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-Mod [Phys. Plasmas 1, 1511 (1994)] can be characterized by an effective particle diffusivity ${\mathrm{(D}}_{\mathrm{eff}})$ that increases markedly with distance from the separatrix. As a consequence, recycling onto the main-chamber walls is large compared to plasma flows into the divertor volume. The SOL exhibits a two-layer structure: Steep gradients and moderate fluctuation levels are typically found in a ∼5 mm region near the separatrix (near SOL) where parallel electron conduction typically dominates energy losses. Small gradients and larger fluctuation levels with longer correlation times are found outside this region (far SOL). $D_{\mathrm{eff}}$ in the near SOL increases strongly with local plasma collisionality normalized to the magnetic connection length. As the discharge density limit is approached, $D_{\mathrm{eff}}$ and associated fluctuation levels become large across the entire SOL and cross-field heat convection everywhere exceeds parallel conduction losses, impacting the power balance of the discharge.