  The results of a numerical simulation of the generation of runaway electrons
in pressurized nitrogen and helium gases are presented. 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.
In addition, the influence of the initial conditions, such as voltage rise time
and amplitude, gas pressure, and the type of gas, on the processes that
accompany runaway electrons generation is presented.
