X-ray Emissions in a Multiscale Fluid Model of a Streamer Discharge

We use a three-specie fluid model of electric discharge in air to simulate streamer evolution from the avalanche-to-streamer transition to the collision of opposite-polarity streamers. We estimate the upper limit on the production of thermal runaway electrons, which is dominant during the second of these processes. More thermal runaways are produced if the ionization due to natural background and photoionization is reduced, due to possibility of creation of higher electric fields at streamer tips. The test-particle simulation shows, however, that these thermal runaway electrons have insufficient energies to become relativistic runaways. The simulations are done in constant uniform background fields of $\mathit{E}_{0}$ = 4 and 6 MV/m. A simulation was also performed in $\mathit{E}_{0}$ = 2 MV/m after formation of streamers in 4-MV/m field, in order to approximate the average background field created by $\sim$1-MV voltage over a $\sim$1-m electrode gap used in laboratory spark experiments. We conclude that the used fluid model is insufficient to explain X-ray observations during such experiments. We discuss the possible role of mechanisms which were not included in this or previous modeling but may play the deciding role in the electron acceleration and X-ray production during a streamer collision.