Density Shoulder Formation in DIII-D

Midplane density profiles in DIII-D plasmas develop a long decay length in the SOL as discharge average density increases and divertor detachment progresses. Length increase is accompanied by enhanced radial transport, decreased parallel transport and increased local neutral pressure, the latter dependent on divertor ExB direction. As line averaged density increases from 1 to 4E13 cm-3, the near scrape-off layer (SOL) decay length stays constant at ~1.6 cm while the far SOL increases from 1.8 to 3.5 cm. Measured radial turbulent particle and heat transport at the midplane increase by factors of 10 with average discharge density, as expected from a filamentary transport model due to the collisionality changes along flux tubes. The measured radial transport at the midplane increases strongly as collisionality at the divertor region varies from 0.05 and surpasses 1. New findings are that the density shoulder is correlated with the active divertor's ExB drift direction pushing particles to the outer strike point, which also can increase the chamber neutral pressure by a factor of 3. These results indicate that the shoulder formation is a combination of increased radial transport and ionization source coupled with a reduction in parallel transport.