Greifswald 2024 – scientific programme

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P: Fachverband Plasmaphysik

P 20: Magnetic Confinement VI

P 20.4: Talk

Thursday, February 29, 2024, 12:15–12:30, ELP 6: HS 3

Experimental investigation of the turbulent drive of the shear flow at the stellarator TJ-K — •Nicolas Dumérat and Mirko Ramisch — IGVP, University of Stuttgart, Germany

Drift wave turbulence has been found to be the dominant instability in the edge of the stellarator TJ-K. Naturally driven by the density gradient, drift waves play a key role in the turbulent transport of particles and energy at the edge of magnetically confined experiments. Inherently related to the coupling between density and potential fluctuations, the drift waves become unstable in case of a non-adiabatic response of the electrons to a density perturbation. Another key agent in such two- dimensional turbulence systems, the zonal flows (ZF) is tied to this cross-coupling. Its interplay with background turbulence is investigated in this work. To this end, convergent cross mapping, a method measuring the causal coupling between variables measured in the same dynamical system is used. By means of multi-dimensional Langmuir probe measurements, and conditional sampling, the plasma fluctuations can be resolved and studied from a new perspective: causality. The causal coupling between density and potential fluctuations during ZF occurrence indicates a clear causality of the density over potential while penetrating the ZF shear layer. Both fluctuations are shown to cause the growth of the ZF, following the drift wave character of the turbulence in the edge of TJ-K. Extending this analysis to wave-number space, the coupling between k_θ modes of plasma fluctuations, unveils the non-locality of the turbulence drive of the ZF as well as evidence of an inverse energy cascade.

Keywords: Causality; Stellarator TJ-K; Zonal flow; Drift wave; Turbulence