Particle transport in the scrape-off layer and its relationship to discharge density limit in Alcator C-Mod

Cross-field particle transport in the scrape-off layer (SOL) of Alcator C-M od [Phys. Plasmas 1, 1511 (1994)] can be characterized by an effective part icle diffusivity (D-eff) that increases markedly with distance from the sep aratrix. As a consequence, recycling onto the main-chamber walls is large c ompared to plasma flows into the divertor volume. The SOL exhibits a two-la yer structure: Steep gradients and moderate fluctuation levels are typicall y found in a similar to5 mm region near the separatrix (near SOL) where par allel electron conduction typically dominates energy losses. Small gradient s and larger fluctuation levels with longer correlation times are found out side this region (far SOL). D-eff in the near SOL increases strongly with l ocal plasma collisionality normalized to the magnetic connection length. As the discharge density limit is approached, D-eff and associated fluctuatio n levels become large across the entire SOL and cross-field heat convection everywhere exceeds parallel conduction losses, impacting the power balance of the discharge. (C) 2001 American Institute of Physics.