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LNF: (2022-2024) Estudio experimental de flujos, turbulencia y modos MHD, y su impacto en confinamiento en los stellarators TJ-II y W7-X: Difference between revisions

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TJ-II is the flagship of the National Laboratory for Fusion (LNF) and part of the Spanish ICTS catalogue. As members of the LNF, the proponents have full access to TJ-II, where the several diagnostics relevant for the study are available. In particular, a steerable Doppler reflectometry (DR) system provides the simultaneous measurement of fluctuations and flows, allowing for very detailed studies of turbulence, critical for the characterization of drift modes. As well, a helical array of Mirnov coils has been recently added to TJ-II, allowing for a detailed study of AEs, including their complex 3D structure in non-axisymmetric devices. W7-X is sited in the Max-Planck-Institut für Plasmaphysik (IPP) in Greifswald. W7X is the largest stellarator in the world and was built to reproduce a number of reactor-relevant features such as optimized magnetic field, high beta operation or actively cooled island divertor. Since 2015, the LNF has collaborated with IPP in the development of a DR system, which has already been successfully operated in previous experimental campaigns. Presently, this system is being refurbished including a number of improvements which will greatly expand the scope of the experimental measurements which can be carried out. As well, a new diagnostic has been included in the proposal: A Charge eXchange Recombination Spectroscopy (CXRS) system, which will complement the investigation of plasma flows carried out by the DR. These diagnostics will ensure access to experimental data in reactor-relevant conditions during the forthcoming OP2.1-OP2.3 campaigns, scheduled for the 2022-2024 period. On top of these purely experimental activities, we outline the data analysis and comparison of physical results to numerical simulations of turbulence (gyrokinetic codes), AE stability (gyrofluid codes), plasma profiles (neoclassical codes) or synthetic DR response to plasma conditions (2D full wave code).
TJ-II is the flagship of the National Laboratory for Fusion (LNF) and part of the Spanish ICTS catalogue. As members of the LNF, the proponents have full access to TJ-II, where the several diagnostics relevant for the study are available. In particular, a steerable Doppler reflectometry (DR) system provides the simultaneous measurement of fluctuations and flows, allowing for very detailed studies of turbulence, critical for the characterization of drift modes. As well, a helical array of Mirnov coils has been recently added to TJ-II, allowing for a detailed study of AEs, including their complex 3D structure in non-axisymmetric devices. W7-X is sited in the Max-Planck-Institut für Plasmaphysik (IPP) in Greifswald. W7X is the largest stellarator in the world and was built to reproduce a number of reactor-relevant features such as optimized magnetic field, high beta operation or actively cooled island divertor. Since 2015, the LNF has collaborated with IPP in the development of a DR system, which has already been successfully operated in previous experimental campaigns. Presently, this system is being refurbished including a number of improvements which will greatly expand the scope of the experimental measurements which can be carried out. As well, a new diagnostic has been included in the proposal: A Charge eXchange Recombination Spectroscopy (CXRS) system, which will complement the investigation of plasma flows carried out by the DR. These diagnostics will ensure access to experimental data in reactor-relevant conditions during the forthcoming OP2.1-OP2.3 campaigns, scheduled for the 2022-2024 period. On top of these purely experimental activities, we outline the data analysis and comparison of physical results to numerical simulations of turbulence (gyrokinetic codes), AE stability (gyrofluid codes), plasma profiles (neoclassical codes) or synthetic DR response to plasma conditions (2D full wave code).
== Main Results ==
This project advanced the experimental and theoretical understanding of plasma turbulence, flows, and magnetohydrodynamic (MHD) activity in the stellarators W7-X and TJ-II through coordinated diagnostic upgrades, systematic experimentation, and integrated modeling.
At W7-X, upgraded Doppler Reflectometer (DR) systems were installed, commissioned, and successfully operated during the OP2 experimental campaigns. Two systems delivered optimal performance, providing high-resolution measurements of turbulence amplitude, perpendicular flows, and long-range correlations, while a third enabled correlation studies despite alignment limitations. A wide range of magnetic configurations—including reduced mirror, low rotational transform, low magnetic shear, and island-chain scenarios—was explored using standardized density and heating power scans. Turbulence levels were found to increase with plasma density and ECH power, while edge turbulence decreased with increasing rotational transform. A clear correlation emerged between reduced edge turbulence, enhanced radial electric field shear, and improved global energy confinement time. Post-pellet enhanced confinement regimes were successfully reproduced, showing turbulence stabilization and increased plasma flow. Dedicated correlation experiments enabled the first direct measurement of zonal flows in a large stellarator, confirming their dependence on density and heating power and validating gyrokinetic predictions. CXRS flow measurements, refined through improved self-calibration, showed significantly better agreement with neoclassical calculations.
In TJ-II, three experimental campaigns ensured full operation of the DR and the helical Mirnov coil array. Dedicated calibration experiments and advanced numerical tools supported reliable magnetic fluctuation analysis. A comprehensive database of NBI-driven Alfvén Eigenmodes was obtained under varying heating and rotational transform conditions. Synthetic magnetic diagnostics and linear MHD simulations improved mode interpretation. While no direct impact of Alfvén activity on turbulence was confirmed, indirect effects linked to injection direction and fast-ion losses were identified. Pellet experiments reproduced enhanced confinement regimes similar to W7-X, though modeling suggests different underlying mechanisms. Overall, the project achieved near-complete fulfillment of its objectives and delivered substantial advances in stellarator turbulence and flow physics.


== References ==
== References ==
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