Simulation of Current Free Double layers in highly electronegative plasma using Monte Carlo Collision in OOPIC Pro and XOOPIC

Abstract

The existence of Current Free Double Layers (CFDL) in high electronegativity plasmas has not been experimentally verified, and the validation of theoretical predictions of these structures is required. As part of this experimental verification a numerical simulation of electronegative plasma using OOPIC Pro and XOOPIC was developed to determine the required experimental apparatus and plasma parameters. Electronegative plasma in this context is defined as a high ratio of negative ions as the charge carriers versus electrons, with theory predicting a required ratio of 3 for CFDL formation. Monte Carlo Collision (MCC) features in OOPIC Pro were configured to produce negative SF6 ions from background gas after collisions with low-energy electrons using cross-section data for attachment. Positive ion formation was also modeled using the existing MCC code, along with secondary electron emission from surfaces. Steady-state conditions were achieved in the simulation allowing analysis of plasma electronegativity, positive ion velocity profiles, electron energy distribution functions, and other plasma parameters for various conditions. Introduction of conductors into the simulation at various potentials allowed analysis of the potential profile in a search for the expected CFDLs. The experimental parameters required to produce electronegative plasmas at a sufficiently high ratio in a physical machine, and achieve experimental verification of CFDLs, were determined.