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

Abstract

Runaway electrons (RAEs) are believed to affect the dynamics of ultra-fast gas breakdown significantly. In this work, considering the field enhancement effect near the micro-protrusion on the cathode surface, the formation of RAEs and diffuse discharge in atmospheric pressure air is investigated by two-dimensional particle-in-cell/Monte Carlo collision simulation. It is found that the beam amplitude of RAEs is dictated by the field enhancement factor and the initial energy of electrons obtained near the micro-protrusion is decisive for their converting to RAEs, which precede the low energy electrons and guide the discharge propagation by improving pre-ionization. As a result, the discharge transfers from the filamentary mode without RAEs to the diffuse mode under the high pre-ionization degree due to RAEs and a wide streamer with a diameter comparable with the gap distance is formed, which transfers from spherical to conical shape. The results of this study illustrate the fundamental process of RAE formation and how RAEs influence streamer dynamics during ultra-fast gas breakdown process.

Optimal Conditions for Generation of Runaway Electrons in High Pressure Gases

“…Among the main directions of RAEs and X-rays investigations, considering only a small part of the well-known publications over the last few years, are works [71][72][73] aimed at obtaining thermonuclear fusion in which the runaway electrons damaged walls of vacuum chambers, thereby limiting plasma heating; works [74][75][76][77][78][79] considering atmospheric discharges, including high-altitude ones in which X-ray and gamma radiation were registered and reasons for their occurrence were discussed; works [80][81][82][83][84][85] in which megavolt voltage discharges emitting X-rays and modeling lightning evolution in meter gaps were registered; works [86][87][88][89] considering discharges in a uniform electric field at relatively low voltages; and, of course, discharges in a non-uniform electric field registered in centimeter gaps at high voltages in air and other gases for which the runaway electrons were experimentally registered. To confirm the relevance of these studies on electron beam generation, some other works not mentioned above that have been published in 2022-2023 should be mentioned here, including works [90][91][92][93][94][95][96][97][98] in which the study of the RAEB generation in centimeter gaps was continued. In works [99,100], attention was focused on the study of the RAEs in TOKAMAK-type installations and devices for their diagnostics in these conditions.…”

Optimal Conditions for the Generation of Runaway Electrons in High-Pressure Gases

“…The works [74][75][76][77][78][79] considered atmospheric discharges, including high-altitude ones in which X-ray and gamma radiation were registered and reasons for their occurrence were discussed; works [80][81][82][83][84][85] in which megavolt voltage discharges emitting X-rays and modeling lightning evolution in meter gaps were registered; works [86][87][88][89] considered discharges in a uniform electric field at relatively low voltages; and, of course, discharges in a non-uniform electric field registered in centimeter gaps at high voltages in air and other gases for which the runaway electrons were experimentally registered. To confirm the relevance of these studies on electron beam generation, some other works not mentioned above that have been published in 2022-2023 should be mentioned here, including works [90][91][92][93][94][95][96][97][98] in which the study of RAEB generation in centimeter gaps was continued. In works [99,100], attention was focused on the study of RAEs in TOKAMAK-type installations and devices for their diagnostics in these conditions.…”

Breakdown modes in nanosecond pulsed micro-discharges at atmospheric pressure

“…Furthermore, in the second breakdown following the removal of the pulse, as depicted in figures 6(b3) and (c3), a similar trend of reduced breakdown time is evident. The underlying cause for the different breakdown timing may be attributed to the pre-ionization effect of runaway electrons, specifically the rapid breakdown of the gas under nanosecond pulse conditions induced by runaway electrons [84,85].…”