On the reliability of a modification of the fluid model of a glow discharge incorporating the calculation of the local EEDF at each point in space

An analysis of the approach named “Space-Dependent Electron Energy Distribution Function (EEDF) Modeling” in the COMSOL Multiphysics's Plasma Module is carried out. This modeling approach allows a wide range of users to determine profiles of glow discharge parameters, including the EEDF, in the entire discharge volume. Comparison of computed results for a short (without a positive column) glow discharge, obtained from this model and from kinetic simulations, displays not only significant quantitative but also qualitative differences in the EEDFs. The analysis showed that in this model, as well as in the previous models provided by the COMSOL Multiphysics's Plasma Module, the derivation of the main equations is based on the factorization of the EEDF, which implies the use of a local approximation when solving the Boltzmann kinetic equation. However, as has been repeatedly shown in the literature, this approximation is fulfilled under rather harsh conditions, namely, when the scale of plasma inhomogeneity is small compared to the electron energy relaxation length, and the ambipolar field is small compared to the external electric field heating the electrons. These restrictions significantly limit the applicability range of the analyzed model and make it impossible to use it in the near-electrode and near-wall regions of any gas discharges. Comparison with the corresponding data from the kinetic simulation and the subsequent analysis reveals the existence of fundamental disagreement and internal contradiction within this model that cast doubt on the reliability of the results obtained using this approach, which we address to the “computational plasma community.”