Modeling of a New Electron Acceleration Mechanism Ahead of Streamers
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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 particle in cell 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 subbreakdown 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 similar to 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 vertical bar E-0 vertical bar = (35, 40, 45) kV/cm, respectively. We observe the formation of a secondary short propagating negative streamer with a strong peak electric field >250 up to 378 kV/cm over a time duration of similar to 0.16 ns at the moment of the collision. The mechanism produces up to 10(6) runaway electrons with an upper energy limit of 24 keV.