Numerical simulations of runaway electron generation in pressurized gases

Abstract

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.

Electron emission mechanism during the nanosecond high-voltage pulsed discharge in pressurized air

“…It was supposed that RAE consist of electrons emitted from the cathode as well as of electrons emitted from the moving boundary of the plasma channel. 3,4,15 In addition, it was suggested, 4 and confirmed by the results of numerical simulations, 16 that the termination of the RAE generation occurs as a result of either the shielding of the electric field that causes the field emission (FE) by the space charge generated inside the CA gap 15,17 or the transformation of FE to explosive emission (EE). 16 It is important to note that the time delay in the EE beginning in gas-filled diodes is still debatable issue (see, for instance, Refs.…”

“…This leads to an increased number and energy of the electrons emitted from the anode side of the VC and reaching the anode. 15 Thus, the results of our PIC simulations showed that the EEDF of RAE depends on the type of electron emission phenomenon, i.e., either the FE or the EE governs the electron emission from the cathode.…”

“…The latter occurs due to the increase in the electric field caused by EE-emitted electrons. 15 In the case of the HV pulse with T/4 ¼ 0.5 ns, one also obtains a change in the EEDF [see Fig. 1(c)], namely, the turning on of the EE for values t d > t VC leads to an increase in the number of RAE in the highenergy tail of the EEDF.…”

“…When the EE was turned on with a variable time delay t d with respect to the beginning of the HV pulse, the results of PIC simulations 15 showed that the value of t d significantly influences the EEDF. Namely, it was shown that if the EE is turned on prior to the VC formation, one obtains screening of the electric field in the cathode vicinity.…”

Response to “Comment on ‘Early stage time evolution of a dense nanosecond microdischarge used in fast optical switching applications’” [Phys. Plasmas 23, 034705 (2016)]

“…Injection of these electrons in the cathode-anode gap can significantly increase the plasma density. 7 In our opinion, this process may possibly be responsible for the generation of fully ionized plasma at the nanosecond time scale and merits a more detailed study.…”