Interaction of electrons with plasma walls

The interaction of electrons with the walls of the discharge vessel is an important surface process in technological low-temperature plasmas. It affects, for instance, the operation modii of dielectric barrier discharges, Hall thrusters, and divertor plasmas in fusion devices. Little is however known quantitatively about the process because it typically occurs at energies below 50 eV which are hard to access experimentally. There are only a few attempts to measure probabilities for electron absorption, backscattering, or secondary emission in this energy range. From a theoretical point of view the description is also challenging, because it cannot be entirely based on an ensemble of individual atomic scattering centers occupying a region of space with a certain density. Instead genuine solid state effects, such as Bragg scattering on crystal planes and collective excitations of the ensemble become important. In my talk I will give a tutorial introduction into the semiempirical microscopic modeling we introduced a few years ago for calculating aforementioned probabilities [1]. It is based on an invariant embedding principle for a function Q summing up the backscattering trajectories arising from the interaction of the incoming electron with the excitations and imperfections of the wall. We substantially augmented this approach to make it applicable to metallic walls. Besides describing the augmentations, I will present for a number of metals the secondary electron emission yields we obtained from our theory. They turn out to be in good agreement with experimental data indicating that our approach captures the low-energy scattering physics rather well.

[1] F. X. Bronold and H. Fehske, Plasma Phys. Control. Fusion 59, 014011 (2017)