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

TLDR

In Alcator C‑Mod, cross‑field particle transport in the scrape‑off layer is characterized by an effective diffusivity that rises sharply with distance from the separatrix. The SOL consists of a near‑separatrix layer with steep gradients and moderate fluctuations where parallel electron conduction dominates, and a far‑SOL layer with shallow gradients, larger fluctuations, and longer correlation times. Effective diffusivity grows with collisionality and, as the discharge density limit is approached, becomes large throughout the SOL, causing cross‑field heat convection to exceed parallel conduction and altering the discharge power balance.

Abstract

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 (Deff) 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). Deff in the near SOL increases strongly with local plasma collisionality normalized to the magnetic connection length. As the discharge density limit is approached, Deff 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.