Time evolution of nanosecond runaway discharges in air and helium at atmospheric pressure

Abstract

Time-and space-resolved fast framing photography was employed to study the discharge initiated by runaway electrons in air and He gas at atmospheric pressure. Whereas in the both cases, the discharge occurs in a nanosecond time scale and its front propagates with a similar velocity along the cathode-anode gap, the later stages of the discharge differ significantly. In air, the main discharge channels develop and remain in the locations with the strongest field enhancement. In He gas, the first, diode "gap bridging" stage, is similar to that obtained in air; however, the development of the discharge that follows is dictated by an explosive electron emission from micro-protrusions on the edge of the cathode. These results allow us to draw conclusions regarding the different conductivity of the plasma produced in He and air discharges. V C 2012 American Institute of Physics. [http://dx.

Spectroscopic study of plasma evolution in runaway nanosecond atmospheric-pressure He discharges

“…This duration is consistent with the lifetime of the hot spots, whose depletion terminates the radial electric field and, therefore, terminates the radial wave of electrons. However, on the same time scale other explosive centers form somewhere else on the cathode edge, followed by a similar development of the discharge channel [16].…”

“…Let us now consider a possible reason for the appearance of a short outburst of energetic electrons with a duration of 1-2 ns. The results of the recent research [16] showed existence of several resistive plasma channels in the CA gap with a lifetime of ∼ 2 ns. The evanescence of a channel is followed by generation of a new channel from a different emission center on the cathode shortly before or after the first channel ceases to exist.…”

“…Before we start discussing these results, let us recall the main findings from Ref. [16]. In that study the same experimental setup for gas discharge was used.…”

“…Major channels are the ones that advance throughout the C-A gap reaching the anode, while minor channels deviate toward the major channel and overlap with it at some distance from the cathode. Later in a discharge, the major discharge channels mostly appear at the middle of the cathode, apparently due to formation of a significant emission centers at the cathode surface at that location [16]. Therefore, we analyzed the spectral lines emitted from major and minor channel regions separately.…”

“…In the shots in which it could be fitted, densities calculated according to the Stark broadening yield values of ∼ 10 18 -10 19 cm −3 [19]. The latter can be related to the appearance of bright and "hot" spots on the edge of the cathode [16]. Such a hot spot is apparently a very dense and hot plasma region emitting very broad spectral lines, and this spot remains for long times.…”