Electron power absorption in electronegative capacitively coupled discharges

Abstract

The aim of this work is to explore the role of electronegativity and the electron power absorption mechanism in electronegative capacitively coupled oxygen and chlorine discharges. The fundamental mechanisms underlying the electron heating and the electron power absorption have been widely studied and discussed over the past decades. However, a fully consistent and general mathematical-physical explanation of the several physical mechanisms involved in the electron power transfer mechanism is still lacking. This is in particular true for electronegative capacitively coupled discharges. These difficulties are related to the overall complexity of these systems and to the behaviour of the plasma within the sheath regions. Since making analytical calculations is extremely complicated in this context, the main tool used for research on capacitive discharges is particle-in-cell Monte Carlo collision (PIC/MCC) simulations, which provide information on the various physical quantities such as the electron and ion densities, and their velocity and energy distributions, as well as phenomena such as electric field and electron power absorption. In the first part of the thesis the quenching probability of the singlet delta metastable molecule O$_2$(a$^1\Delta_{\rm g}$) on the electrodes is varied in the simulations, along with the secondary emission yield for ion impact and electron reflection for a capacitively coupled oxygen discharge, within the pressure range 0.13 -- 6.66 Pa, in order to explore their influence on the electronegativity and the electron power absorption. In the second part, we explored the behaviour of both the electric field and the electron power absorption in a capacitively coupled oxygen discharge within the pressure range 1.33 -- 13 Pa and in a capacitively coupled chlorine discharge within the pressure range 1 -- 50 Pa, by comparing the physical quantities determined by the simulations to Boltzmann term analysis applied to the simulation outputs. This allows us to determine the processes that contribute to electron power absorption. In the oxygen discharge the electron power absorption mechanism depends on the discharge pressure. The electron power absorption is due to pressure heating and Ohmic heating. At low pressure (1.33 Pa) the electron temperature gradient term contributes to electron heating and the ambipolar term to electron cooling while the opposite is true at 13 Pa. The chlorine discharge is highly electronegative and at pressures > 10 Pa the Ohmic heating contribution to electron heating dominates. At lower pressure there is also a contribution from the electron temperature gradient term.