Validating Divertor Power Exhaust Models with Vacuum Ultraviolet Spectroscopy in DIII-D*

The unique combination of vacuum ultraviolet spectroscopy, impurity flow imaging, and Thomson scattering in the DIII-D divertor has enabled direct measurement of the components of radiated power, significantly advancing our understanding of dissipative processes and the root causes of an under-prediction of radiative power by fluid simulations compared to experiments. Radiative exhaust experiments in H-mode at DIII-D, corroborated with multi-fluid simulations, show that the intrinsic carbon  impurity, CIV (1550 Å), line dominates the radiated power and peaks close to the X-point in detached divertor conditions with ion B×∇B drift toward the X-point (fwd. B_T). In contrast operating with ion drift away from the X-point (rev. B_T), the Deuterium Ly-a (1215 Å) dominates over CIV in detached conditions and peaks in front of the outer target revealing the important role of cross-field drifts for detachment onset and detachment front characteristics. UEDGE simulations with drifts qualitatively capture the dominant radiating lines in fwd. and rev. B_T. The simulations, however, both under-predict the total radiated power and predict the profile to be a factor of three more localized than measured, indicating models are not capturing mechanisms that expand the radiating volume. Cross-field drifts and parallel flows measured in 2D with coherence imaging are examined as possible root causes for the radiation shortfall. Divertor exhaust predictions for ITER and design efforts for future fusion devices rely on simulations with 2D fluid codes, such as SOLPS. This work provides a significant contribution in the effort to improve confidence of the code predictions.