Search for zonal flows in the edge turbulence of Alcator C-Mod

The time-dependent radial and poloidal velocity of edge turbulence is evaluated using a 2D time-delayed cross-correlation analysis of fast camera data from the gas puff imaging (GPI) diagnostic viewing the outer midplane region in Alcator C-Mod. The local poloidal velocity fluctuations are averaged over the poloidal viewing region of the GPI diagnostic and radially resolved over ±2cm around the separatrix. The resulting poloidal velocity usually has a broadband frequency spectrum in the range 120kHz, and a radial correlation width which decreases with increasing line-averaged density. In some cases with ICRH heating there was also a coherent poloidal velocity oscillation at 67kHz which was highly correlated with a poloidal magnetic field oscillation at the same frequency. Some of these results are similar to the geodesic acoustic modes and/or zonal flows described in previous experiments and theory.