A HR-XPS STUDY OF LITHIATED HIGHLY ORDERED PYROLYTIC GRAPHITE

Abstract

The deposition of lithium (Li) on the graphite “first wall” in experimental nuclear fusion reactors has been shown to improve plasma performance, primarily through the enhanced retention of incident deuterium (D) ions. However, the exact mechanism of this retention is not well understood. It has been proposed that oxygen (O) from residual gases in the reactor is captured by Li and that this O subsequently binds with incoming D. However, the relevant X-ray photoelectron spectroscopy (XPS)surface studies have used non-monochromated X-ray sources and O-containing ATJ graphite as a substrate, making it difficult to discern the exact source and chemical environment of O in the system. By probing the fundamental chemical characteristics of lithiated graphite with high-resolution XPS (HR-XPS) using O-free highly ordered pyrolytic graphite (HOPG) as substrate, this study offers a clearer overall picture of the chemical speciation present at this surface of interest. In addition, by bombarding the lithiated graphite surface with high-energy Ar(^{+}) ions (1 keV) and examining the chemical effects, this study isolates changes due to D-chemistry and not just induced by the process of high-energy bombardment.