New Experimental observations of sheared Zonal Flows and validation by Synthetic Diagnostic

A. Jamann¹, G. Dif-Pradalier¹, F. Clairet¹, A. Glasser², S. Hacquin¹, F. Orlacchio³, L. Vermare³, R. Varennes¹, G.D. Conway⁴, the WEST team¹ and the ASDEX Upgrade team⁴
1) CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France
2) Aix-Marseille University, CNRS, Centrale Méditerranée, M2P2, France
3) Ecole Polytechnique, LPP, CNRS UMR 7648, 91128 Palaiseau, France
4) Max-Planck-Institut für Plasmaphysik, 85748 Garching, Germany

In tokamaks, while turbulence typically degrades confinement, it can self-organize into large-scale structures known as Zonal-Flows (ZFs). These poloidal-toroidal flows can shear turbulence (via ExB shear), regulate turbulent transport and nonlocal spreading, and steepen kinetic profiles. ZFs play a major role in transitions to improved confinement regimes.

Measuring turbulence self-organization remains challenging, as it requires diagnostics capable of resolving a wide range of spatiotemporal scales. Ultra-fast sweeping reflectometry can measure electron density fluctuations with millimetre resolution from the edge to the plasma core and was previously deployed on AUG. The data revealed unexpected features, potentially indicating successive radially sheared ZFs (i.e., ExB Staircase). Nevertheless, flow identification is currently limited to indirect measurements and requires further validation.

A synthetic reflectometry diagnostic was employed to support the interpretation of these measurements. The results show remarkable agreement with AUG data, confirming new signatures of sheared ZFs. In particular, we demonstrate that a standard reflectometer can be used as a “velocimeter”, providing ExB velocity measurements similar to those obtained with a Doppler Backscattering System (DBS). These findings pave the way for in-depth investigations of ZF dynamics, turbulence–flow interactions, and their role in improved confinement regimes.