Impurity sources at the first wall of Alcator C-Mod and their effect on the central plasma

In this paper we present data concerning impurity sources and transport in a tokamak with a divertor and molybdenum first wall. A spectrometer, viewing visible and UV light, has been used to take time dependent spectra of C, O, and Mo from both the divertor and the inner limiter surfaces. In the divertor the sputtering threshold for Mo is not normally reached (T_e ≤ 25 eV). Chord measurements from four filtered photo-diode arrays have enabled us to invert the measured brightness profiles tomographically and obtain 2D emissivity information for H_α and CIII. The total C flux sputtered at the divertor target can be twice as high as from the inner wall surface, increasing with density to about 2 × 10¹⁸ s⁻¹. However, modeling the SOL, we find that the penetration efficiency for C is roughly 10 times smaller for the divertor than for the inner wall. Oxygen fluxes are generally observed to be 1/3 of the C flux.