristics (e.g. spatial distribution of electric potential, ionization source, and ion density). At low pLvalues the simulations also indicated the existence of oscillating electrons. The spatial distribution of light intensity calculated for different pressures shows good qualitative agreement with the experimentally observed distributions.

The peak electron density (N_e ) in the ablative dense plasma produced by the pulsed discharge through a Teflon-(CF₂ )_ncapillary has been measured using Stark's spectroscopy of the atom emission lines in the range N_e= 10¹⁶ -10¹⁸cm^-3 . The time-resolved profiles of F(I) (683.43, 685.60, 687.02, 690.25, 690.98 nm) lines were detected for the different discharge energy densities and compared with the theoretical data. The results show that the method can be used for the diagnostic of plasmas with peak electron density up to about 3 × 10¹⁷cm^-3 , which corresponds to a discharge energy density of 9 kJ cm^-3 .

This paper describes the work undertaken to investigate the electrode erosion mechanisms and lifetime-limiting parameters of a sealed, high pulse repetition frequency (PRF), triggered, corona-stabilized (TCS) switch. The switch, which had a rod-plane geometry, was filled with air and operated at repetition rates of 10 Hz and 1 kHz. The erosion rates of anode-cathode electrode materials such as brass, stainless-steel, aluminium and elkonite were measured, and their surface conditions were studied optically and photographed. The experimental results presented reflect the different mechanisms responsible for the erosion of the rod (anode) electrode of the TCS switch, when operated at a PRF of 10 Hz and 1 kHz. The erosion of the anode electrode at a PRF of 10 Hz varied from 5.44 × 10^-6cm³C^-1for elkonite to 17.37 × 10^-6cm³C^-1for aluminium. Additionally, at a PRF of 1 kHz, anode erosion ranged from 9.82 × 10^-6cm³C^-1for brass to 16.58 × 10^-6cm³C^-1for elkonite. Cathode erosion rates as high as 36.6 × 10^-6cm³C^-1for brass at 10 Hz and 19.55 × 10^-6cm³C^-1for elkonite at 1 kHz were measured. The lifetime of the TCS switch was found to be inversely proportional to the erosion rate of the rod electrode. Additionally, gas deterioration and anode surface conditions were found to have a pronounced influence on the performance and operational behaviour of the TCS switch.

A plasma is produced by pulsed microwaves in a circular tube with a conducting wall and a radius below the cut-off value for the fundamental circular waveguide mode (TE₁₁ ). The discharge tube is surrounded with a multicusp magnetic field constructed with permanent magnets which provide a minimum-B configuration. Short pulse (0.05-1.0 µs) microwaves of 3 GHz and 60-100 kW peak power with a repetition frequency of 10-500 Hz are launched into the narrow tube with the propagation vector perpendicular to the cusped magnetic field in a pressure range of 10^-2 -10 Torr. It has been found that, in addition to occurring at the entrance, a plasma is produced at a location of about one-third length from the tube exit. Plasmas generated at the two locations diffuse mostly along the tube axis with the radial diffusion strongly suppressed by the multicusp field. The density uniformity is better at lower pressures. The spatio-temporal evolution of electrons and ions shows a density growth after the end of a microwave pulse for a few to tens of microseconds depending on the axial position, followed by decay. The profiles have been explained by numerical simulation based upon a model that charged particles are driven by electrostatic and ponderomotive forces and diffuse along the longitudinal direction. The interpulse regime has an electron temperature of about 10 eV. This indicates that the plasma is still active between the pulses, in distinct contrast with the usual afterglows.

Because outdoor insulating materials in service are subjected to numerous wet and dry cycles, it is necessary to establish their performance in acid rain. The erosion effect of acid rain on atactic polystyrene insulating material is investigated using accelerated ageing by artificial acid rain. The degradation mechanisms of material structure and tracking resistance are discussed. The experimental results reported here show that the erosion of acid rain causes the degradation of the surface chemical and physical structures of polystyrene (PS) material so that the surface conductivity of aged material increases. Under electrical stress, the surface discharge current increases, and the local surface is deteriorated so that the tracking is initiated earlier and the material rapidly fails. The degradation rate of PS material in acid rain varies as function of the ion concentration, pH and conductivity of acid rain. Only the severe acid rain of high acidity and conductivity can exert an influence on PS insulating material. The concentration of actual rainwater is insufficient to lead to obvious erosion on PS insulation. PS insulation could resist on the erosion of normal acid rainwater. Even though clear degradation occurs with strong acid rain, PS could behave quite well in mild acid rain conditions.

Following the modified model used in our previous paper for the study of laser-induced breakdown of molecular nitrogen, in this work we present the influence of secondary ionization processes on the breakdown phenomenon. The analysis is based on the calculation of the electron energy distribution function and its parameters for laser wavelengths covering the range 266-1064 nm, in irradiated molecular nitrogen at atmospheric pressure. The result of computations revealed that no evidence is observed for collision and photoionization processes of ground-state molecules. Under the experimental conditions applied in this analysis, collision processes lead only to the population of excited states. Breakdown of nitrogen at laser wavelengths 532, 355 and 266 nm proceeds totally via photoionization of the excited states. For = 1064 nm, electron growth is obtained due to multi-photoionization of the excited states, in addition to the contribution of collisional ionization of these states.

It is shown that the electric field of any polarized resonant capacitive transducer cannot exceed some maximum value and equations calculating this are obtained. It is found that the field inhomogeneity on the transducer gap edge should cause the decrease of the transducer resonant frequency with its polarizing voltage. Equations describing this effect are obtained. There is a good agreement with experimental results.