Pulsed Streamer Discharge Development in Inhomogeneous Media and External Magnetic Fields*

A numerical modeling is used to simulate the properties of positive and negative streamers emerging from a high-voltage electrode in a long air gaps with density gradients and external magnetic fields. It is shown that photoionization in front of the streamer head is important not only for the development of strong positive discharges, but for the development of strong negative discharges as well. An increase in the photoionization rate increases the propagation velocity of the positive streamer and retards the propagation of the negative streamer. A completely new mechanism was proposed to explain the generation of runaway electrons in atmospheric discharges. Conditions were found when the electric field at the streamer head exceeds the breakdown threshold by a factor of 30–50 and significantly exceeds the critical values for electron runaway. As a result, the majority of electrons could transform into the runaway regime. It was shown that positive streamer deceleration in the undisturbed atmosphere by itself can generate extremely high electric fields exceeding the runaway threshold. In such fields, intensive relativistic electron beams, as well as electron-induced x-ray radiation, could be formed. The numerical simulation of the development of a streamer discharge in a gap with an external longitudinal magnetic field was used to demonstrate the self-focusing of such discharges. Self-focusing is caused by a sharp deceleration of the radial ionization wave due to a change in the electron energy distribution function, a decrease in the average electron energy, the rate of gas ionization and the electron mobility in crossed electric and magnetic fields as compared to the case of the discharge development without a magnetic field. The self-focusing effect of a streamer discharge in an external longitudinal magnetic field is observed for both polarities.