Study of the electric field in a diffuse nanosecond positive ionization wave generated in a pin-to-plane geometry in atmospheric pressure air

International audienceThe dynamics of a nanosecond positive ionization front generated in a pin-to-plane geometry inatmospheric pressure air is simulated using a 2D axisymmetric drift-diffusion fluid model. For a16 mm gap and a sharp pin electrode, the plateau of the applied voltage is varied between 40and 60 kV and the rise time is varied between 0.5 and 1.5 ns or a DC voltage is applied. Thedischarge ignition time and the voltage at ignition are shown to depend mostly on the voltagerise time. The connection time, i.e. the time for the ionization wave to ignite, propagate andconnect to the plane is shown to strongly depend on both the values of the voltage plateau andrise time. For all cases, the discharge has a conical shape with a maximal radius of about 8 mmas it connects to the grounded plane. The average propagation velocity of the ionization front isfound to vary in the range 3.1 to 8.5 mm ns−1. These values are in rather good agreement withexperiments. Temporal evolutions of the electric field are recorded on the symmetry axis atdifferent positions in the gap. At each location, an increase and decrease of the electric field isobserved 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 risetime of 0.5 ns, a temporal sampling of 100 ps is shown to be sufficient to capture the dynamicsof the electric field during the ionization front propagation when it passes close to the middle ofthe gap. Conversely, a temporal sampling of 10 ps is required when the ionization wave is closeto both electrodes, or during the fast redistribution of the electric field after the connection of theionization front at the cathode

Morphology of positive ionization waves in atmospheric pressure air: influence of electrode set-up geometry

International audienceA numerical parametric study on positive diffuse discharges in pointto-plane geometry in air at atmospheric pressure is presented. Different discharge characteristics are studied: ignition time, connection time to the grounded cathode plane, shape of the discharge and its maximum radius at the connection time, evolution of the maximum electric field in the discharge front and velocity of the ionization front during its propagation. First, a case at a DC voltage of 50 kV applied on a rod anode ended by a semi-sphere with a radius of 100 µm set at 1.6 cm from a grounded cathode plane is considered. The influence of the rod radius, the position of a disc holder, the shape of the anode electrode and the radial extension of the computational domain are studied. The radius of curvature of the anode tip (varied between 100 and 1000 µm) and the shape of the anode electrode (rod or hyperbola) are shown to have a negligible influence on discharge characteristics. Conversely, the presence of a disc holder or a small radial computational domain lead to a decrease of the maximum discharge radius at the connection time and a change in the discharge shape from a conical to an ellipsoidal shape. These changes on the discharge morphology have only a limited impact on the propagation velocity of the discharge front and maximum electric field on the discharge axis. Then, a point-to-plane geometry with a rod electrode of 50 µm radius, in a 1.6 cm gap, with a 100 kV voltage applied with a rise time of 1 ns is studied. The influence of a disc holder on the discharge characteristics is the same as for lower DC voltages. Finally, the time evolution of the absolute value of the electric field at different test points on the discharge axis is studied. Close to the anode tip, rapidly after the peak of electric field due to the passage of the ionization front, the electric field in the discharge channel is shown to increase to values higher than the breakdown field

Numerical study on the time evolutions of the electric field in helium plasma jets with positive and negative polarities

International audienceThis paper presents 2D simulations of atmospheric pressure discharges in helium with N2 and O2 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 23 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% He1% N2 and 99% He1% O2 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