Reduction of divertor carbon sources in DIII-D

The evolution of carbon release from the DIII-D lower divertor tiles is studied using atomic and molecular spectroscopy. Newly installed virgin graphite tiles in 1992 are found to have had a chemical erosion yield, Y_chem≤3-5%, consistent with both laboratory results and similar experiments in other tokamaks. The average Y_chem measured in the DIII-D lower divertor decreased approximately a factor of ten between 1992 and 2000. The presumed cause of this reduction is the cumulative effect of >30 wall-conditioning boronizations and 10⁵ s of plasma exposure, although the relative importance of these two mechanisms is unknown. This result indicates that a substantial reduction in carbon chemical erosion, and its relative importance as a source of carbon, can be obtained by long-term in situ wall conditioning techniques. The total carbon source sputtered into the DIII-D lower divertor has also apparently decreased over the same period. However, there has been no significant decrease in the average core carbon contamination with the decreasing lower divertor carbon source.