Streak investigations of the dynamics of subnanosecond discharge developing in nitrogen at a pressure of 6 atm with the participation of runaway electrons

Abstract

The results of an investigation of a self-sustained subnanosecond discharge in nitrogen at a pressure of 6 atm are presented. A high voltage pulse with a front of approximately 770 ps at the level of 0.1-0.9 in amplitude was applied to the studied gas gap. In this case, the voltage rise rate in the discharge gap at the prebreakdown stage was 1.45 × 10 14 V s −1 . A breakdown occurs at the end of the front of the voltage pulse. The cathode geometry used provided a 3.3-fold enhancement of an electric field in the near-cathode spatial domain. It is shown that at the initial stage the discharge is of a volumetric form which turns into a spark form further. The discharge contraction starts from a cathode and an anode almost simultaneously. The propagation rate of ionization waves accompanying the spark channel development is 4.2 × 10 8 cm s −1 . The initial volumetric form of the discharge is provided by preliminary ionization of a gas medium by runaway electrons. The generation of runaway electrons takes place in the enhanced electric field area formed near a microprotrusion on the cathode surface, while the macro-geometry of the discharge gap does not ensure enhancing of the electric field up to the value which is sufficient to the implementation of the runaway electron generation criterion. Numerical simulation of a formation process of the electron avalanche initiated by an electron field-emitted from the top of the cathode microprotrusion was carried out taking into account the motion of each electron in the avalanche. To simulate electron motion through the discharge gap, the 3D Monte-Carlo technique was employed. The following runaway electron parameters were calculated: characteristic runaway electron trajectories; runaway electron energy gained during the motion through the discharge gap; and times required for runaway electrons to reach the anode.

The critical effect of electron acceleration under enhanced electric field near cathode on the formation of runaway electrons and diffuse discharge in atmosphere

“…If the micro-protrusion experiences an electrical explosion or it is destructed under the impact of the electric field, the emission of FE electrons is eliminated [20,32]. The explosive electron emission (EEE) process can be divided into two stages that, the initial stage involves the heating of metal in the solid state through FE current with the metal density changes slightly, and then the explosion itself occurs (second stage) [51].…”

“…Without external pre-ionization sources, diffuse discharge can also be generated and propagate as the ionization wave under extreme field enhancement near the sharpended electrode [15][16][17][18]. Runaway electrons (RAEs) are widely observed in a pin-to-plate geometry (with the pin as the cathode), and are thought to have a decisive effect on the breakdown process and the spatial structure of the discharge [19][20][21]. Furthermore, the photoionization produced by vacuum ultraviolet (VUV) radiation is responsible for the appearance of seed electrons ahead of the discharge front and is thought to play a critical role during the discharge inception [22,23].…”

Investigation of the switching characteristics of high-pressure subnanosecond gas dischargers with the purpose of a sharp increasing of the breakdown voltages and the switching speed

“…It is known that the field emission current density and the impact ionization coefficient depend exponentially on the electric field. In [28], at the initial stage of development, the discharge has a volumetric form initiated by RAEs, which later turns into a spark (see this article and [25,26,88]). Nitrogen is a molecular gas with a large number of possible electronically excited states.…”

“…In [25,[84][85][86][87][88], using the method of high-speed streak photographic recording, the glow accompanying a subnanosecond discharge in nitrogen in the pressure range from 4.5 atm to 40 atm was investigated. It was shown that a volume discharge is almost instantaneously ignited in the discharge gap, which then passes into the spark stage.…”

“…This preionization is inhomogeneous. This preionization takes place only in a narrow region of the gas volume bounded by the growing conducting column [25,88]. The maximum of ionizations due to RAEs will be near the boundary of the IW front, because the RAEs energy in this region is the lowest, since they continue to accelerate even in a relatively weak average electric field of the discharge gap.…”