Zonal flows are radially localised, poloidally and toroidally symmetric potential structures (see figure, movie from DALF3 a simulation). They can be described as low-frequency ExB shear flows driven by turbulent Reynolds stress (RS). Therefore, they play an important role for turbulent transport. Zonal flows are considered as a candidate for triggering transport barriers.
In the power spectrum of the poloidal mean flow from simulation data, two types of zonal flows can be observed: long-lived flows at a frequency of about zero and flows at the geodesic acoustic mode (GAM) frequency. The zero-frequency flows are difficult to detect experimentally since for an identification, the long-range correlation on a whole flux surface has to be taken into account. The GAM, on the other hand, provides a frequency scaling with the sound speed c s.
Currently, the correlation of turbulent RS and poloidal ExB flows is investigated by means of the RS array.