Stratification of Capacitively Coupled Plasma in Noble Gases

Standing striations previously observed experimentally in Capacitively Coupled Plasma (CCP) in noble gases have been reproduced in self-consistent computer simulations using the recently developed hybrid kinetic-fluid code [1]. The plasma model assembled in COMSOL contains a kinetic equation for electrons in (x,u) phase space (u is the electron kinetic energy), drift-diffusion equations for ions, and the Poisson equation for the electric field. The radial loss of the particles to the wall via ambipolar diffusion is included, and periodic boundary conditions are used to simulate the stratified positive column plasma. For the first time, we obtained standing striations in CCP due to nonlocal electron kinetics. We confirmed the kinetic nature of these striations at low currents, which resemble moving s striations in DC discharges. They are associated with enhanced electron heating by the RF electric field in the regions of low plasma density and the electron diffusion in phase space with conservation of total energy (kinetic plus potential). The nonlocal kinetics in phase space results in the maximum excitation/ionization rates at the points of maximal plasma density. We have shown that lower average electric fields can maintain stratified plasma compared to striation-free plasma. This substantiates the principle of minimal power for the kinetic striations in DC and RF discharges.

[1] Vladimir I Kolobov, Juan Alonso Guzman and Robert R Arslanbekov, A self-consistent hybrid model of kinetic striations in low-current argon discharges, Plasma Sources Sci. Technol. 31 (2022) 035020