Direct measurement of divertor exhaust neon enrichment in DIII-D

We report first direct measurements of divertor exhaust gas impurity enrichment, η_exh = (exhaust impurity concentration) ÷ (core impurity concentration), for both unpumped and D₂ puff-with-divertor-pump conditions. The experiment was performed with neutral beam heated. ELMing H-mode, single-null diverted deuterium plasmas with matched core and exhaust parameters in the DIII-D tokamak. Neon gas impurity was puffed into the divertor. Neon density was measured in the exhaust by a specially modified Penning gauge and in the core by absolute charge exchange recombination spectroscopy. Neon particle accounting indicates that much of the puffed neon entered a temporary unmeasured reservoir, inferred to be the graphite divertor target, which makes direct measurements necessary to calculate divertor enrichments. D₂ puff into the SOL with pumping increased η_exh threefold over either unpumped conditions or D₂ puff directly into the divertor with pumping. These results show that SOL flow plays an important role in divertor exhaust impurity enrichment.