Monte Carlo study of charged-particle collisions in low-temperature plasmas

Charged-particle (Coulomb) collisions are often neglected when studying low-temperature plasmas, due to their low ionization degrees ∼ 10⁻⁷-10⁻³. However, Coulomb collisions are known to significantly alter the electron kinetics in various gases, above ionization degrees ∼ 10⁻⁶-10⁻⁵.

This work examines the effect of charged-particle interactions on the electron kinetics of low-temperature plasmas, implementing electron-electron and electron-ion collisions in the open-source LoKI-MC code. This numerical tool simulates the classical motion of numerous electrons, interrupted by collisions with neutrals, using the Monte Carlo method. Several techniques are employed to describe Coulomb collisions, with results being compared to theory and other modelling tools.

Nanbu's method is argued as the most computationally efficient and accurate approach for Monte Carlo simulation of Coulomb collisions. The effects of electron-electron and electron-ion collisions are then extensively studied and documented for various gases, ionization degrees, and a wide range of DC electric fields. This includes the analysis of the electron energy distribution functions, transport coefficients, the phenomenon of negative differential conductivity and perturbations to Coulomb collision theory caused by electron-neutral collisions.

The impact of Coulomb collisions is also accessed on time-dependent electron kinetics subject to a model nanosecond electric-field pulse. In particular, electron-electron collisions are shown to facilitate energy relaxation to neutrals. Finally, an adjacent study on improvements to the Monte Carlo method is presented, which enables significant computational gains in the simulation of electron-neutral collisions.