Chemical erosion of DIII-D lower divertor tiles

Through spectroscopic analysis, it has been observed that the brightness of the CD emission band in the lower divertor of General Atomic's DIII-D tokamak fusion reactor has decreased by an order of magnitude over the course of eight years of plasma discharges. These observations could imply a reduction of the chemical erosion of the carbon component in the divertor. To test this hypothesis, chemical erosion measurements of specimens from the DIII-D lower divertor inner and outer strike points were performed at the University of Toronto using a mass-analyzed, low-energy ion gun. The measured erosion yields were compared to results obtained from pyrolytic graphite and unexposed DIII-D lower divertor material. The results indicate that all of the DIII-D specimens have similar erosion characteristics to pyrolytic graphite. Surface analysis of the specimens revealed clusters of impurities but their effect on the chemical erosion of the tiles appears to be insignificant.