Fine structure of streamer-to-filament transition in high-pressure nanosecond surface dielectric barrier discharge

Abstract

The fine structure of a streamer–to–filament transition in a single–shot high–voltage nanosecond surface dielectric barrier discharge (nSDBD) in molecular nitrogen at pressure $P = 6$~bar was studied with the help of ICCD microimaging. An intermediate discharge structure, existing for only a few nanoseconds, was observed in the time interval between two discharge modes: streamer discharge, with a typical electron density of $n_e \sim 10^{15}$~cm$^{-3}$, and filamentary discharge, with $n_e \sim 10^{19}$~cm$^{-3}$. The structure was observed for both polarities of the high–voltage electrode. The structure can be briefly described as a stochastic appearance of thin channels propagating a bit faster than the main ionization front of merged surface streamers, transforming in a few nanoseconds in a bi–directional ionization wave. One wave, which we associate with a feather–like structure in optical emission, propagates further away from the high–voltage electrode, and another, a backward wave of emission, propagates back towards the edge of the high–voltage electrode. When the backward wave of emission almost reaches the high–voltage electrode, the filament appears. Plasma properties of the observed structure were studied to better understand the nature of a streamer–to–filament transition. Theoretical analysis suggests that the instability of a flat front of ionization wave (Laplacian instability) triggers the streamer–to–filament transition, and that a surface stem (a tiny region with enhanced electron density) should be in the origin of the bi-directional ionization wave.

Kinetic mechanism and sub-ns measurements of the thermal spark in air

“…The same group measured electron number densities n e = 10 18 -10 19 cm −3 in similar configurations and carefully documented the propagation of the surface fully ionized filaments [43]. More recently, they measured surface filament diameters of 10 µm [46]. These values are comparable to those of Parkevich et al [35][36][37][38]47] observed in pin-to-pin configuration and mentioned earlier.…”

Streamer-to-filament transition in pulsed nanosecond atmospheric pressure discharge: 2D numerical modeling

“…The streamer-to-filament transition is accompanied by a sharp increase in the electron density, up to about 10 19 cm -3 [17,18,21]. The baseline scheme mentioned above is enough for simulations at low ionization degree, with electron densities of about 1.0 × 10 15 cm -3 , and should be extended to higher ionization degree.…”

“…ˆS(ne⩾10 17 ) |q e |n e (t) v dr (t) dS (10) where I filament is the integral of the current density over the cathode surface where the electron density exceeds 1.0 × 10 17 cm −3 . The currents of the filament(s) and the glow are shown in figure 7.…”

“…The typical time of filamentation was about 1 ns or less in the pressure range of 2-10 bar. The electron density increased from about 1.0 × 10 15 cm −3 in the streamer phase to 1.0 × 10 19 cm −3 in the filament phase, and the diameter of the channel decreased from 200 to 20 µm [17]. The emission spectrum during the transition was first dominated by the molecular bands of N 2 and then continuous wavelength (cw) emission with the distinct appearance of broadened lines of atoms (N, O) and ionized atoms (N + ) [8,17].…”

Impact of surface charges on energy deposition in surface dielectric barrier discharge: a modeling investigation

“…Nevertheless, in the reality of both low-frequency AC and repetitively pulsed SDBD, there is the existence of residual surface charges with complex distribution and evolution dynamics, which will significantly influence the following breakdown [17,36,[38][39][40]. A series of self-consistent models coupling discharge and fluid flow have been established to study the AC SDBD [16,41,42], showing the strong impact of residual surface charges on the discharge evolution between the positive and negative phases [16,17].…”