Velocity fields of edge/Scrape-Off-Layer turbulence in Alcator C-Mod

Using high speed movies of edge/Scrape-Off-Layer emission, velocity fields of the turbulent fluctuations have been measured in a small region of the poloidal/radial plane at the plasma's outboard midplane. Gas-puff-imaging is used to localize the emission in the toroidal dimension. The measured Dα line emission responds to the underlying spatio-temporal dynamics of density and temperature. Time-delay cross-correlation analysis is applied to the 64 × 64 pixel movie images and yields time-averaged velocity fields for the emission perturbations. The spatial resolution of the velocity determinations is a few mm over the viewed region. It is found that the velocities inside the separatrix are almost purely poloidal. In the SOL the radial velocity component is outward, and significant radial acceleration is observed. The poloidal and radial components of the SOL velocities vary with the discharge conditions. The magnitudes of all the velocity components measured this way range from 0 to ∼1000 m/s.