  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.

Applied Physics