UC San Diego

At the onset of detachment, measurements show that radial turbulent particle transport (Γ~r) increases by a factor of 2−4 above the dissipative region in the near scrape-off layer (SOL) of both single-null (SN) and snowflake (SF) divertors. This leads to a fourfold increase in the density decay length, with broadened density profiles and flattened electron temperature (Te) profiles along the divertor leg. These profiles are convected uniformly through the divertor by parallel flows, with parallel heat transport shifting from conduction- to convection-dominated. Within the dissipative divertor, the flattening and collapse of the plasma potential (Vp) profiles weaken electric field gradients, reducing the E × B drifts. Turbulent spreading upstream of the dissipative region thus emerges as the dominant mechanism for radial transport at the detachment onset. Radial and poloidal drifts are shown to move respectively ∼10%–20% and ∼15%–30% of the target-normal particle and heat flux delivered to the outer strike point in attached conditions, but their contribution drops to a few percent at the detachment onset.