Table of contents

The high-pressure caesium discharge has a favourable spectral distribution consisting of a smooth recombination continuum in the visible range. When operated on a continuous power source, the spectrum in the visible region is close to blackbody radiation; however, the lamp efficacy is restrained by the self-reversed resonance lines occurring at 825 nm and 894 nm. Pulse modulation significantly increases the core plasma temperature, suppresses the near-infrared segment of the spectrum, strongly enhances the continuous radiation in the visible region, and successfully avoids overloading (<40 W cm^-2) the arc tubes. The spectrum in the visible appears to have the same shape as blackbody radiation when the lamp is operated on a multiple pulse modulated power source. The arc tube geometry, caesium/mercury compositions, and power supply waveforms were optimized for photometric performance through a series of comparison tests. The lamp efficacy increased with narrower diameter arc tubes, higher lamp currents, as well as higher current crest factors (ratio of current pulse peak to RMS current). The highest efficacy achieved for the lamp operated on the pulse modulated power supply was 46 lpw. The lamp exhibits excellent dimming characteristics and has a colour rendering index (CRI) very close to a thermal source such as a tungsten halogen lamp. This study provides a framework for the design of a new lamp/ballast system which features excellent dimming characteristics, a near-perfect CRI, an efficacy above 40 lpw, and long life. The application for this light source could be a replacement for a high-end tungsten halogen or a white high-pressure sodium lamp.

In this paper we present a study concerning the influence of the electrode gap on the erosion of a copper cathode under the sole action of an electric arc in air at atmospheric pressure. The causes of the variations of the erosion rate with this parameter are studied. It is shown that, certainly, the variations of the erosion rates with the electrode gap should be imputed to two factors: the arc mobility which changes strongly with the electrode gap and a `supplementary' heating of the cathode surface due to interactions that occur at small electrode gap values between the cathode surface and the anode plasma jets.

We report on time-dependent population distributions of excited rotational states of hydrogen in a capacitively coupled RF discharge. The common model to obtain the gas temperature from the rotational distribution is not applicable at all times during the discharge cycle due to the time dependence of the EEDF. The apparent temperature within a cycle assumes values between 350 K and 450 K for the discharge parameters of this experiment. We discuss the optimum time window within the discharge cycle that yields the best approximation to the actual temperature. Erroneous results can be obtained, in principle, with time-integrated measurements; we find, however, that in the present case the systematic error amounts to only approximately 20 K. This is due to the fact that the dominant contribution to the average intensity arises during that time window for which the assumptions underlying the analysis are best fulfilled. A similar analysis can be performed for N⁺₂ rotational bands with a small amount of nitrogen added to the discharge gas. These populations do not exhibit the time variations found in the case of H₂.

The interaction of a vacuum arc plasma jet (VAPJ) with the neutral atmosphere filling the plasma duct in a vacuum arc plasma coating system was modelled. The VAPJ velocity, ion density and neutral gas density were calculated using a one-dimensional approximation in steady state, as a function of distance from the duct entrance. Three cases were considered, with (1) the duct entrance, (2) the duct exit and (3) the average neutral gas pressure fixed. It was found that, generally, the VAPJ velocity decreases as it propagates along the duct, while the neutral density increases as a function of the distance from the duct entrance. The jet deceleration is steepest when the pressure is fixed at the duct entrance. When the pressure is fixed at the duct exit, the deceleration of the VAPJ is greater for less-dense jets.

A plasma model of a dc magnetron glow discharge of an Ar-N₂ mixture, with an aluminium target, is extended in order to explicitly include plasma-surface interactions. Theoretical results are compared with experimental data from optical emission spectroscopy of Al. The effect of the nitrogen concentration is studied. The theoretical calculation trends are in good agreement with the experimental results. The present work allows (1) the theoretical evaluation of the concentration of sputtered Al species and (2) the evaluation of the relative roles of the sputtering mechanisms and the contribution of the various species.

Nitrogen plasmas at atmospheric pressure produced by 2.45 GHz microwaves at a power density of approximately 10 MW m^-3 have a degree of ionization less than about 10^-7. Nevertheless they have interesting and potentially important effects on polymer and metal surfaces exposed to them. An experimental programme is underway to identify the active species in the plasma and its afterglow. This paper describes a simplified model of the chemical kinetics in the plasma that allows species concentrations to be estimated in a range of conditions, for comparison with experimental data. It predicts a high degree of dissociation combined with low gas temperature in microwave-generated plasmas.

Time resolved current and voltage measurements have been made on a pulsed radio frequency (rf) inductively coupled plasma (ICP) at 13.56 MHz in argon. Measurements were made on the rf coil using a high-voltage probe, a Rogowski current probe, and a high-performance digital oscilloscope. Relative phase information was also obtained so that time resolved rf power measurements could be made. Due to the inductive nature of the load, measurement of the phase had to be better than 0.6 mrad at 13.56 MHz in order that the power measurements were accurate to 10%. This accuracy in phase measurement was achieved by careful positioning of the probes and by establishing accurate phase calibration procedures. The power was calculated by three methods: discrete Fourier transform, integral of the current voltage product over N periods, and least-squares fits of a sine wave to the measured data. Time-resolved measurements of the system complex impedance, power loss in the ICP planar coil, and the actual amount of rf power delivered to the plasma were made. These measurements give details during plasma breakdown and show the transition from capacitive to inductive discharge. The results are compared with both time-resolved plasma emission and time-resolved Langmuir probe measurements.

Tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and four stable molecules, CH₄, C₂H₂, C₂H₄ and C₂H₆, in a H₂ surface wave discharge (f = 2.45 GHz and power density ≈10-50 W cm^-3) containing up to 10% methane under different flows (Φ = 22-385 sccm) and pressures (p = 0.1-4 Torr). The degree of dissociation of the methane precursor varied between 20% and 85% and the methyl radical concentration was found to be in the range of 10¹² molecules cm^-3. The methyl radical concentration and the concentrations of the stable C-2 hydrocarbons C₂H₂, C₂H₄, C₂H₆, produced in the plasma, increased with an increasing amount of added CH₄ as well as with increasing pressure. For the first time, fragmentation rates of methane (R_F(CH₄) = 1×10¹⁵-2.5×10¹⁶ molecules J^-1) and conversion rates to the measured C-2 hydrocarbons (R_C(C₂H_y): 5×10¹³-3×10¹⁵ molecules J^-1) could be estimated with dependence on the flow and pressure in a surface wave discharge. The influence of diffusion and convection on the spatial distribution of the hydrocarbon concentration in the discharge tube was considered by a simple model.

We present a modelling study of pulsed H₂/CH₄ microwave plasmas obtained under moderate pressure discharge conditions in a tubular quartz reactor. The transport in the reactor was described using a Nusselt model for a radially quasi-homogeneous plasma. The thermal behaviour of the plasma was modelled by distinguishing a single heavy species energy mode and the electron translation mode. The chemistry was described using a 30 species-130 reaction model. The time variations of the electron energy distribution function, the species concentrations and the gas temperature were determined by solving the coupled set of the electron Boltzmann equation, species kinetics equations and a total energy equation. Some of the results obtained from the present model were compared to measurements previously carried out on the plasmas considered. Good agreement was obtained for the time variations of the gas temperature, the relative concentration of the H-atom and the intensities of the H_α and the argon 750 nm emission lines. The effect of the duty cycle on the time-averaged composition and temperatures of the discharge was also studied. Results showed that moderate pressure H₂/CH₄ pulsed discharges obtained at duty cycles of less than 20% show different behaviour than those obtained at higher duty cycles. In particular, while the plasma reaches the permanent periodic regime in less than 2 pulse-periods, i.e. 60 ms, for duty cycle values of less than 20%, long-time-scale density variations of hydrocarbon species, ions and electrons are obtained when this parameter is greater than 20%. The model was also used to determine if the use of a pulsed regime may bring some improvements in plasma-assisted diamond deposition processes. For this purpose we analysed the variation with duty cycle of the time-averaged populations of the H-atom and CH₃ that represent the key species for diamond deposition. Results showed that pulsed discharges with small duty cycle, of typically less than 20%, lead to a substantial enhancement of the time-averaged dissociation yield. On the other hand, the CH₃ concentration exhibits a strong decrease with the duty cycle. The methyl concentration in the investigated pulsed discharge is generally smaller than in continuous wave discharges obtained in the same reactor. These results indicate that short-pulse discharges would favour the formation of films with higher Raman quality, while long duty cycle pulsed discharges would enable deposition at higher growth rates.

A global (volume-averaged) model is applied to investigate the effect of the electron energy distribution function on the plasma parameters of a high-density, low-pressure, argon discharge. The effective electron temperature increases and the electron density decreases as the electron energy distribution is varied from being Maxwellian to become Druyvesteyn like. The sheath voltage decreases as the electron energy distribution function is varied from being Maxwellian to become Druyvesteyn like for low pressures ({<}2 mTorr) and increases for higher pressures as the electron energy distribution function is varied. Simple global model calculations demonstrated that increasing the operating pressure does not necessary lead to a higher electron density if the electron energy distribution evolves from Maxwellian to become more Druyvesteyn like as the operating gas pressure is increased.

There is a continuing need for the development of effective, cheap and environmentally friendly processes for the disinfection and degradation of organic pollutants from water. Ozonation processes are now replacing conventional chlorination processes because ozone is a stronger oxidizing agent and a more effective disinfectant without any side effects. However, the fact that the cost of ozonation processes is higher than chlorination processes is their main disadvantage. In this paper recent developments targeted to make ozonation processes cheaper by improving the efficiency of ozone generation, for example, by incorporation of catalytic packing in the ozone generator, better dispersion of ozone in water and faster conversion of dissolved ozone to free radicals are described. The synthesis of ozone in electrical discharges is discussed. Furthermore, the generation and plasma chemical reactions of several chemically active species, such as H₂O₂, O^•, OH^•, HO₂^•, O₃*, N₂*, e^-, O₂^-, O^-, O₂⁺, etc, which are produced in the electrical discharges are described. Most of these species are stronger oxidizers than ozone. Therefore, water treatment by direct electrical discharges may provide a means to utilize these species in addition to ozone. Much research and development activity has been devoted to achieve these targets in the recent past. An overview of these techniques and important developments that have taken place in this area are discussed. In particular, pulsed corona discharge, dielectric barrier discharge and contact glow discharge electrolysis techniques are being studied for the purpose of cleaning water. The units based on electrical discharges in water or close to the water level are being tested at industrial-scale water treatment plants.}

This research concerns the development of a SF₆ RF discharge at low pressure in a small reactor for industrial applications. The plasma is produced in the pressure range 0.05-1 mbar by a RF supply. The pumping system sustains a flowrate of about 50 cm s^-1, with residence time in the discharge of about 0.2 s at a pressure of 0.1 mbar. The discharge parameters were measured at a low operation power. Measurements were performed by means of movable electrostatic probes and a photodiode. Particular care in the analysis of the data proved to be necessary due to the presence of a substantial amount of negative ions. The reactor has been employed for textile treatment in order to modify the surface properties of the fibres. Favourable operating conditions leading to an improved hydrophobicity of the textiles were achieved.

The influence of the excitation frequency f = ω/2π of the applied electric field on the period average electron energy distribution function (EEDF) and on the atomic hydrogen concentration found near the deposited diamond films (substrate) and in the bulk of CH₄(⩽5%)/H/H₂ plasmas produced in RF and MW discharges is estimated. This is done through the solution, as a function of the reduced effective electric field, of a stationary homogeneous electron Boltzmann equation (EBE) and the solution, in terms of the atomic hydrogen mole fraction, of a simple kinetic model for the plasma mechanisms underlying the production and loss of atomic hydrogen. The physical basics underlying the approach followed to solve the EBE, including discussion of EEDF time-modulation effects, are discussed in the light of recent results by Loureiro (1993 Phys. Rev. E 47 1262) on time-dependent kinetics of pure H₂ plasmas. Correlations are established between the results, obtained under various discharge conditions, from plasma-enhanced chemical vapour deposition (PECVD) experiments of diamond-like carbon (DLC) and diamond thin films, and the calculated EEDF, atomic hydrogen concentrations (in the plasma and near the substrate) and mechanisms underlying the production and loss of atomic hydrogen in the plasma.