Speaker
Matthias Otte
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.1007.pdf
Overview of Magnetic Flux Surface Measurements at W7-X
M. Otte1, S. Bozhenkov1, V. Bykov1, T. Andreeva1, M. Endler1, S. A. Lazerson2 and the
W7-X team
1
Max-Planck-Institut für Plasmaphysik, Greifswald, Germany
2
Princeton Plasma Physics Laboratory, Princeton, USA
The existence of closed and nested magnetic flux surfaces is mandatory in magnetic
confinement plasma machines and is one of the optimization criteria at the Wendelstein 7-X
(W7-X) stellarator. The existence and quality of vacuum magnetic flux surfaces was already
confirmed during the initial operation phase OP1.1, and the error field was found in the
expected range as well [1, 2]. However, a comparison with field line tracing simulations
taking into account the ideal but also the as-built geometry of the coils revealed small but
measurable deviations in the radial profile of the rotational transform /2. A systematic
decrease of iota of / ~ 1-2% with respect to the ideal coil geometry could be derived from
the measurements. The effect, also observed at the predecessor experiment W7-AS [3], is
related to the deformation of the non-planar coils of up to ~ 10-15 mm under electromagnetic
loads, which are generating the rotational transform.
In the first divertor campaign OP1.2a, performed in the second half of 2017, the accessible
magnetic configuration space was extended. Additional flux surface measurements were
performed for new magnetic configurations. The experimental results are compared with
different field coil models. Furthermore, measurements addressing the potential error field
have been conducted again. The dominating n, m = 1 Fourier harmonic was characterised in
amplitude and phase for different magnetic configurations. Utilizing the flux surface
diagnostic, it could be shown that this error field component can be compensated with the
trim coil set installed outside the plasma vessel, which results in a symmetrisation of the heat
loads on the divertors during plasma operation [4].
References
[1] M. Otte et al., Plasma Phys. Control. Fusion 58 (2016)
[2] T.S. Pedersen et al., Nature Comm. 7 (2016)
[3] M. Otte et al., Stellarator News 100 (2005)
[4] S.A. Lazerson et al., this conference
