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 set-up. 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 .

This paper presents 2D simulations of atmospheric pressure discharges in helium with N 2 and O 2 admixtures, propagating in a dielectric tube between a point electrode and a grounded metallic target. For both positive and negative polarities, the propagation of the first ionization front is shown to correspond to a peak of the absolute value of the axial electric field inside the tube, but also outside the tube. After the impact on the metallic target, a rebound front is shown to propagate from the target to the point electrode. This rebound front is 2-3 times faster than the first ionization front. Close to the high voltage point, this rebound front corresponds to a second peak of the absolute value of the axial electric field. Close to the target, as the first ionization and rebound fronts are close in time, only one peak is observed. The dynamics of the absolute value of the radial component of electric field outside the tube is shown to present an increase during the first ionization front propagation and a fast decrease corresponding to the propagation of the rebound front. These time evolutions of the electric field components are in agreement with experiments. Finally, we have shown that the density of metastable He * in 99% He-1% N 2 and 99% He-1% O 2 atmospheric pressure discharges are very close. Close to the grounded target, the peak density of reactive species is significantly increased due to the synergy between the first ionization and rebound fronts, as observed in experiments. Similar results are obtained for both voltage polarities, but the peak density of metastable He * close to the target is shown to be two times less in negative polarity than in positive polarity.

The dynamics of a nanosecond positive ionization front generated in a pinto-plane geometry in atmospheric pressure air is simulated using a 2D axisymmetric drift-diffusion fluid model. For a 16 mm gap and a sharp pin electrode, the plateau of the applied voltage is varied between 40 and 60 kV and the rise time is varied between 0.5 and 1.5 ns or a DC voltage is applied. The discharge ignition time and the voltage at ignition are shown to depend mostly on the voltage rise time. The connection time, i.e. the time for the ionization wave to ignite, propagate and connect to the plane is shown to strongly depend on both the values of the voltage plateau and rise time. For all cases, the discharge has a conical shape with a maximal radius of about 8 mm as it connects to the grounded plane. The average propagation velocity of the ionization front is found to vary in the range 3.1 to 8.5 mm ns −1 . These values are in rather good agreement with experiments. Temporal evolutions of the electric field are recorded on the symmetry axis at different positions in the gap. At each location, an increase and decrease of the electric field is observed as the ionization front, propagating from the pin to the plane, passes the studied point, in accordance with experimental observations. Finally, for a voltage plateau of 55 kV and a rise time of 0.5 ns, a temporal sampling of 100 ps is shown to be sufficient to capture the dynamics of the electric field during the ionization front propagation when it passes close to the middle of the gap. Conversely, a temporal sampling of 10 ps is required when the ionization wave is close to both electrodes, or during the fast redistribution of the electric field after the connection of the ionization front at the cathode.

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.

In this paper we propose a representative simulation test-case of E × B discharges accounting for plasma wall interactions with the presence of both the Electron Cyclotron Drift Instability (ECDI) and the Modified-Two-Stream-Instability (MTSI). Seven independently developed Particle-In-Cell (PIC) codes have simulated this benchmark case, with the same specified conditions. The characteristics of the different codes and computing times are given. Results show that both instabilities were captured in a similar fashion and good agreement between the different PIC codes is reported as main plasma parameters were closely related within a 5% interval. The number of macroparticles per cell was also varied and statistical convergence was reached. Detailed outputs are given in the supplementary data, to be used by other similar groups in the perspective of code verification.

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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 cm 4 3 Keywords: fast-pulsed discharges, dielectric barrier discharges, streamer discharge in air at atmopsheric pressure, 2D fluid model, 3D Monte Carlo model, surface emission processes