Abstract. Experiments in Alcator C-Mod have uncovered a direct link between the character and scaling of edge transport and the empirical Greenwald density limit (nG). In low to moderate density discharges, the scrape-off layer (SOL) exhibits a two-layer structure: a near SOL (5 mm zone) with steep density and temperature gradients and a far SOL with flatter profiles. In the far SOL, the transport fluxes exhibit large transport events (`bursts' which carry particles to main-chamber structures. In the near SOL, transport fluxes appear to be less `bursty' particle diffusivities in this region is found to increase strongly with local plasma collisionality. As n/nG (or collisionality) is raised, cross-field heat convection begins to compete with parallel conduction to the divertor. At N/nG 0.5, TE at the separatrix is reduced. As n/nG approaches 1, regions inside the separatrix exhibit flatter profiles with `bursty' transport behavior; cross-field heat convection to main-chamber structures becomes comparable to the radiated power. Thus as n/nG is increased, cross-field edge transport physics progressively changes, ultimately impacting the power balance of the discharge near N/nG 1.
IAEA 2003