Magnetic configuration effects on the Wendelstein 7-X stellarator

Abstract

The two leading concepts for confining high-temperature fusion plasmas are the tokamak and the stellarator. Tokamaks are rotationally symmetric and use a large plasma current to achieve confinement, whereas stellarators are non-axisymmetric and employ three-dimensionally shaped magnetic field coils to twist the field and confine the plasma. As a result, the magnetic field of a stellarator needs to be carefully designed to minimize the collisional transport arising from poorly confined particle orbits, which would otherwise cause excessive power losses at high plasma temperatures. In addition, this type of transport leads to the appearance of a net toroidal plasma current, the so-called bootstrap current. Here, we analyse results from the first experimental campaign of the Wendelstein 7-X stellarator, showing that its magnetic-field design allows good control of bootstrap currents and collisional transport. The energy confinement time is among the best ever achieved in stellarators, both in absolute figures (τE > 100 ms) and relative to the stellarator confinement scaling. The bootstrap current responds as predicted to changes in the magnetic mirror ratio. These initial experiments confirm several theoretically predicted properties of Wendelstein 7-X plasmas, and already indicate consistency with optimization measures.

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Fig. 1: Outline of Wendelstein 7-X and its magnet system.
Fig. 2: Radial profiles of important magnetic field properties.
Fig. 3: Waveforms of plasma discharges.
Fig. 4: Energy confinement times of Wendelstein 7-X.
Fig. 5: Temperature and density profiles for different magnetic configurations.
Fig. 6: Neoclassical modelling: radial electric field, energy fluxes and bootstrap current profiles.

Change history

  • 11 September 2018

    In the version of this Article originally published, and in the associated Publisher Correction, the members of the W7-X Team were not included. All versions of the Article, and the Publisher Correction, have now been amended to include these team members.

  • 03 July 2018

    In the version of this Article originally published, A. Mollén’s affiliation was incorrectly denoted as number 10; it should have been 1. Throughout the Article, some technical problems in typesetting meant that the tilde symbol above b and one instance of a superscript 2 were too high to be visible; see the correction notice for details. Finally, the citation to ref. 35 on page one of the Supplementary Information was incorrect; it should have been to ref. 36. These issues have now been corrected.

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Acknowledgements

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 under grant agreement 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. This work is partially supported by the US Department of Energy under a project agreement with the Max Planck Institute for Plasma Physics.

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A.D., C.D.B., P.H., T.S.P., R.C.W. and T.K. wrote the paper. A.D., T.S.P., S.B., F.E. and J.G. prepared the configuration changes and the discharge program. A.D., H.M., Y.T., T.A., A.A., C.D.B., F.E., Y.F., J.G., A.M., N.M., H.M.S. and O.S. did modelling and data validation. K.R., B.B., B.B., A.C., G.F., M.H., U.H., M.J., J.K., G.K., A.K.F., M.K., A.L., H.L., U.N., H.N., E.P., N.P., L.R., T.S., T.S., G.W., T.W., G.W. and D.Z. did measurements and data analysis.

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Correspondence to A. Dinklage.

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Dinklage, A., Beidler, C.D., Helander, P. et al. Magnetic configuration effects on the Wendelstein 7-X stellarator. Nature Phys 14, 855–860 (2018). https://doi.org/10.1038/s41567-018-0141-9

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