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A moment model that captures non-Maxwellian electron energy distribution function effects in partially-ionized plasmas
Researchers of the low-temperature plasma group at LPP working on electric propulsion have proposed a fluid model that self-consistently captures non-Maxwellian electron energy distribution functions (EEDFs) in partially-ionized plasmas. The work is published in the “Editor’s pick” collection of Physics of Plasmas.
Electrons in partially-ionized plasmas often do not follow a Maxwellian distribution due to the collisions with a much colder gas, spatial inhomogeneities, and the presence of electromagnetic fields (See figure 1). However, most of the fluid models for plasma discharges are based on the local approximation (that assumes that the electric forces are locally balanced by the collisions with the gas) or simplify the collisional exchange terms (by assuming a constant collision frequency or a Maxwellian distribution).
- Figure 1
- Experimental measurements of the EEDF carried out at LPP of an argon discharge. By assuming a Maxwellian distribution function (left) the tail of the distribution is overestimated. Alternatively, the high-order moment model is able to capture the shape of the EEDF.
In this work, we propose a macroscopic model that, in addition to electron particle, momentum and energy conservation equations, solves the evolution equations for the heat flux vector and the contracted fourth moment. The article shows that by solving fourth moment equation, we are able to self-consistently capture non-Maxwellian distribution functions as seen in experiments and in kinetic simulations (see fig. 2). In addition, novel non-local transport phenomena are found by the model. They are due to spatial gradients of the EEDF, which is beyond the local-field assumption. The collisional terms in the equations are solved exactly by considering elastic, inelastic and ionization collisional processes. The model is computationally less expensive than kinetic solvers.
- Figure 2
- Comparison between kinetic simulations (red), high-order moment model (green), and a model that assumes a Maxwellian distribution function (blue). The proposed model quantitatively captures the distribution function as simulated with a kinetic solver.
View online : A. Alvarez Laguna, B. Esteves, A. Bourdon, and P. Chabert, Phys. Plasmas 29, 083507 (2022)
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