Investigation of zonal flows with toroidal correlation analysis in TJ-K

In toroidal fusion plasmas, zonal potential perturbations associated with zonal flows can appear. These potential structures have toroidal and poloidal symmetry. Zonal flows are intrinsically connected to flow shear and interact with drift-wave turbulence through the shearing of eddies. By this mechanism, the zonal flow gains energy and the transport through drift-wave turbulence is reduced. Zonal flows have been found to evolve naturally in the drift-wave dominated turbulence of low-temperature plasmas in the stellarator TJ-K. In order to capture their long-range feature through the symmetry properties, Langmuir-probe measurements are performed at two well separated toroidal positions. Long-range correlations are studied with respect to their spatio-temporal properties. In a first step, the radial structure is analyzed by means of a rake-like probe with radially separated pins. Thereby, the influence of zonal flows on radial correlation lengths of the ambient turbulence is shown. Furthermore, a stationary poloidal probe array is used in combination with a 2D-movable probe system in order to uncover the turbulent dynamics in the full poloidal cross-section, conditionally, when zonal flows evolve.