Study of relativistic electron avalanche enhancement in the atmosphere at low overvoltages due to avalanche bremsstrahlung

An effective technique for numerical simulation, using the Monte Carlo technique, of the kinetics of relativistic runaway electron (RE) avalanche and its bremsstrahlung in a dense gas in the external electric field, based on successive generations of REs multiplied due to electron impact, and bremsstrahlung, which is the source of the next RE generation, has been developed. This technique is especially effective at low overvoltages (delta) relative to a relativistic minimum of the drag force (218 keV m(-1) atm(-1)). It has been indicated that the characteristic time of avalanche enhancement is slightly smaller than the time obtained during the previous studies. The numerical simulation of an avalanche in the air has been performed for delta = 1.5. It has been indicated that the processes of secondary RE generation by bremsstrahlung substantially contribute to the rate of avalanche development. In spite of participation of photons, avalanche is concentrated in a relatively small volume, with similar to 1% of electrons outside this volume, which indicates that the feedback mechanism in the development of ascending atmospheric discharges is actual.