We propose a setup enabling electron energy loss spectroscopy to determine the density of the electrons accumulated by an electro-positive 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. We demonstrate the feasibility of the proposal by calculating the loss spectrum for an sapphire film on top of a CaO layer. We find a reflection maximum strongly shifting with the density of the surplus electrons and suggest to use it for its diagnostics.
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