Stationary Zonal Flows during the Formation of the Edge Transport Barrier in the JET Tokamak

Abstract

High spatial resolution Doppler backscattering measurements in JET have enabled new insights into the development of the edge E-r. We observe fine-scale spatial structures in the edge E-r well with a wave number k(r rho i) approximate to 0.4-0.8, consistent with stationary zonal flows, the characteristics of which vary with density. The zonal flow amplitude and wavelength both decrease with local collisionality, such that the zonal flow E x B shear increases. Above the minimum of the L-H transition power threshold dependence on density, the zonal flows are present during L mode and disappear following the H-mode transition, while below the minimum they are reduced below measurable amplitude during L mode, before the L-H transition.