The emission of energetic electrons from the complex streamer corona adjacent to leader stepping

Abstract

We here propose to model the production of energetic electrons serving as a source of X-rays and γ-rays, associated to electric discharges in preionized and perturbed air. During its stepping, the leader tip is accompanied by a corona consisting of multitudinous streamers perturbing the air in its vicinity and leaving residual charge behind. We explore the relative importance of air perturbations and preionization on the production of energetic runaway electrons by 2.5D cylindrical Monte Carlo particle simulations of streamers in ambient fields of 16 and 50 kV cm-1 at ground. We explore preionization levels between 1010 and 1013 m-3, channel widths between 0.5 and 1.5 times the original streamer widths and air perturbation levels between 0% and 50% of ambient air. We observe that streamers in preionized and perturbed air accelerate more efficiently than in non-ionized and uniform air with air perturbation dominating the streamer acceleration. We find that in unperturbed air and in fields above breakdown strength preionization levels of 1011 m-3 are sufficient to explain significant runaway electron rates. In perturbed air, the production rate of runaway electrons varies from 1010 to 1017 s-1 with maximum electron energies from some hundreds of eV up to some hundreds of keV in fields above and below the breakdown strength with only a marginal effect of the channel radius. Conclusively, the complexity of the streamer zone ahead of leader tips allows explaining the emission of energetic electrons and photons from streamer discharges in fields below and above the breakdown magnitudes.