Chemical erosion of DIII-D lower divertor tiles

Spectroscopic measurements in the DIII-D lower divertor have shown an order of magnitude decrease in the brightness of the CD emission band 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 using a low-energy D2+ ion beam, with energies extending down to ∼15 eV/D+. The results indicate that the erosion characteristics of the DIII-D lower divertor plasma-exposed specimens are similar to those of pyrolytic graphite and the virgin DIII-D tile material. The total chemical erosion yields for all specimens peak at ∼2-4% at 700 K for 15 eV/D+, and reach ∼7% for 200 eV/D+.