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

Research on the scrape-off-layer 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 scrape-off-layer 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 scrape-off-layer, and these profiles show qualitative differences between near scrape-off-layer and far scrape-off-layer regions. Particle transport in the near scrape-off-layer exhibits a strong scaling with collisionality, while transport in the far scrape-off-layer 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 ~1cm and their characteristic lifetime is ~1-50 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 scrape-off-layer. For single-null magnetic configurations, plasma in the inboard scrape-off-layer 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 scrape-off-layer flows and core toroidal rotation.