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

Abstract

A numerical parametric study on positive diffuse discharges in point-to-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.

Positive and negative streamers in air and nitrogen in a sharply inhomogeneous electric field under conditions of runaway electron generation

“…The same streamers were observed in Refs. [12,18,19,22,25,26,38,44,46,[49][50][51][62][63][64][65][66]. It is seen that the streamer diameter can reach the order of the length of the interelectrode distance d. Figure 2c shows that the streamer front moves in the direction of the grounded electrode at different velocities as in our previous studies [51,67].…”

“…The same streamers were observed in Refs. [12, 18, 19, 22, 25, 26, 38, 44, 46, 49–51, 62–66]. It is seen that the streamer diameter can reach the order of the length of the interelectrode distance d .…”

“…They are formed at high gas pressures, in particular at atmospheric pressure. Due to the prospects of using atmospheric pressure discharges in a wide variety of fields of science and technology, significant success has been achieved both in the modelling of streamer discharges using particle-in-cell methods [11][12][13], fluid models [14][15][16][17] including nanosecond atmospheric discharges [18][19][20], experimental studies [21][22][23][24], and their combinations [25][26][27][28][29]. Special attention has been paid to nanosecond discharges in gases of atmospheric pressure in an inhomogeneous electric field, as a fairly simple method for producing a low-temperature non-equilibrium plasma.…”

Formation of wide negative streamers in air and helium: the role of fast electrons

“…The diffuse discharges can be both positive [32][33][34][35][36][37][38] and negative [39][40][41]. It was assumed that the characteristic radiation could produce effective pre-ionization of the gas for both voltage polarities.…”

“…With a positive polarity of a sharp electrode, the shape of the streamer does not differ significantly from the shape of the streamer at a negative polarity [36,37]. It is assumed that, analogous to the positive diffuse discharges, the positive wide streamers are initiated by the initial electrons ahead of the IWs.…”