Zonal flows, GAM, and radial electric field in the H-1 heliac

The ubiquity of zonal flows in toroidal plasma is now well recognized (see [1] for review on zonal flow theory and a special March issue of Plasma Physics and Controlled Nuclear Fusion 87 (2006) for the latest experimental papers). Two main branches of zonal flows have been identified. Both have zero poloidal and toroidal mode numbers, m = n = 0. One of them is a zero-frequency, finitebandwidth zonal flow, often referred to as stationary zonal flow while the other is the finite-frequency mode often referred to as a geodesic acoustic mode, or GAM. The latter originates from the geodesic curvature in toroidal geometry and develops as a result of coupling between mean zonal flow and the m = 1, n = 0 pressure (density) perturbation [2]. Zonal flows are found in both low and high confinement modes. There is growing evidence that zonal flows play important roles in generation of transport barriers in the plasma. Since structures of transport barriers are closely related to the global stability, confinement and hence the plasma performance, understanding zonal flows formation and their spatial localization becomes crucial for the physics of transport barriers. Zonal flows have been studied in the H-1 heliac in the context of confinement improvement and pedestal formation. These results are reviewed in this paper. Emphasis is made on the interplay between turbulence, electric field, zonal flows and geodesic acoustic modes. Relevance of the H-1 results to other experiments is justified by dimensional similarity of the low-temperature plasma in the H-1 and plasma in larger tokamaks and stellarators. A concept of the dimensionally similar plasmas proposed by Kadomtsev [3] and developed by Connor and Taylor [4] has been extensively used with regard to both tokamaks and stellarators to develop and justify the trans-