Modeling of a New Electron Acceleration Mechanism Ahead of Streamers

Abstract : Head‐on collisions between negative and positive streamers have been proposed as a mechanism behind X‐ray emissions by laboratory spark discharges. Recent simulations using plasma fluid and PIC models of a single head‐on collision of two streamers of opposite polarities in ground pressure air predicted an insignificant number of thermal runaway electrons >1 keV and hence weak undetectable X‐ray emissions. Because the current available models of a single streamer collision failed to explain the observations, we first use a Monte Carlo model coupled with multiple static dielectric ellipsoids immersed in a sub‐breakdown ambient electric field as a description of multiple streamer environment and we investigate the ability of multiple streamer‐streamer head‐on collisions to accelerate runaway electrons >1 keV up to energies ~200‐300 keV instead of just one single head‐on collision. The results of simulations show that the streamer head‐on collision mechanism fails to accelerate electrons, instead they decelerate in the positive streamer channel. In a second part, we use a streamer plasma fluid model to simulate a new streamer‐electron acceleration mechanism based on a collision of a large negative streamer with a small neutral plasma patch in different Laplacian electric fields|E0|=(35, 40, 45) kV/cm, respectively. We observe the formation of a secondary short propagating negative streamer with a strong peak electric field >250 kV/cm up to 378 kV/cm over a time duration of ~0.16 ns at the moment of the collision. The mechanism produces up to 106 runaway electrons with an upper energy limit of 24 keV.
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Kevin M. A. Ihaddadene, Joseph R. Dwyer, Ningyu Liu, Sébastien Celestin, Feng Shi. Modeling of a New Electron Acceleration Mechanism Ahead of Streamers. Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, In press, ⟨10.1029/2018JA026084⟩. ⟨insu-02263528⟩

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