Electron emission mechanism during the nanosecond high-voltage pulsed discharge in pressurized air

Abstract

A comparison between the results of x-ray absorption spectroscopy of runaway electrons (RAEs) generated during nanosecond timescale high-voltage (HV) gas discharge and the simulated attenuation of the x-ray flux produced by the runaway electron spectrum calculated using particle-in-cell numerical modeling of such a type of discharge is presented. The particle-in-cell simulation considered the field and explosive emissions (EEs) of the electrons from the cathode. It is shown that the field emission is the dominant emission mechanism for the short-duration (<2.5 ns) high-voltage pulses, while for the long-duration (>5 ns) high-voltage pulses, the explosive emission is likely to play a significant role. V C 2012 American Institute of Physics.[http://dx.doi.org/10.1063/1.3689010] Today, nanosecond and sub-nanosecond high-voltage (HV) and high-current electrical discharges in pressurized gas are used in various applications, such as x-ray generation, laser pumping, and gap spark switches. 1 This type of discharge 2-4 is accompanied by the generation of runaway electrons (RAEs), which presumably pre-ionize the gas inside the cathode-anode (CA) gap and lead to the generation of a fast propagating ionization wave. 5,6 Because of the sub-nanosecond timescale of the discharge, investigation of the RAE generation inside the CA gap with the required time and space resolution is problematic. Today, the parameters of RAE behind the thin anode foil are studied using Faraday cups, collectors, foil spectrometry, and the time-of-flight method. [2][3][4][5][7][8][9] Since the 1960s (see, for instance, Refs. 10-14), the detection and analysis of the x-rays produced by the RAE's interaction with the anode have remained a reliable method of obtaining important data on RAE generation. Nevertheless, none of these diagnostic methods allows one to determine the RAE source(s) inside the CA gap or the phenomena responsible for RAE generation. It was supposed that RAE consist of electrons emitted from the cathode as well as of electrons emitted from the moving boundary of the plasma channel. 3,4,15 In addition, it was suggested, 4 and confirmed by the results of numerical simulations, 16 that the termination of the RAE generation occurs as a result of either the shielding of the electric field that causes the field emission (FE) by the space charge generated inside the CA gap 15,17 or the transformation of FE to explosive emission (EE). 16 It is important to note that the time delay in the EE beginning in gas-filled diodes is still debatable issue (see, for instance, Refs. 4, 18 and 19). The separation between space charges of electrons and ions generated in non-uniform electric field in the cathode vicinity changes drastically the electric field at the cathode surface and, for instance, in Refs. 17 and 18, it was shown that in gas-filled diode, the EE could start at 50 ps with respect to the beginning of the HV pulse.In this paper, it will be shown that attenuation dependencies of experimentally obtained and simulated x-rays fluxes generate...

Effect of photoionisation on initial discharge in air under nanosecond pulse voltage

“…Wilson first proposed in 1924 that electrons might enter a state of continuous acceleration under a strong electric field [15]. Since then, many studies have reported the theory of electron runaway under a strong electric field [12, 16, 17], that is, the energy the electrons lose between collisions is less than the energy electrons accumulate from the strong electric field. The condition can be simplified as follows: $$$\frac{\mathrm{d}\varepsilon }{\mathrm{d}x}=e\cdot E(x)-F(\varepsilon ) > 0$$$ where, e is the charge of the electron, E is the electric field where the electron is, x is the position of the electron, ε is the energy of the electron, and F ( ε ) is the effective friction force [18], that is, the energy loss function of the inelastic collision between electrons and gas molecules or atoms in the process of motion.…”

“…Wilson first proposed in 1924 that electrons might enter a state of continuous acceleration under a strong electric field [15]. Since then, many studies have reported the theory of electron runaway under a strong electric field [12,16,17], that is, the energy the electrons lose between collisions is less than the energy electrons accumulate from the strong electric field. The condition can be simplified as follows:…”

Effect of cathode and anode materials on the high-energy electron beam in the nanosecond-pulse breakdown in gas-filled diodes

“…It has a long history and it is widely used in the areas of high-energy electron beam generation, vacuum switching and illumination industry [2,3]. Recently, dynamical emission characteristics of gas-filled or vacuum diodes with cold cathode have been extensively investigated, especially in an inhomogeneous electric field provided by subnano-and nanosecond voltage pulses [4][5][6][7][8]. They are viable for the further studies aimed at generation of electron beams and x-ray radiation in air-filled diodes under high pressure [9][10][11].…”

Parameters of Nanosecond Overvoltage Discharge Plasma in a Narrow Air Gap between the Electrodes Containing Electrode Material Vapor

“…The results of studies of the high-current subnanosecond discharge in gases, generation mechanisms of runaway electrons and accompanying X-rays, as well as the parameters of this plasma and the main applications of such discharges, were summarized in works [21][22][23][24]. The conditions and mechanisms of formation of homogeneous plasma aggregates with a high density and a large volume in strongly nonuniform electric fields of the high-pressure multielectrode corona discharge were considered in [25].…”