Three-dimensional numerical modelling of gas discharges at atmospheric pressure incorporating photoionization phenomena

International audienceA three-dimensional (3D) numerical model for the characterization of gas discharges in air at atmospheric pressure incorporating photoionisation through the solution of the Helmholtz equation is presented. Initially, comparisons with a two-dimensional (2D) axi-symmetric model are performed in order to assess the validity of the model. Subsequently several discharge instabilities (plasma spots and low pressure inhomogeneities) are considered in order to study their effect on streamer branching and off-axis propagation. Depending on the magnitude and position of the plasma spot, deformations and off-axis propagation of the main discharge channel were obtained. No tendency for branching in small (of the order of 0.1cm) overvolted discharge gaps was observed

Simulation of the discharge propagation in a capillary tube in air at atmospheric pressure

International audienceThis paper presents simulations of an air plasma discharge at atmospheric pressure initiated by a needle anode set inside a dielectric capillary tube. We have studied the influence of the tube inner radius and its relative permittivity ε r on the discharge structure and dynamics. As a reference, we have used a relative permittivity ε r = 1 to study only the influence of the cylindrical constraint of the tube on the discharge. For a tube radius of 100 µm and ε r = 1, we have shown that the discharge fills the tube during its propagation and is rather homogeneous behind the discharge front. When the radius of the tube is in the range 300 to 600 µm, the discharge structure is tubular with peak values of electric field and electron density close to the dielectric surface. When the radius of the tube is larger than 700 µm, the tube has no influence on the discharge which propagates axially. For a tube radius of 100 µm, when ε r increases from 1 to 10, the discharge structure becomes tubular. We have noted that the velocity of propagation of the discharge in the tube increases when the front is more homogeneous and then, the discharge velocity increases with the decrease of the tube radius and ε r. Then, we have compared the relative influence of the value of tube radius and ε r on the discharge characteristics. Our simulations indicate that the geometrical constraint of the cylindrical tube has more influence than the value of ε r on the discharge structure and dynamics. Finally, we have studied the influence of photoemission processes on the discharge structure by varying the photoemission coefficient. As expected, we have shown that photoemission, as it increases the number of secondary electrons close to the dielectric surface, promotes the tubular structure of the discharge

Mesure directe par élargissement stark des raies de l'hydrogène de la densité de courant dans un gaz ionisé ou règne un champ macroscopique quasi uniforme

A theorical analysis of the hydrogen line broadening for an ionised gas in the presence of a macroscopic field Fc (for example, a space charge field within a pre-breakdown discharge) shows that the broadening depends not only on the charged particle density Ne but also on the macroscopic field Fc. A consequence developed here is that in a certain range the broadening is pratically dependant only on the product Ne F c. It is thus possible to evaluate directly the current density by measuring the line broadening induced in hydrogen introduced into a gas in minute concentration when the mobilities of particles in that gas are known. An example of an application is given (corona discharge).L'analyse théorique de l'élargissement de raies de l'hydrogène pour un gaz ionisé où règne un champ macroscopique Fc (champ de charge d'espace dans les décharges prédisruptives par exemple) montre que l'élargissement ne dépend plus seulement de la densité électronique Ne mais également de la valeur du champ macroscopique Fc. Une conséquence est développée ici : dans un certain domaine, l'élargissement est pratiquement fonction du produit N e Fc. Il devient alors possible d'évaluer directement la densité de courant en introduisant une petite quantité d'hydrogène dans un gaz où la mobilité des particules est connue. Un exemple d'application est donné (décharge couronne)

A model of the streamer-induced spark formation based on neutral dynamics

The breakdown of a positive point to plane gap in air near atmospheric pressure begins with the formation of a low-conductivity filament by the space-charge-controlled streamer process. Within the filament, the rate of electron attachment exceeds that of ionization, and the external current decreases. However, a sudden rise of current leading to spark breakdown can be observed. The following explanation is proposed. Owing to the current flow the temperature increases within the discharge. A radial flow of neutral species will thus appear, which decreases the neutral density near the discharge axis. In turn E/N increases until the ionization rate becomes greater than that of attachment, leading to the final current growth. Journal of Applied Physics is copyrighted by The American Institute of Physics

2D positive streamer modelling in NTP air under extreme pulse fronts. What about runaway electrons?

International audienc

DETERMINATION OF MACROSCOPIC QUANTITIES FOR A TOWNSEND DISCHARGE IN HELIUM BY BOTH BOLTZMANN EQUATION AND MONTE CARLO METHODS

No abstract availabl

ELECTRON DISTRIBUTION FUNCTION IN A VERY NON UNIFORM ELECTRIC FIELD

No abstract availabl