Reduction of divertor carbon sources in DIII-D

The evolution of carbon release from the DIII-D lower divertor tiles is stu died using atomic and molecular spectroscopy. Newly installed virgin graphi te tiles in 1992 are found to have had a chemical erosion yield, Y-chem les s than or equal to 3-5%, consistent with both laboratory results and simila r experiments in other totamaks. The average Y-chem measured in the DIII-D lower divertor decreased approximately a factor of ten between 1992 and 200 0, The presumed cause of this reduction is the cumulative effect of >30 wal l-conditioning boronizations and 10(5) s of plasma exposure, although the r elative importance of these two mechanisms is unknown. This result indicate s 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 wal l conditioning techniques. The total carbon source sputtered into the DIII- D lower divertor has also apparently decreased over the same period. Howeve r, there has been no significant decrease in the average core carbon contam ination with the decreasing lower divertor carbon source, (C) 2001 Elsevier Science B.V. All rights reserved.