Highlights from the division Stellarator Edge and Divertor Physics

2016

The evidence: the fluorescent rod makes closed, nested magnetic surfaces visible – the magnetic field cage for the plasma is exactly as it should be.

Magnetic field of Wendelstein 7-X exact to a hundred-thousandth
Evaluation of magnetic field measurement / Overview in “Nature Communications”

The shape of the magnetic field needed in the Wendelstein 7-X stellarator fusion device was achieved to a precision of a hundred-thousandth. This evaluation of the first physical results with the new machine – these having been achieved before Wendelstein 7-X went into operation at Max Planck Institute for Plasma Physics in Greifswald in December 2015 – appeared today in the online journal, “Nature Communications”.

The article describes the scientific path to the complex field structure of Wendelstein 7-X tailored for good confinement of the plasma. How precisely the field structure needed – a setup of closed magnetic surfaces nested in one another – can be generated by the specially shaped superconducting stellarator coils is now clear. Deviations from the calculated target shape are within a hundred-thousandth: A magnetic field line traced for a distance of 100 metres, i.e. the extent of a football pitch, will be spot on target to within a millimetre.

This was confirmed with a highly sensitive measuring method: For this purpose, a thin electron beam is injected and moves along a field line in circular tracks through the evacuated plasma vessel. If a fluorescent rod is moved through the vessel cross-section, light spots are created when the electron beam hits the rod. In the camera recording, the structure of the magnetic field gradually becomes visible – the expected elliptical surfaces nested in one another.

Proof of exact magnetic surfaces thus achieves the first aim of Wendelstein 7-X. There are, however, many questions that still have to be answered by the Wendelstein 7-X team to clarify whether the stellarator is the right concept for fusion energy: “That task”, according to the authors, “has just started”.

    

1.
T. Sunn Pedersen, M. Otte, S. Lazerson, P. Helander, S. Bozhenkov, C. Biedermann, T. Klinger, R. C. Wolf, H. -S. Bosch & The Wendelstein 7-X Team
Confirmation of the topology of the Wendelstein 7-X magnetic field to better than 1:100,000
Nature Communications 7, 13493 (2016)
2.
Otte, M., Assmus, D., Biedermann, C., Bozhenkov, S., Bräuer, T., Dudek, A., et al.
Setup and initial results from the magnetic flux surface diagnostics at Wendelstein 7-X
Plasma Physics and Controlled Fusion, 58: 064003 (2016)
3.
Lazerson, S. A., Otte, M., Bozhenkov, S., Biedermann, C., Pedersen, T. S., & W7-X Team
First measurements of error fields on W7-X using flux surface mapping
Nuclear Fusion, 56: 106005 (2016)

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