Modeling the thermalization of electrons in conditions relevant to atmospheric pressure He-O2 nanosecond pulsed discharges

Abstract

The electron thermalization process is significant in nanosecond pulsed discharges due to the applied voltage pulse's short duration and rapid rise and fall times. In this contribution a comparison was made between two approaches to modeling the electron kinetics of electron thermalization in atmospheric pressure helium plasma with an oxygen admixture. Modeling based on the direct solution of the local time-dependent electron Boltzmann equation was compared with modeling based on the commonly used but less general local mean energy approximation. For modeling based on the local time-dependent electron Boltzmann equation, a temporary faster decay in the population of electrons in the high energy tail, and a slower decay in the population of intermediate energy electrons was observed while the electron swarm cooled from an average energy of above 8 eV, without an electric field present. During that period, the electron impact reaction rate coefficients of helium direct ionization and electronic excitation decreased by more than 3 orders of magnitude as compared to the modeling based on the local mean energy approximation. Global modeling of the evolution of plasma species densities in response to an electric field typical of atmospheric pressure pulsed discharges was performed with the two approaches to electron kinetics. Differences in the species densities were observed between the two approaches, with an 100% increase in the maximum density of electrons found with the modeling based on the local mean energy approximation.

Investigation of O atom kinetics in O2 plasma and its afterglow

“…Note that the effect of the negative ions on the mobility is neglected, as our modelling calculations indicate that their contribution was found to be negligible under the conditions investigated in this work. We use a stationary EEDF since the electric field varies on time scales much slower than the thermalization time of the electrons [42]. Furthermore, the EEDF is calculated at the beginning of each timestep.…”

The formation of O and H radicals in a pulsed discharge in atmospheric pressure helium with water vapour admixtures

“…As a result of employing the steady state approximation when deriving the electron Boltzmann equation, these transient features of the EEDF were not modelled. It was shown that the rapidly varying electric fields in pulsed discharges, without space charge being considered, can lead to transient EEDF structures that, for example, alter the density of O by around 15% in comparable discharges [55]. Other time-dependent EEDF effects in atmospheric pressure pulsed discharges, for example, occurring as a result of electron-electron collisions, to the authors knowledge, remain uninvestigated in these conditions.…”