Description
Plasma turbulence in tokamaks can self-organize into large-scale structures known as Zonal-Flows (ZFs). These axisymmetric flows can shear turbulence, steepen kinetic profiles and are expected to play a major role in transitions to improved confinement regimes. However, measuring turbulence self-organization is challenging and requires diagnostics capable of resolving a wide range of spatiotemporal scales. Ultra-fast sweeping reflectometry (UFSR), measures electron density fluctuations with millimetre resolution from the edge to the plasma core.
In this contribution, we present measurements from AUG that reveal spectral broadening and frequency shifts in the density fluctuation spectra, suggesting the existence of sheared flows distributed from the core to the edge. To support the interpretation of the data, we employ a synthetic reflectometry diagnostic based on 2D full-wave scheme, coupled to modified Hasegawa-Wakatani turbulence simulations with strong and stable flows. Results show remarkable agreement with AUG data, confirming the new signatures of sheared-flows. First, frequency spectra recorded along the radius systematically contract at shear locations and broaden at maximum ExB velocity. In addition, at slight angle between the beam and the flux surfaces, quantitative measurements of ExB velocity are possible. On WEST, measurements from both Doppler Backscattering System and UFSR show good agreement.
This confirms the possibility to identify and track sheared-flows by UFSR.