The Scrape-Off Layer in Alcator C-Mod: Transport, Turbulence, and Flows

Abstract

Research on the scrape-off layer (SOL) plasma of the Alcator C-Mod Tokamak is reviewed. The research has focused on understanding the transport of energy and particles both parallel and perpendicular to the magnetic field. Large differences between the inboard, high-field side SOL and the outboard, low-field side are found. On the outboard side large levels of anomalous cross-field transport of heat and particles exist, with important and far-reaching consequences on recycling, power handling, plasma flows, and possibly core-plasma density limits and rotation. The phenomenon of main chamber recycling is discussed. Parallel and perpendicular transport, together with the heat and particle sources, determine the plasma profiles in the SOL, and these profiles show qualitative differences between near- and far-SOL regions. Particle transport in the near SOL exhibits a strong scaling with collisionality, while transport in the far SOL is clearly convective, with little obvious dependence on collisionality. The anomalously large magnitudes of perpendicular transport are the result of turbulence. Turbulent structures, “blobs,” are largely responsible, and their characteristics have been examined. The turbulent structures are approximately aligned with the field and have k ≪ kperp. Their characteristic size perpendicular to the field is ~1 cm, and their characteristic lifetime is ~1 to 50 μs. The turbulent structures move both radially outward and poloidally at speeds up to ~1 km/s. Evidence that this turbulent transport may play an important role in the core-plasma density limit is presented. Much lower levels of turbulence and no blobs are observed in the high-field-side SOL. For single-null magnetic configurations, plasma in the inboard SOL appears to be almost entirely a result of plasma flow along field lines from the low-field side. Strong parallel flows with sensitivity to magnetic topology are found, along with strong evidence for momentum coupling between these SOL flows and core toroidal rotation.