On the use of ultra-high resolution PIC methods to unveil microscale effects of plasma kinetic instabilities: electron trapping and release by electrostatic tidal effect

Abstract

Ultra-high resolution Particle-in-Cell coupled to Monte-Carlo collisions modelling unveils microscale instabilities in non-equilibrium plasmas fulfilling Penrose's instability criterion. The spontaneous development of ion turbulence in the phase-space generated by charge exchange collisions leads to finite amplitude modulations of the local electric field. The latter are responsible for the trapping of low energy electrons and their transport from the plasma volume to the sheath vicinity. Electrostatic tidal effect occuring near the sheath is responsible for the release of the trapped electrons as a monochromatic bunch, accelerated back towards the source. This instability provides an additional theoretical ground for the anomalous enrichment of low-energy electrons observed by Langmuir probes in similar conditions. \hl{The present results demonstrate that marginally fulfilling PIC criteria is insufficient to study the microscale instabilities effects on the electrons dynamics in non-equilibrium low temperature plasmas.

Numerical simulation and experimental diagnostics of fast electron kinetics and plasma parameters in a microhollow cathode discharges in helium

“…Note that in recent years, hybrid models of gas-discharge plasma have gained great popularity, in which heavy particles (ions, excited particles, neutrals) are described by the fluid approximation equations, and electrons by the PIC/MCC method [34][35][36][37][44][45][46][47][48][49][50][51][52][53]. The idea of this method is as follows: the trajectory of an electron starting from the cathode is calculated according to the equations of motion mechanics in a given electric field; in this case, the elastic and inelastic scattering of an electron is taken into account as a random process.…”

“…As a result of such numerical solutions, the empirical distribution of the ionization source is determined and the system of fluid equations for electrons and ions is closed. Despite the success of numerical calculations [34][35][36][37][44][45][46][47][48][49][50][51][52][53], such models are often limited by computational resources and do not allow taking into account the completeness of the processes occurring in a nonequilibrium plasma: a large number of elementary processes, gas heating, etc.…”

Ion-acoustic waves in weakly ionized plasmas with charge-exchange collisions

“…Nevertheless, charge-exchange collisions may give rise to potential perturbations and energy losses of positive ions when colliding with neutrals, causing the Ion Velocity Distribution Function (IVDF) to become linearly unstable 11 . From this point of view, a self-consistent numerical analysis by Schiesko et al 12 showed that the ion-acoustic instability was triggered by a non-monotonically decreasing ion’s VDF (i.e., satisfying Penrose’s criterion 13 ) inducing potential modulations. Furthermore, as demonstrated by the convective nature of this instability, no net damping was observed for the chosen parameters 12 .…”

“…From this point of view, a self-consistent numerical analysis by Schiesko et al 12 showed that the ion-acoustic instability was triggered by a non-monotonically decreasing ion’s VDF (i.e., satisfying Penrose’s criterion 13 ) inducing potential modulations. Furthermore, as demonstrated by the convective nature of this instability, no net damping was observed for the chosen parameters 12 .…”