Georges Zissis

A study of the convective flow as a function of external parameters in high-pressure mercury lamps

This paper deals with the modelling of the convection processes in high-pressure mercury arcs. Temperature and velocity fields have been calculated by using a 2D semi-implicit finite-element scheme for the solution of conservation equations relative to mass, momentum and energy. After validation, this model was applied to the study of the influence of arc external parameters such as mercury charge and tube diameter on the convective processes. It was found that stable laminar mono-cellular convection flow occurs at low values of tube diameter for a cylindrical burner and/or a low mercury charge. Finally, we considered in detail the region behind the electrodes, where an accumulation of mercury is observed. The evaluation of this amount of mercury trapped in these regions is of prime importance for a good description of the distribution of mercury in the burner and for a correct evaluation of the total discharge pressure.

Definition of a high intensity metal halide discharge reference lamp

The design of a reference metal halide discharge lamp is presented. This lamp is meant as a common study object for researchers working on metal halide discharge lamps, who by using the same design will be able to compare results between research groups, diagnostic techniques and numerical models. The lamp exhibits all interesting plasma physical, chemical and material science problems, which are currently under investigation in various laboratories. The lamp filling is relatively simple and the design symmetric to allow realistic numerical modelling. Furthermore, it is adapted to enable the use of various diagnostics.

Electrical modeling of an homogeneous dielectric barrier discharge (DBD)

Dielectric barrier discharge (DBD) excimer or exciplex lamps are known as efficient UV and VUV sources but their power supply operates at a high voltage (some kV) and at a frequency of some tens of kHz. Consequently, for industrial applications, DBD lamps need specific power supplies, designed for one lamp geometry and gas filling. The aim of this work is to provide an improved electrical model for a homogeneous DBD which can provide relevant electrical parameters such as gas voltage drop, total current, sheath voltage and current, surface charge deposited on the dielectric surfaces but can allow for fast simulation times.

Dynamic thermal and radiative behaviors of a high pressure sodium lamp plasma

The main purpose of the present work consists in the study of supply frequency and sodium vapor pressure effects on the sodium lamp properties. The retained model is a two-temperature channel type that reproduces quite well the electrical and thermal behaviors as well as the main radiative characteristics of mercury-sodium discharge plasma and can be easily coupled with the lamp circuitry software.

Spatio-temporal study of the deviations from thermal equilibrium in a high-pressure mercury plasma working under an ac power supply

This work deals with the study of deviations from thermal equilibrium in high-pressure mercury discharges operating in ac mode. A one-dimensional time-dependent fluid model assuming a non-equilibrium plasma (two-temperature code) has been developed. This code solves, self-consistently, the set of hydrodynamic equations describing the discharge plasma, coupled with an equation describing the power supply circuit. This code is based on a finite-element method and it has been optimized in order to keep CPU times realistic. Our calculations at an industrial 50 Hz frequency, validated by using experimental data from the literature, allow us the possibility to better understand the mechanisms that are responsible for deviations from thermal equilibrium within an ac cycle. Furthermore, a parametric study allows us to study in detail the influence of some key parameters, such as frequency and mercury load on those deviations.

Experimental investigation of deviations from local thermodynamic equilibrium in high-pressure mercury discharges

The determination of the radial profile of the ground state density without assuming local thermodynamic equilibrium (LTE) conditions in around atmospheric pressure (0.01u200aMPa<p<0.3u200aMPa) discharges used as light sources is a worthy investigation subject. This work deals with the high-pressure mercury discharge which could be considered as a “test case.” Particularly useful for the diagnostics of these plasmas is the self-reversed resonance mercury line 253.7 nm. In this article, two independent experimental methods were used: emission spectroscopy, called the “ΔλR method,” and interference shift measurements “hook method.” Using the Hg-253.7 nm resonance line, both experimental methods indicate similar deviations from LTE in particular for the lower pressure discharges (p<0.04u200aMPa). In those cases, the experimental errors for both methods are significantly lower than the detected deviations. Furthermore, the measured deviations are in good agreement with predicted values from a two-temperature, two-dimensi...

Numerical modelling of a HgTlI discharge lamp: transport coefficients and thermodynamic properties

A LTE chemical model is developed to determine the plasma composition and transport coefficients of a thallium iodide discharge. Collision integrals, diffusion coefficients, thermal and electrical conductivity as well as conductance have been computed as functions of temperature at different atomic ratios using the Chapman Enskog theory. This chemical model is then coupled with a one-dimensional time-dependent fluid model to describe the temperature variation and electrical behaviour of the lamp.

Simulation of the high-pressure mercury discharge lamp during the middle phase of start-up (medium mercury pressure)

This paper deals with the modelling of the medium pressure Hg and Hg-Ar positive column (Hg pressure range 5-350 Torr). The aim of this work is to simulate, in a first approximation, the middle phase of the Hg high-pressure lamp warm-up. In this approach, the main assumption is that time evolution of the discharge can be divided into a succession of stationary sub-phases characterized by the Hg partial pressure. Thus, we present here a self-consistent steady-state collisional-radiative model describing the middle sub-phase. This model includes volume recombination of Hg atomic and molecular ions, as well as several atom-atom inelastic scattering mechanisms. Calculations are carried out for both pure Hg and Hg-Ar discharges. Our results, which are in good agreement with experimental data from the literature, confirm that plasma thermalization occurs in the middle start-up phase (electron and gas temperatures become equal during this phase). Furthermore, this simulation shows the importance of different elementary processes, like atom-atom inelastic scattering, for the medium pressure plasma description. In fact, these mechanisms cannot be neglected until LTE conditions are reached.

A dynamic study of the warm-up phase of a high-pressure mercury lamp

A time-dependent two-dimensional computational fluid model has been adopted to investigate the dynamic behavior of the high-pressure mercury lamp during the last phase of the warm-up period. The model solves the combined momentum, continuity, energy, and electric field equations for the plasma and the energy equation for the wall. Two models have been compared. The first takes convection into account and is called “convection model.” The second, which neglects this term, is termed “convectionless model.” Good agreement between the predictions and experimental data from literature has been obtained. It is found that the convection affects the lamp performance by increasing the mercury losses behind the electrodes and the mercury-evaporation time.

Calculation of the impedance of an axisymetric DBD lamp for power supply design purposes

An electrical model of an axisymmetric multifilament mode dielectric barrier discharge (DBD) is proposed. The model relies on a simplified filament model extended by a function simulating multifilaments.