First measurements of error fields on W7-X using 
flux surface mapping

Authors:  Samuel A. Lazerson, Matthias Otte, Sergey Bozhenkov,
 Christoph Biedermann, Thomas Sunn Pedersen and the
W7-X Team

Abstract:  Error fields have been detected and measured using the flux surface mapping diagnostic system on Wendelstein 7-X (W7-X). A low-field  magnetic configuration , sensitive to error fields, was developed in order to detect their presence using the flux surface mapping diagnostic. In this configuration, a vacuum flux surface with rotational transform of n / m = 1 / 2 is created at the mid-radius of the
configuration. If no error fields are present a small n / m = 5 / 10 island chain should be present. Modeling indicates that if an n = 1 perturbing field is applied by the trim coils, a large n / m = 1 / 2 island chain will be opened. This island chain is used to create a perturbation large enough to be imaged by the diagnostic. Phase and amplitude scans of the applied field allow the measurement of a small ~ 0.04 m intrinsic island chain with a 140 ° phase relative to the first module of the W7-X experiment. These error fields are determined to be small and easily correctable by the trim coil system.
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Submitted to:  Nuclear Fusion
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Download PPPL-5233 (pdf 8.1 MB 21 pp)
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