Comparing Definitions of Detachment on DIII-D

DIII-D’s broad range of divertor diagnostics are used to characterize both the onset and trajectory of H-mode detachment for a variety of detachment definitions. Varying definitions of detachment focus on pressure loss, dropping electron temperature, rising neutral pressure, ion flux roll-over, and heat-flux reduction. Clearly defining the state of detachment therefore relies on combining diagnostics so that the relative importance of each physical process can be compared. Identifying the sensitivity of these processes to upstream conditions and radiative power fraction facilitates identifying unstable detachment regimes. On DIII-D, detachment identification is achieved using Langmuir probes, Thompson scatting, EUV/VUV, visible and near-infrared spectroscopy, visible and IR imaging, coherence imaging, neutral pressure gauges, and bolometry. ELM “burn through” in partially detached plasmas is discussed whereby periodic reattachment can occur with associated rises in heat flux and high energy particle transport to the divertor whose magnitude depends on an interplay between degree of detachment and pedestal characteristics.