Derek D. Monahan

Global models of electronegative discharges: critical evaluation and practical recommendations

A first step towards understanding a complex plasma is usually to develop a zero-dimensional or global model. This is difficult when the plasma is electronegative, because the literature contains many proposed models with different and sometimes contradictory detailed assumptions, and different domains of applicability. The appropriateness of such models in a given context is often hard to assess. In this paper, we present a set of detailed kinetic simulations spanning a wide of range of parameters, especially with respect to electronegativity, collisionality and dominant negative ion destruction mechanism. We use these simulations as a benchmark to investigate the validity of the fundamental global model assumptions when used to model electronegative discharges. We reach two important conclusions: (1) that an accurate electron kinetics model is more important than detailed considerations relating to plasma dynamics in the presence of negative ions and (2) that there exists a simple and robust transport model that is in reasonable agreement with all of our benchmark simulations, when the electrons are treated properly.

On the global model approximation

Global models are commonly employed to study chemically complex low temperature, low-pressure plasma discharges. However, due in part to the inherent non-uniformity of charged species density in confined plasmas, these models are often regarded as crude first-order approximations. This view is supported by the literature, where global models typically give only broad qualitative agreement when compared with experimental measurements. Recently, the present authors have argued against such comparisons being a useful indicator of the validity of the underlying model assumptions. Choosing instead to quantify the validity of a relatively simple global model by comparing it with a one-dimensional particle-in-cell simulation, they found the two treatments to be in quite good agreement over an extended region of parameter space. This paper is an attempt to offer further insight into this result.