The results of a numerical simulation of the nanosecond timescale high-voltage breakdown of the nitrogen at atmospheric pressure are presented. The role of runaway electrons in breakdown is studied. It was shown that runaway electrons generation occurs in two stages. In the first stage, runaway electrons are composed of the electrons emitted by the cathode and produced in gas ionization in the vicinity of the cathode. This stage is terminated with the formation of the virtual cathode, which becomes the primary source of runaway electrons in the second stage. Also, it was shown that runaway electrons current is limited by both the shielding of the field emission by the space charge of the emitted electrons and the formation of a virtual cathode.

The role of transition of field emission of electrons from the cathode to explosive emission with a variable time delay was studied as well. It is shown that the time when the explosive emission turns on influences significantly the generation of runaway electrons. Namely, an explosive emission turn-on prior to the formation of the virtual cathode leads to an increase in the current amplitude of the runaway electrons and in a decrease in its duration. Conversely, an explosive emission turn-on after the formation of the virtual cathode and during the high-voltage pulse rise time does not influence the generation of runaway electrons significantly. When the explosive emission turns on during the fall of the high-voltage pulse and also after the virtual cathode formation, one obtains additional runaway electrons generation.