M Yousfi

Comparisons between different methods of solution of the Boltzmann equation adapted to the calculation of swarm parameters in a weakly ionised medium

Two numerical solutions of the Boltzmann equation are developed and applied to the calculations of swarm time-of-flight parameters in molecular model gases. It is shown that the two methods not only give near similar results but are in good agreement with calculations carried out by some other authors for the same situations. Errors already pointed out in some other works are confirmed and the need of using high-order methods of solution of the Boltzmann equation is stressed.

Effect of order fluid models on flue gas streamer dynamics

The present paper shows that in the case of a micro-discharge modelling using the hydrodynamics assumption, the second order fluid model involving the complete electron momentum conservation equation must be used in order to better quantify the radical formation in a micro-discharge applied to air pollution control. The present results show large differences in the micro-discharge parameters (such as velocity and electron density) between the three tested hydrodynamics models: the classical first order model using the local electric field approximation and two second order models using the local energy approximation with or without the drift–diffusion approximation. The tests have been carried out in the case of a wire-to-plane corona reactor filled with a typical flue gas (76% N2, 12% CO2, 6% O2, 6% H2O) at atmospheric pressure and ambient temperature. The simulation of the micro-discharge dynamics is performed using a 1.5D numerical streamer model coupled with a simple chemical kinetics model involving 31 species (charged and neutral particles in their fundamental or metastable state) reacting following 29 selected chemical reactions.

The application of a modified form of theSN method to the calculation of swarm parameters of electrons in a weakly ionised equilibrium medium

A numerical method is developed for the solution of the Boltzmann equation in order to calculate the swarm parameters of a weakly ionised gas. A brief description is given of the three main types of swarm experiment in which the measurements of these parameters are usually carried out. Then it is shown how these experiments can be simulated by calculation in order to explain the discrepancies observed between the parameters measured under the same conditions but in different experiments. A description is given of the present method of solution which uses both Lesaints SN method and an iterative process based on the so-called Eddington factor. The efficiency and the accuracy of this treatment is shown in two model cases which have already been investigated by several authors. It is observed that the number of iterations is drastically reduced and that the results obtained are of high accuracy.

Electrical and spectroscopic analysis of mono- and multi-tip pulsed corona discharges in air at atmospheric pressure

This work is devoted to the analysis of experimental results obtained in dry air at atmospheric pressure in a positive point-to-plane corona discharge under a pulsed applied voltage in the cases of anodic mono- and multi-tips. In the mono-tip case, the peak corona current is analysed as a function of several experimental parameters such as magnitude, frequency and duration of pulsed voltage and gap distance. The variation of the corona discharge current is correlated with the ozone production. Then in the multi-tip case, the electrical behaviour is analysed as a function of the distance between two contiguous tips and the tip number in order to highlight the region of creation active species for the lowest dissipated power. Intensified charge-coupled device pictures and electric field calculations as a function of inter-tip distance are performed to analyse the mutual effect between two contiguous tips. The optical emission spectra are measured in the UV–visible–NIR wavelength range between 200 nm and 800 nm, in order to identify the main excited species formed in an air corona discharge such as the usual first and second positive systems with first negative systems of molecular nitrogen. The identification of atomic species (O triplet and N) and the quenching of NOγ emission bands are also emphasized.

Basic data for atmospheric pressure non-thermal plasma investigations in environmental and biomedical applications

The aim of this paper is to discuss some aspects of the optimization of the active species generated by corona or DBD discharges at atmospheric pressure which are very useful in the field of plasma environmental and biomedical applications. For such an optimization, this paper targets, in particular, the use of discharge modeling tools and the problem of accuracy of the required basic data. First of all, an overview on the different experimental diagnostics used for the characterization of these non-thermal plasmas is given followed by a short description of the different models (streamer dynamics, gas dynamics and chemical kinetics coupled with models of basic data calculation) required for complementing such experimental investigations. Then, emphasis is placed on the basic data of charged particles (electrons and ions) needed for streamer dynamics modeling and particularly on the necessity to use accurate and validated basic data in order to have a quantitative (not only qualitative) description of the phenomena and processes occurring in such discharges. An overview is given on the calculations and the fitting methods of collision cross sections and swarm coefficients of the data of charged particles and their validation using, in particular, pulsed Townsend measurements for experimental comparisons. Swarm coefficients are calculated from a multi-term solution of the Boltzmann equation or from Monte Carlo simulation. Some illustrative results are given in the case of the simulations of a dc positive point-to-plane corona discharge in air at atmospheric pressure. The effect of consideration of some basic data, particularly those of polyatomic ions, is shown on the discharge development and the radical production. (Some figures in this article are in colour only in the electronic version)

Drift and reactions of positive tetratomic ions in dry, atmospheric air: Their effects on the dynamics of primary and secondary streamers

The ion swarm data, namely, the reduced mobility, diffusion, and reaction rates of the positive tetratomic ions O4+ and N2O2+ in N2 and O2 have been determined from a Monte Carlo simulation using calculated and fitted elastic and inelastic cross sections. The elastic momentum transfer cross sections have been determined from a semiclassical Jeffreys-Wentzell-Kramers-Brilouin (JWKB) approximation based on a rigid core potential model well adapted for polyatomic ions. The inelastic cross sections have been approximated from considerations based on the N4+/O2 and N4+/N2 systems. The validated cross section sets in pure N2 and O2 have been used to determine the O4+ and N2O2+ swarm data in dry air over a large E/N range up to 1000 Td. However, due to the lack of experimental ion transport coefficients necessary for a more rigorous cross section validation, the present data, validated only at low E/N, should be regarded as a first approximation, susceptible to improvements as soon as measurements of ion transpo...

Full multi grid method for electric field computation in point-to-plane streamer discharge in air at atmospheric pressure

Streamers dynamics are characterized by the fast propagation of ionized shock waves at the nanosecond scale under very sharp space charge variations. The streamer dynamics modelling needs the solution of charged particle transport equations coupled to the elliptic Poissons equation. The latter has to be solved at each time step of the streamers evolution in order to follow the propagation of the resulting space charge electric field. In the present paper, a full multi grid (FMG) and a multi grid (MG) methods have been adapted to solve Poissons equation for streamer discharge simulations between asymmetric electrodes. The validity of the FMG method for the computation of the potential field is first shown by performing direct comparisons with analytic solution of the Laplacian potential in the case of a point-to-plane geometry. The efficiency of the method is also compared with the classical successive over relaxation method (SOR) and MUltifrontal massively parallel solver (MUMPS). MG method is then applied in the case of the simulation of positive streamer propagation and its efficiency is evaluated from comparisons to SOR and MUMPS methods in the chosen point-to-plane configuration. Very good agreements are obtained between the three methods for all electro-hydrodynamics characteristics of the streamer during its propagation in the inter-electrode gap. However in the case of MG method, the computational time to solve the Poissons equation is at least 2 times faster in our simulation conditions.

Transport properties of {\rm SF}_{6}^{-} in SF6?Ne, SF6?N2 and SF6?O2 mixtures

The mobility of has been calculated for the gas mixtures SF6?Ne, SF6?N2 and SF6?O2 using an optimized Monte Carlo code for the ion transport simulation in a drift tube. The elastic momentum transfer collision cross sections needed for the calculation were determined from a semi-classical JWKB approximation, while the inelastic ones (detachment, dissociative charge transfer and conversion to and F?) were taken from the literature for the case of the collision system. The resulting sets of collision cross sections were validated by comparing the calculated mobilities with those measured in the above mixtures with a time-resolved pulsed Townsend technique. The longitudinal and transverse density-normalized diffusion coefficients were calculated for these mixtures for the case where the share of SF6 in the mixture was 50%. Finally, the validity of Blancs law was discussed at low and high electric fields, whereby we show that it fails at high fields, where inelastic processes are dominant.

Power dissipation analysis in N2O RF discharges using Monte Carlo modelling

In this paper, a microscopic approach for the calculation of partial and total power dissipation from energy losses by collisions is considered and applied in the case of N2O low pressure RF discharges. This approach is based on a Monte Carlo technique in a particle model permitting sampling of the energy deposited by different inelastic electron?N2O collisions. The calculated power densities presented in this paper are in good agreement with the experimental results and those obtained by the classical macroscopic formula based on spatio-temporal integration of the product of current density and electrical field. This microscopic approach presents, however, a major advantage in comparison with the classical method (which only offers the possibility to calculate the global power dissipation) by making possible the calculation of all the power density terms, thereby permitting one to examine the relative contribution of each collision process in the power dissipation. Its application to N2O electronegative discharges, at 503?K gas temperature, several RF voltages and two different gas pressures shows how the power is dissipated through electron?gas processes. The power density variation is found to be proportional to the electron density variation brought about by the changes in attachment (i.e. e + N2O ? N2 + O?), detachment (i.e. NO? + N2O ? NO + N2O + e) and ionization (i.e. e + N2 O ? N2O+ + 2e) processes. The role of each of these processes is fully studied with our particle model in order to explain the dissipated power variation.