Physical and Chemical Erosion of Li Treated ATJ Graphite for NSTX

Plasma-facing component (PFC) candidate materials must have characteristics allowing for high temperature resilience while limiting deuterium recycling and core contamination from erosion. Graphite is a good choice of material for its high temperature tolerance, but can negatively influence the plasma through chemical sputtering and has a high recycling rate once saturated with deuterium. To reduce recycling, lithium has been used extensively as a coating on plasma facing surfaces, though many issues still remain. In particular at NSTX, lithium pellet injection (at mg levels) was utilized to create a lithium coating on the plasma wetted surfaces (walls of ATJ graphite), which was noted to significantly pump neutral-beam-heated plasmas. Results were initially good in reducing the deuterium recycling, though pre-lithium conditions returned in a short time. After a time, the lithium intercalates into the graphite lattice, requiring a repeated application of lithium. Additionally, the lithium film and any exposed graphite are subject to erosion by incident deuterium ions, both through physical sputtering chemical erosion. The present work involves the Ion-surface InterAction experiment (IIAX) at the University of Illinois (UIUC), which uses a low-energy ion beam (primarily deuterium) to bombard samples and monitors particular effects of the bombardment. This research is being done in conjunction with the IMPACT experiment at Argonne National Laboratory, which is involved in researching the effects of light ion bombardment of carbon and carbon/lithium surfaces to help explain the NSTX experimental results. Central to the current research aim are the problems of intercalation of lithium into the bulk carbon and the release of methane from the surface, as well as overall erosion rates of various treated graphite surfaces. It is proposed to first test surfaces of plain graphite and lithiated graphite, followed by surface treatments such as Boronization, He glow cleaning, or others to find overall erosion rates and specific treatments' effects on carbon sputtering (physical vs. chemical) and deuterium retention.