Electron energy loss spectroscopy of wall charges in plasma-facing dielectrics

Abstract

We propose a setup enabling electron energy loss spectroscopy to determine the density of the electrons accumulated by an electropositive dielectric in contact with a plasma. It is based on a two-layer structure inserted into a recess of the wall. Consisting of a plasma-facing film made out of the dielectric of interest and a substrate layer, the structure is designed to confine the plasma-induced surplus electrons to the region of the film. The charge fluctuations they give rise to can then be read out from the backside of the substrate by near specular electron reflection. To obtain in this scattering geometry a strong charge-sensitive reflection maximum due to the surplus electrons, the film has to be most probably pre-n-doped and sufficiently thin with the mechanical stability maintained by the substrate. Taking electronegative CaO as a substrate layer we demonstrate the feasibility of the proposal by calculating the loss spectra for Al2O3, SiO2, and ZnO films. In all three cases we find a reflection maximum strongly shifting with the density of the surplus electrons and suggest to use it for charge diagnostics.