Comparison of mechanisms of the plasma generation by nanosecond discharge at extremely high overvoltage

Two mechanisms of the nanosecond timescale discharge initiation in nitrogen at extremely high overvoltage are compared using a one-dimensional Particle-in-Cell Monte Carlo collisions model. In the first mechanism, the discharge is ignited by the electrons seeded initially in the vicinity of the cathode, while in the second mechanism, the discharge is initiated by the electron field emission from the cathode. It was shown that both approaches predict discharge evolution governed by the runaway electrons generating in the cathode-anode gap. The latter promotes the propagation of the fast ionization wave from the cathode to the anode. However, there are some distinctive features which are explained by different numbers of runaway electrons presented in the cathode-anode gap. In the case, when the field emission is considered, one obtains the virtual cathode formation, the generation of electrons with the anomalous energy, and the fast ionization wave velocity comparable to the speed of light.