Description
Turbulence-driven transport and its self-regulation by plasma flows play a central role in confinement in magnetised fusion plasmas [1,2]. The perpendicular mean flow velocity (v_⊥) and its radial shear facilitate turbulence reduction and transport barrier formation [3]. Radial profiles of v_⊥ are accessible with Doppler Backscattering (DBS) [4], using density fluctuations as tracers. The measured v_⊥ is typically interpreted as the local E×B flow [5–7], but actually includes a contribution from the phase velocity of turbulent structures, usually neglected. Previous experiments have shown a systematic velocity increase at lower wavenumbers, suggesting a scale-dependent contribution on top of the mean flow [8,9].
Characterising the turbulent phase velocity provides access to the turbulence regime and flow-turbulence interaction. Here, such a characterisation is achieved for the first time, with the k_⊥ dependence of v_⊥ from DBS compared against quasilinear predictions using TGLF with saturation rule 2 [10,11]. A systematic increase of v_⊥ at lower wavenumbers is observed across L-mode configurations on WEST [12] and TCV, consistent with a finite phase velocity. This interpretation is verified using a synthetic DBS diagnostic based on full-wave simulations [13] applied to Kolmogorov-like spectra and Hasegawa-Wakatani turbulence [14], consistently recovering phase velocity signatures. The comparison with TGLF-sat2 simulations shows good quantitative agreement, establishing a robust framework for characterising turbulence phase velocity and interpreting DBS measurements.