Radially propagating fluctuation structures in the scrape-off layer of Alcator C-Mod

Radially propagating spatiotemporal fluctuation structures are observed in the scrape-off layer of Alcator C-Mod [I. H. Hutchinson et al. , Phys. Plasmas 1, (1994)] using the combination of electric probes, a radial array of views measuring D α emission, and two-dimensional imaging of D α emission. For a specific magnetic-field configuration the electric probe and the D α array measured plasma density and potential fluctuations along the same magnetic-flux tube. Calculations of the cross-correlation functions of D α intensity fluctuations with ion saturation current fluctuations and floating potential fluctuations, respectively, reveal that the potential associated with fluctuation structures is of dipole type, consistent with fundamental models for radial blob propagation. Radial and poloidal velocities of fluctuation structures are obtained by two-dimensional spatiotemporal turbulence imaging using an ultrafast framing camera observing the D α emission intensity in the poloidal plane. In the poloidal direction the fluctuation structures are predominantly propagating in direction of background E × B velocity. Measured radial velocities are directed outwards with a magnitude of typically 1% of the ion sound speed.