Low-energy electron production by relativistic runaway electron avalanches in air

This paper investigates the production of low-energy (few eV) electrons by relativistic runaway electron avalanches. This work is motivated by a growing body of literature that claims that runaway electron avalanches produce an anomalous growth of low-energy electrons and hence an anomalously large electrical conductivity, a factor of 50 larger than expected from standard calculations. Such large enhancements would have a substantial impact on properties of runaway electron avalanches and their observable effects. Indeed, these purportedly large conductivities have been used to argue that runaway electron avalanches result in a novel form of electrical breakdown called runaway breakdown. In this paper, we present simple analytical calculations, detailed Monte Carlo simulations, and a review of the experimental literature to show that no such anomalous growth of low-energy electron populations exists. Consequently, estimates of the conductivity generated by a runaway electron avalanche have been greatly exaggerated in many previous papers, drawing into question several of the claims about runaway breakdown.