Verification of Zeff Profiles Using the Visible Bremsstrahlung Diagnostic on DIII-D

Comparison of the effective impurity content, Z_eff, derived from visible bremsstrahlung (VB) and charge exchange recombination (CER) diagnostics demonstrate good agreement when carbon is the dominant impurity in the plasma. Accurate characterization of Zeff profiles is important during impurity seeding experiments to monitor impurity accumulation within the core and to measure impurity transport. Zeff determined through CER drops after impurities are injected, since the measurement is based on C6+ concentration, while Zeff from VB remains relatively constant. The VB diagnostic measures plasma emissivity at 523 nm through a 3 nm band-pass filter, utilizing 16 tangentially facing spatial views at a 20 kHz sample rate. Emissivity contributions within the scrape-off-layer, sensitivity to electron density and temperature profile fits, and comparison of Z_eff determined through Abel inversion and a least-squares minimization will be shown.