Geant4 Monoenergetic Electron Beam Simulations of Runaway Electrons in Streamer Discharges

Runaway electrons are a central pillar in the emerging field of high-energy atmospheric physics. An important source of runaway electrons is the so-called thermal runaway electron acceleration mechanism, where electrons are accelerated from the thermal population to runaway energies by a strong electrostatic field, in excess of 250 kV/cm Such strong electric fields may exist during a short period of time at the tips of streamer channels. This mechanism is the most accepted source of runaway electrons causing X-ray emissions in association with stepping negative leaders, and it also a plausible source behind the more powerful terrestrial gamma-ray flashes observed at satellite altitudes. There are a number of difficulties associated with studying runaway electrons in nature, due to the stochastic nature of lighting. For this reason, we look a the results of laboratory experiments where the source location of X-rays and the electrical discharges properties can be easily pinpointed. In this work, we introduce a computer simulation tool to probe the runaway electron distribution energy from measured X-ray emissions. The simulation code has been created with the powerful Geant4 software toolkit, and it mirrors an 8-cm rod-to-plane gap experimental lab discharge setup [da Silva et al., GRL, 44, 11174, 2017]. In this work we study the bremsstrahlung X-ray emission of monoenergetic electron beams. The beams are injected in a region with uniform electric field of 12.5 kV/cm at a distance of 2 cm from the anode, and have initial electron energies in the range between 20 and 75 keV. The lower energy value is chosen to be around the runaway electron energy threshold at the applied electric field, while the upper limit is chosen so that the maximum kinetic energy of electrons is 100 keV, corresponding to the peak applied voltage in the discharge gap, of 100 kV. The simulations confirm that in a short gap X-rays are produced when runaway electrons hit the anode, and that radial diffusion due to elastic collisions substantially affects the electron propagation. Detailed analysis of the X-ray emission impulse response sheds some light on how to unwrap photon pile up, and confidently infer the energy distribution of runaway electrons emitted by streamer discharges.