Absolute Impurity Concentrations During Attached and Detached Divertor Conditions in DIII-D

The inter-ELM intrinsic carbon impurity fraction in DIII-D is measured to be 4.5 +/-1.0% in attached H-mode conditions, falling to 0.4 +/-0.1% in detached conditions, a 10X drop and a significant departure from a fixed fraction assumption. Intensity calibrated, vertically-viewing EUV/VUV spectroscopy provides line data for dominant radiation emissions in the divertor. Spectroscopic data is interpreted using Divertor Thomson scattering (DTS) for direct measurement of electron temperature and density and ADAS for simulation of emission intensities. The EUV/VUV spectrum suggests that ~20% of the measured spectrum is unaccounted-for by line emissions alone, suggesting that molecular emissions (D₂ Lyman-Werner bands) may be present. These results provide critical benchmarks for code validation and detachment scalings, insight into divertor/scrape-off-layer (SOL) impurity transport, and reveal how efficiently the intrinsic impurity can be complemented with extrinsic sources.

Work supported by US DOE under DE-FC02-04ER54698, DE-AC52-07NA27344, DE-FG02-07ER54917, LLNL LDRD project 17-ERD-020.