Speaker
Description
Tokamak plasmas are complex non-equilibrium systems where turbulence plays a critical role in the transport of heat and particles. These turbulent processes span a wide range of spatial and temporal scales, making their observation particularly challenging. Ultra-fast sweeping reflectometry is a diagnostic technique capable of measuring electron density fluctuations with very high spatial and temporal resolution, capturing phenomena driven by both MHD activity and microturbulence. This technique has been used to observe meso-scale structures, such as the ExB staircase — a successive radial pattern of poloidal flows and avalanches [1]. This tertiary structure emerges from the self-organization of turbulence and regulates turbulent transport by alternating spatially between phases of free energy accumulation (zonal mean flows) and bursts of outward transport. Such organization is believed to enhance plasma confinement and could be of significant importance for future fusion devices.
In this work, we propose to probe turbulence maps generated by the gyrokinetic code GYSELA using the synthetic reflectometry diagnostic FeDoT [2]. FeDoT is based on a two-dimensional (2D) Finite Difference Time Domain (FDTD) full-wave numerical scheme with absorbing boundary conditions. It supports arbitrary antenna configurations, and can simulate both O-mode and X-mode polarizations at any probing incidence angle. This flexibility enables the extraction of key turbulence characteristics, including frequency spectra, fluctuation levels and radial correlation lengths.
The turbulence data analyzed in this study originates from flux-driven simulations with kinetic electrons. The external source can be modulated to vary the distance to “marginality” (see G. Dif-Pradalier et al., this conference), allowing us to investigate its influence on the formation of the ExB staircase. This approach enables a direct comparison of synthetic diagnostic results obtained in scenarios both with and without staircase formation. The simulated reflectometry signals are then compared to experimental measurements to identify and validate ExB staircase signatures. Additionally, we propose new diagnostic signatures that may further enhance the detection and characterization of these meso-scale structures.
This integrated approach, coupling synthetic diagnostics and flux-driven gyrokinetic simulations, advances the interpretation of experimental measurements and paves the way toward a deeper understanding of turbulent self-organization mechanisms in tokamak plasmas.
References
[1] G. Dif-Pradalier et al. Phys. Rev. Lett. 114, 085004 (2015) ; G. Hornung et al. Nuclear Fusion: 57.1 (2016).
[2] A. Glasser et al. Plasma Phys. Control. Fusion 67 035022 (2025).
