A Detailed Study of Carbon Chemical Erosion in L-Mode Plasmas in the DIII-D Divertor

A series of quiescent L-mode discharges have been used with varying degrees of divertor attachment to study carbon erosion in the DIII-D tokamak divertor. Spectra of atomic and molecular carbon plasma emissions are measured across the divertor. Predictions of carbon emission resulting from chemical and physical sputtering at the outer strikepoint are provided by the WBC transport code. For attached ionizing plasmas ( T e ~ 20 eV and carbon surface T ~ 350 K) the CD and C 2 carbon radical emissions are consistent with a total chemical sputtering yield, Y chem ~ 0.3%. During sweeps of the inner divertor leg, CD and C 2 emissions indicate the main-wall tiles has a six times higher Y chem than the divertor tiles despite identical incident plasma conditions. Emission of atomic (CI) and singly ionized carbon are dominated by physical sputtering with a measured yield Y phys ~ 2% as expected from laboratory data, indicating that chemical sputtering plays a minor role as a divertor carbon source. The Doppler broadening of the CI emission agrees with the physical sputtering model, but the predicted Doppler shift is a factor three larger than the experiment. CD and C 2 emission are reduced below detection during divertor detachment ( T e ~ 1–2 eV), an indication of the suppression of chemical erosion with Y chem < 10 -4 based on expected photon intensity from WBC.