Transport phenomena in the edge of Alcator C-Mod plasmas

Full text: Investigations into particle transport in the edge and Scrape-Off-Layer of Alcator C-Mod have found: 1) in plasmas with Enhanced Dα H-mode confinement, the transport appears to be regulated by the presence of a quasi-coherent (QC) density and potential fluctuation localized within the edge pedestal; and 2) in the SOL, the cross-field transport is convective, dominates the parallel transport, and occurs predominantly as intermittent events that propagate both radially and poloidally. Study of the QC fluctuation shows that it exists in a narrow radial region on the outboard, bad-curvature side, is not present at the inboard midplane, and has a poloidal wavenumber of 1-2cm-1 at the outboard midplane. It is located within the edge pedestal and moves with it. Movies of the edge/SOL turbulence show generation of the intermittent events ('blobs') at and outside of the steep edge gradient region, with clear radial and poloidal propagation. Near the density limit the region of generation is seen to move with the steep gradient to inside the separatrix. Radial tracking of the events shows that the largest amplitude fluctuations move with radial speeds of ∼ 0.5-2km/s, typically accelerating with radius.(author)