François Pechereau

Numerical and experimental study of the dynamics of a s helium plasma gun discharge with various amounts of N 2 admixture

This paper presents a combined 2D numerical and experimental study of the influence of N 2 admixture on the dynamics of a He–N 2 discharge in the 10 cm long dielectric tube of a plasma gun setup. First, the comparison between experiments and simulations is carried out on the ionization front propagation velocity in the tube. The importance of taking into account a detailed kinetic scheme for the He–N 2 mixture in the simulations to obtain a good agreement with the experiments is put forward. For the μs driven plasma gun, the two-and three-body Penning reactions occurring in the plasma column behind the ionization front, are shown to play a key role on the discharge dynamics. In the experiments and simulations, the significant influence of the amplitude of the applied voltage on the ionization front propagation velocity is observed. As the amount of N 2 varies, simulation results show that the ionization front velocity, depends on a complex coupling between the kinetics of the discharge, the photoionization and the 2D structure of the discharge in the tube. Finally, the time evolution of axial and radial components of the electric field measured by an electro-optic probe set outside the tube are compared with simulation results. A good agreement is obtained on both components of the electric field. In the tube, simulations show that the magnitude of the axial electric field on the discharge axis depends weakly on the amount of N 2 conversely to the magnitude of the off-axis peak electric field. Both, simulations and first measurements in the tube or within the plasma plume show peak electric fields of the order of 45 kV·cm −1 .

Simulation of the reignition of a discharge behind a dielectric layer in air at atmospheric pressure

This paper presents simulations of an air plasma discharge at atmospheric pressure in a point-to-plane configuration with a dielectric layer in the path of the discharge. First, the dielectric layer is placed on the cathode plane and we study the influence of the permittivity and thickness of the dielectric on the positive streamer discharge dynamics and the dielectric surface charging. We show that the velocity of the surface discharge on the dielectric surface depends on the capacitance of the dielectric layer and decreases as this capacitance increases. Conversely, the amount of positive surface charge deposited by the positive surface discharge on the dielectric surface is not directly related to the value of the capacitance of the dielectric layer. However, the amount of surface charge deposited increases as the capacitance of the dielectric layer increases. Second, the dielectric layer is placed in the air gap as an obstacle for the propagation of the first streamer discharge ignited at the point electrode. In this case, after the impact on the dielectric, the first discharge spreads along the upper dielectric surface and we show that, depending on the location of the dielectric layer, its permittivity, its thickness and its opacity to radiation, a second discharge may reignite or not below the dielectric layer. During the discharge dynamics, positive charges are deposited on the upper surface of the dielectric and negative charges are deposited on its bottom surface. For all conditions studied in this work, we show that surface charge deposition on both faces of the dielectric layer has a small influence on the discharge reignition below the dielectric layer. Finally, with two closely spaced dielectric layers in the path of the discharge, a series of spreading/reignition for each dielectric layer is observed.

Influence of the polarity of the applied voltage on the reignition of a discharge below a dielectric layer in air at atmospheric pressure

The dynamics of an atmospheric pressure air discharge in a point-to-plane geometry with a dielectric layer obstacle on the discharge path is investigated numerically for different applied voltages. Whatever the polarity of the voltage applied, first, a streamer discharge of the same polarity ignites at the point and propagates towards the dielectric layer. After the impact on the dielectric surface, the streamer discharge spreads along the upper dielectric surface and charges it positively or negatively depending on its polarity. On the bottom surface of the dielectric layer, charges with an opposite polarity are deposited. Surface charges on both faces of the dielectric layer are shown to have a significant influence on the discharge reignition for a negative applied voltage, but not for a positive one. Furthermore, it is shown that the dynamics of the discharge reignition below the dielectric layer depends on the polarity of the applied voltage at the point electrode. For a positive applied voltage, the reignited discharge is a positive ionization wave propagating towards the grounded plane. For a negative applied voltage, a double headed discharge is observed with positive and negative fronts propagating in opposite directions. Finally, the minimal value of the ionization integral to have a discharge reignition below the dielectric obstacle is found to be less for a negative applied voltage than for a positive one.

Influence of surface emission processes on a fast-pulsed dielectric barrier discharge in air at atmospheric pressure

This paper presents simulations of an atmospheric pressure air discharge in a point-to-plane geometry with a dielectric layer parallel to the cathode plane. Experimentally, a discharge reignition in the air gap below the dielectrics has been observed. With a 2D fluid model, it is shown that due to the fast rise of the high voltage applied and the sharp point used, a first positive spherical discharge forms around the point. Then this discharge propagates axially and impacts the dielectrics. As the first discharge starts spreading on the upper dielectric surface, in the second air gap with a low preionization density of 10^4~\textc\textm^-3 , the 2D fluid model predicts a rapid reignition of a positive discharge. As in experiments, the discharge reignition is much slower, a discussion on physical processes to be considered in the model to increase the reignition delay is presented. The limit case with no initial seed charges in the second air gap has been studied. First, we have calculated the time to release an electron from the cathode surface by thermionic and field emission processes for a work function φ ∈ ≤ft[3,4\right] eV and an amplification factor β ∈ ≤ft[100,220\right] . Then a 3D Monte Carlo model has been used to follow the dynamics of formation of an avalanche starting from a single electron emitted at the cathode. Due to the high electric field in the second air gap, we have shown that in a few nanoseconds, a Gaussian cloud of seed charges is formed at a small distance from the cathode plane. This Gaussian cloud has been used as the initial condition of the 2D fluid model in the second air gap. In this case, the propagation of a double headed discharge in the second air gap has been observed and the reignition delay is in rather good agreement with experiments.