Runaway Electron Generation by Decelerating Streamers in Inhomogeneous Atmosphere

The dynamics of positive and negative streamers is numerically simulated in atmospheric pressure air. It is shown that positive and negative streamers behave in radically different ways when decelerating and stopping. When the head potential drops, the negative streamer transits to the mode in which the propagation is due to the forward electron drift. In this case, the radius of the ionization wave front increases, whereas the electric field at the streamer head decreases further and the streamer stops. The only advancement mechanism for a positive streamer with decreasing head potential is a decrease in the effective radius of the ionization wave, leading to a local increase in the electric field. This mechanism compensates for the decrease in the efficiency of gas photoionization at small head diameters. The local electric field at the streamer head can exceed the runaway threshold when the head potential decreases to ~1.2 kV in atmospheric pressure air. In this case, pulsed generation of a beam of runaway electrons directed into the channel of a stopping positive streamer can occur. The energy of the formed pulsed electron beam can vary from 700 V (when increasing the photoionization rate by a factor of 10 with respect to the value in atmospheric pressure air) to 2.6 kV (in air with an absorbing additive that reduces the photoionization rate by a factor of 1000). It is possible that this behavior of decelerating positive streamers can explain the observed bursts of X-ray radiation during the streamer propagation.