Double layer formation in a two-region electronegative plasma

Abstract

Summary form only given. We describe the formation of a double layer (DL) in a two- dimensional (2D) electronegative plasma with a source (heating) section connected to a larger diameter downstream section. A 2D particle-in-cell (PIC) code is used to exhibit the DL, which appears near the transition between the source and downstream chambers, over a range of pressures and electronegativities (ratios of negative ion to electron density). Diagnostics of the PIC code allow the calculation of various plasma parameters, not easily measured in experiments, to be compared to an analytic theory. The theory, modifying a previous calculation to conform to the simulation, consists of a collisionless one-dimensional model of a DL separating 2D source and downstream globally-modeled regions. In the global models the conditions of positive and negative ion balance upstream and downstream, and the downstream energy balance, are used to determine the double layer potential, electron temperatures, and other plasma parameters. Because of computational limitations of the PIC simulation, a rescaled oxygen reaction set is used both for the simulation and for the analytic comparison, to accommodate lower densities and smaller sizes than those of a typical experiment. In contrast to experimentally observed electron distributions, the PIC simulations exhibit a Maxwellian electron distribution in the source region at temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e2</sub> and a bi- Maxwellian distribution downstream, with a low energy population at a lower temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e1</sub> < T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e2</sub> and with a hotter tail also having temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e2</sub> . Using these results in the model, a DL is found in reasonable agreement with that obtained in the simulation. We have also investigated numerically and analytically the range of pressures over which a DL can exist.