Runaway Electrons Emitted by Electron Avalanches in Nanosecond Discharges in Air
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way Electrons Emitted by Electron Avalanches in Nanosecond Discharges in Air G. A. Mesyatsa,*, N. M. Zubareva, and I. V. Vaseninaa a
Lebedev Physical Institute, Russian Academy of Sciences, Moscow, 119991 Russia * e-mail: [email protected]
Received December 25, 2019; revised December 25, 2019; accepted December 25, 2019
Abstract—Nanosecond electric discharges in atmospheric air at high overvoltages are considered. Plasma channels and diffuse plasma glow simultaneously exist in these discharges. Channels are created by subnanosecond streamers formed in the cathode region. Diffuse glow is caused by runaway electrons emitted by electron avalanche heads. The necessary electric field enhancement is created due to the plasma configuration in the avalanche head and body. Keywords: electron avalanches, runaway electrons, nanosecond discharge, streamer, field emission, explosive emission DOI: 10.3103/S1068335620070052
In physics of electric discharge in gases, along with widely known Townsend (avalanche) and streamer discharges, there is a discharge simultaneously featuring properties of one and another. These discharges differ visually. The Townsend discharge exhibits diffuse glow in the cathode—anode gap; the streamer exhibits a bright glowing plasma channel. In pulsed discharges under high overvoltage (two- threefold and more), diffuse glow and plasma channels threading it simultaneously exist [1–3]. Hence, there are the processes characteristic of both streamer and Townsend discharges. Such are referred to as nanosecond diffuse—channel (NDC) discharges. In air at atmospheric pressure, this discharge takes place at E0/p > 102 V/(cm Torr). The duration of processes in these discharges is in the range of nano- and subnanoseconds. Physical processes in such discharges have not been studied in sufficient detail. Although the origin of channels can be understood from the standpoint of the streamer theory, the nature of diffuse glow has not yet been determined. The main question arising in this case is what is the nature of free electrons providing diffuse glow in the cathode—anode gap. This effect cannot be explained by photon emission by avalanche molecules as in ordinary discharges [3–5], because of a sharp decrease in the discharge time by one-two orders of magnitude (from 10–8 to 10–10). Furthermore, the critical number of electrons in the avalanche decreases by one—two orders of magnitude (from 108 to 106) when it transforms to the streamer. In our opinion, free electrons controlling the diffuseness in such a discharge are produced by runaway electrons (RE) emitted by the electron avalanche itself. The existence of the RE effect was discovered in the study of the nanosecond discharge in atmospheric air [6]. The critical field under which this takes place is determined by the formula [7, 2]
Ecr /p = 3.38 × 103 z /I ,
(1)
where I is the average energy of the electron inelastic loss and z is the number of electrons in the molecule. For example, for nitrogen z = 14, I = 80 eV, Ecr/p = 590 V/(cm Torr). At atmospheri
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