Carbon sources in the DIII-D tokamak

Physical and chemical sputtering in the DIII-D divertors are distinguished by comparisons of emission rates of C I, CD, and C 2 radiation and by spectral profiles of C I lines. When flux measurements indicate chemical sputtering dominates, the effective C I temperatures (best fit to a thermal distribution) tend to lie in the range 1.0 ±0.2 eV, as expected for atoms coming from molecular dissociation. When P inj > 7 MW and n e < 5 × 10 13 cm -3 , molecular emissions are very weak, if present at all, and effective C I temperatures reach values 5-7 eV. In these cases, detailed examinations of the C I line profiles often reveal that they are asymmetric with shifted peaks. Such characteristics are consistent with expectations for physical sputtering, and attempts have been made to understand the line shapes by modeling them using Thompson flux distributions.