Investigation of Changes in the Allotropic Structure of Graphite Surface under Plasma Exposure in the EAST Tokamak and PLM

Abstract

This study presents a method of X-ray photoelectron spectroscopy that facilitates the analysis of both photoelectron peaks excited in vacuum without inelastic energy losses, as well as the adjacent energy loss regions. Using this method, the changes in the allotropic modifications of carbon materials subjected to helium plasma exposure, simulating conditions at the plasma-wall interface in nuclear fusion reactors, as well as deuterium plasma exposure in the EAST tokamak, were comprehensively investigated. Additionally, the effects of helium plasma on the fine-grained dense graphite grade FDG-8 were examined. The results indicate that the surface of the FDG-8 graphite sample exposed to plasma transitions to a structure resembling pyrolytic graphite. Furthermore, the graphite utilized as first-wall material in the EAST tokamak was analyzed. Findings demonstrated that operating graphite materials in the EAST tokamak leads to a transformation of the near-surface layers of graphite into amorphous diamond-like structures, which in turn results in the formation of diamond crystals on the surface of the graphite. Consequently, the carbon coating on the EAST tokamak acquires dielectric properties, rendering such coatings unsuitable for use as plasma-facing materials. The study also discusses the potential for developing graphite-based coatings with properties essential for effective first-wall materials.