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

LNF - Nationally funded project

Title: Estudio experimental de flujos, turbulencia y modos MHD, y su impacto en confinamiento en los stellarators TJ-II y W7-X

Reference: PID2021-125607NB-I00

Programme and date: Proyectos de Generación de Conocimiento Año 2021

Programme type (Modalidad de proyecto): Proyectos de investigación no orientada

Area/subarea (Área temática / subárea): Ciencias Físicas / Física y sus aplicaciones

Principal Investigators: Teresa Estrada and Daniel Carralero

Project type: Proyecto individual

Start-end dates: 01/01/2022 - 31/12/2024

Financing granted (direct costs): 160.000,00 €

Description of the project

The main objective of the present proposal is to study a family of instabilities present in the strongly magnetized plasmas required for the development of a practical nuclear fusion reactor, which range from the electrostatic drift turbulence typically dominating transport among thermal populations of confined species, to magneto-hydrodynamic modes destabilized by fast ion populations, such as the Alfvèn Eigenmodes, as well as the potential interactions between them. Finding mechanisms by which these instabilities can be suppressed or controlled in reactor-relevant conditions is critical for the achievement of the high plasma confinement required for an economical exploitation of nuclear fusion. With this aim, we propose the experimental characterization of these instabilities and the plasma conditions driving them, followed by its interpretation under the best available theoretical frameworks. This project can be seen as a continuation and expansion of the previous FIS2017-88892-P grant, in which related research was carried out in the TJ-II and Wendelstein 7-X (W7-X) stellarators, including the commissioning and operation of several relevant diagnostics. From there, our work plan assesses the current state of the research and defines several lines of work such as turbulence stabilization during post-pellet phases, fluctuation and potential asymmetries, flow departure from neoclassical theory, NBI destabilization of AE modes, detection and characterization of zonal flows, etc.

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.

Dissemination of project results (peer-reviewed publications and conference presentations)

Peer-reviewed publications:

[1] D. Carralero, T. Estrada, E. Maragkoudakis, T. Windisch, J. A. Alonso, J. L. Velasco, O. Ford, M. Jakubowski, S. Lazerson, M. Beurskens, S. Bozhenkov, I. Calvo, H. Damm, G. Fuchert, J.M. García-Regaña, U. Höfel, N. Marushchenko, N. Pablant, E. Sánchez, H.M. Smith, E. Pasch, and T. Stange. Plasma Phys. Control. Fusion 64, 044006 (2022)

[2] E. Ascasíbar, F. Lapayese, A. Soleto, A. Jiménez-Denche, Á. Cappa, P. Pons-Villalonga, A. B. Portas, G. Martín, J.M. Barcala, R. García-Gómez, M. Chamorro, L. Cebrián, R. Antón, L. Bueno, C. Reynoso, V. Guisse, and A. López-Fraguas. Rev. Sci. Instrum. 93, 093508 (2022)

[3] J. A. Alonso, O.P. Ford, L. Vanó, S. Äkäslompolo,
 S. Buller, R. McDermott, H. Smith, J. Balzuhn, 
C.D. Beidler, M. Beurskens, S. Bozhenkov, K.J. Brunner, I. Calvo, D. Carralero, A. Dinklage, T. Estrada, G. Füchert, J. Geiger, J. Knauer, A. Langenbert, N. Pablant, E. Pasch, P. Zs Poloskei, J.L. Velasco,
T. Windisch and the W7-X team. Nuclear Fusion 62, 106005 (2022)

[4] Sunn Pederdsen, I. Abramovic, P. Agostinetti, ..., A. Alonso, …, E. Ascasíbar, …, A. Cappa, …, D. Carralero, …, T. Estrada, et al. Nuclear Fusion 62, 042022 (2022)

[5] C. Hidalgo, E. Ascasíbar, D. Alegre, A. Alonso, ..., A. Cappa, D. Carralero, …, T. Estrada, et al. Nuclear Fusion 62, 042025 (2022)

[6] E. Maragkoudakis, D. Carralero, T. Estrada, T. Windisch, Y. Gao, C. Killer, M. Jakubowski, A. Puig Sitjes, F. Pisano, H. Sándor, M. Vecsei, S. Zoletnik, A. Cappa, and the Wendelstein 7-X team. Nuclear Fusion 63, 026011 (2023)

[7] I. García-Cortés, K. J. McCarthy, T. Estrada, V. Tribaldos, D. Medina-Roque, B. van Milligen, E. Ascasíbar, R. Carrasco, A.A. Chmyga, R. García, J. Hernández-Sánchez, C. Hidalgo, A.S. Kozachek, F. Medina, M. A. Ochando, J. L. de Pablos, N. Panadero, I. Pastor, and TJ-II Team. Phys. Plasmas 30, 072506 (2023)

[8] A. González-Jerez, J.M. García-Regaña, I. Calvo, D. Carralero, T. Estrada, E. Sánchez, M. Barnes, and the W7-X team. Nuclear Fusion 64, 076029 (2024)

[9] O. Grulke, C. Albert, J.A. Alcusón, …, A. Alonso, …, E. Ascasíbar, …, A. Cappa, …, D. Carralero, …, T. Estrada, … Nuclear Fusion 64, 112002 (2024)

[10] A. Alonso, D. Alegre, J. Alonso, …, E. Ascasíbar, …, A. Cappa, D. Carralero, …, T. Estrada, …, J.M. Fontdecaba, …, J. Martínez, …, A. Pereira, …, P. Pons, A.B. Portas, …, … J. de la Riva, et al., Nuclear Fusion 64, 112018 (2024)

[11] P. Pons-Villalonga, Á. Cappa, J. Martínez-Fernández, O. S. Kozachok, E. Ascasíbar. Review of Scientific Instruments 96 063502 (2025)

[12] D. Carralero, T. Estrada, J M García-Regaña, E Sánchez, T. Windisch, A. Alonso, E. Maragkoudakis, C Brandt, K J Brunner et al. Physical Review Research ,7, L022009 (2025).

[13] J. de la Riva Villén, J A Alonso, O P Ford, T Romba, E Maragkoudakis, D Carralero, T Estrada, T Windisch, J L Velasco, H M Smith, D Gradic, P Poloskei and the W7-X Team. Plasma Phys. Control. Fusion 68 015015 (2026).

Conference presentations:

[1] D. Carralero et al., (Invited talk) “Recent turbulence investigations in the TJ-II and W7-X stellarators: experimental characterization and 3D code-based interpretation”, 23rd International Stellarator and Helliotron Workshop (ISHW), Warsaw, Poland, June 2022.

[2] A. Cappa et al., (Invited talk) “Linear Stability Analysis of TJ-II stellarator NBI-driven Alfvén Eigenmodes in ECRH and ECCD experiments”, 23rd International Stellarator and Helliotron Workshop (ISHW), Warsaw, Poland, June 2022.

[3] P. Pons et al. (Poster) “New in-vessel helical arrays of magnetic coils in TJ-II, calibration and preliminary results”, 23rd International Stellarator and Helliotron Workshop (ISHW), Warsaw, Poland, June 2022.

[4] E. Maragkoudakis et al.(Poster) “On the SOL radial electric field, divertor heat fluxes and plasma edge turbulence of W7-X”, 23rd International Stellarator and Helliotron Workshop (ISHW), Warsaw, Poland, June 2022.

[5] T. Estrada, et al. (Invited talk) “Radial electric fields, turbulence and transport studies in W7-X and TJ-II”, 48th EPS Conference on Plasma Physics, June 27 - July 1, on-line conference (2022)

[6] T. Estrada, et al. (Oral) “Effect of internal magnetic islands on turbulence and flows in W7-X and TJ-II”, 15th International Reflectometry Workshop IRW15, ITER, St Paul Lez Durance Cedex, France (2022)

[7] E. Maragkoudakis, et al. (Oral) “Use of the field line tracing code for the interpretation of Doppler reflectometry measurements in W7-X”, 15th International Reflectometry Workshop IRW15, ITER, St Paul Lez Durance Cedex, France (2022)

[8] D.Carralero and C. Killer (Oral) “Task Force-III: Wendelstein 7-X optimization” OP2.1 Program Planning Workshop, IPP Greifswald, September, 2022

[9] D.Carralero and M. Nunami (Oral) “Core transport in a stellarator reactor. Learning from present-day experiments and main questions ahead”, 23rd Coordinated Working Group Meeting, Kyoto, Japan, June, 2023

[10] P. Pons et al., (Poster) “Measurements of spatial periodicity and radial structure of NBI-driven Alfvén Eigenmodes in the TJ-II stellarator”, 49th EPS conference on Plasma Physics, Bordeaux, France, July 2023.

[11] J. de la Riva et al. (Poster) “Characteristic profiles of radial electric field and parallel velocity obtained in W7-X using charge exchange recombination spectroscopy”, 49th EPS conference on Plasma Physics, Bordeaux, France, July 2023.

[12] A. Cappa et al., (Poster) “Fast ion physics in the TJ-II stellarator: experiments and model validation activities” 29th IAEA Fusion Energy Conference (IAEA-FEC), London, October 2023.

[13] E. Maragkoudakis, D. Carralero, T. Estrada, T. Windisch et al (Oral). “Characterization of Doppler Reflectometry profiles for various Wendelstein 7-X scenarios” 16th International Reflectometry Workshop, Greifswald, Germany, May 2024.

[14] D. Carralero, T. Estrada, J. M. García-Regaña, E. Sánchez et al. (Invited) “First experimental observation of zonal flows in the optimized stellarator Wendelstein 7-X” 50th EPS Conference on Plasma Physics, Salamanca, Spain, July 2024

[15] T. Estrada, Á. Cappa, ..., J. de la Riva et al (Invited). “Impact of radial electric field, turbulence and impurity transport on plasma performance in co- and counter-NBI heating scenarios in TJ-II”, 24th International Stellarator and Heliotron Workshop (ISHW), Hiroshima, Japan, September 2024.

[16] D. Carralero, T. Estrada, J. M. García-Regaña, E. Sánchez et al (Invited). “First experimental observation of zonal flows in the optimized stellarator Wendelstein 7-X” 24th International Stellarator and Heliotron Workshop, Hiroshima, Japan, September 2024.

[17] P. Pons et al, "Characterization of the spatial structure of NBI driven shear Alfven waves in the TJ-II stellarator". 18th Technical Meeting on Energetic Particles (TMEP2025), Seville, Spain, March 2025.

[18] D. Carralero, on behalf of the W7-X team (invited) “Machine report: W7-X Core Transport after OP2.3” 25th Coordinated Working Group Meeting, Princeton, USA, June, 2025.

[19] S. Vaz Mendez, ...,Á. Cappa, P. Pons villalonga, et al. (Poster) “Discovering Alfvén Mode Excitation by ITG Turbulence”, 51st EPS conference on Plasma Physics, Vilnius, Lithuania, July 2025.

[20] Á. Cappa, P. Pons villalonga et al (Oral), “Spatial structure of NBI-driven shear Alfvén waves in the TJ-II stellarator: modeling vs. experimental results” 51st EPS conference on Plasma Physics, Vilnius, Lithuania, July 2025.

[21] J. de la Riva et al. (Oral) “Systematic study of CXRS ion flow measurements in the W7-X stellarator towards a validation of neoclassical theory”, 51th EPS conference on Plasma Physics, Vilnius, Lithuania, July 2025.

[22] J. M. García-Regaña, J. A. Alonso, …, E. Ascasíbar1, …, A. Cappa, D. Carralero, et al. (Overview) “Transport in high-performance plasmas of the TJ-II stellarator: from first-principles simulations to experimental validation”, 30th IAEA Fusion Energy Conference (FEC2025), Chendu, China, October, 2025

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