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

## TL;DR

This paper proposes a novel electron energy loss spectroscopy setup to measure electron densities in plasma-facing dielectric films, using a specially designed two-layer structure to confine and detect surplus electrons.

## Contribution

It introduces a new method for diagnosing plasma-induced electron densities in dielectrics via a tailored multilayer structure and loss spectrum analysis.

## Key findings

- Calculated loss spectrum shows a strong shift with electron density.
- Feasibility demonstrated with sapphire on CaO layers.
- Potential for non-invasive plasma diagnostics.

## Abstract

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.