Development of a multi-component diffusion fluid solver for a two-temperature argon plasma sheath
Creators: name:  G.M. GANGEMI 
name:   A. ALVAREZ LAGUNA 
name:   K. HILLEWAERT 
name:   T. MAGIN 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: FLIGHT PHYSICS
PLASMA FLOWS
ENERGY DEPOSITION

description: The behaviour of plasmas in a wall-confined environment is usually assessed by dividing the domain in two main different regions: the bulk region, where the quasi-neutral condition is assumed, meaning that the density of negative particles is approximately equal to the density of positive charges, and the sheath region in the proximity of the walls, where a positive space charge is built [1]. This phenomenon is due to the higher mobility of electrons (compared to the one of neutrals and ions) and creates an electric potential structure that confines electrons and repels positive ions. Studying the plasma sheath is fundamental to a number of applications, ranging from arcing in high vacuum electronics to charging of space platforms due to plume contamination of electric thrusters: these fields differ for the variety of the species involved and for the different nature of the wall confining it. The development of numerical solvers that are able to capture the formation of the sheath and, at the same time, describe the transition region to the quasi-neutral bulk present several challenges that are due to the intrinsic multiscale character of the problem: the low inertia of electrons imposes strict constraints on the stability of the time integration scheme that are overcomed by implicit methods; the positive space charge that it is built in front of the solid surface requires that the mesh is fine enough to resolve the Debye length. In this work we investigate the development of a new simulation strategy for the sheath of an argon plasma, constituted by electrons and argon single- charged ions in a background of argon neutrals, in the vicinity of a solid wall using a multicomponent diffusion approach. A density conservation equation is solved for each species with a dedicated treatment for chemical source terms (using the library Mutation++ [2]); two energy equations, one for electrons and one for heavy species, are introduced in order to account for thermal non- equilibrium due to low pressure conditions; the Poisson equation is coupled to the fluid system guaranteeing charge conservation across the domain. In contrast to the more common multifluid model, a multicomponent approach does not solve for the individual momenta of the species, but their relative motion is modeled as interspecies diffusion; potentially a convection speed for the entire fluid and associated momentum equation is introduced. This leads to a significant reduction of computational cost, especially in 3D cases and if a large number of species is considered. Results obtained are compared to state-of-the-art multifluid simulations [3] on a typical testcase taken from literature. The symmetrical one-dimensional discharge provides the ideal configuration to describe the formation of the sheath starting from a neutral mixture in contact with solid walls and developing, as the simulation advances two distinct sheaths separated by a large quasi-neutral zone. Several plasma conditions are investigated as well as different boundary conditions in order to represent the effect of different surfaces. The outcomes of the present work are extremely encouraging and will allow in the future to study more complex mixture and more complex configurations.