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

Error fields have been detected and quantified using the flux surface mapping diagnostic system on Wendelstein 7-X (W7-X). A low-field ‘’ magnetic configuration (), sensitive to error fields, was developed in order to detect their presence using the flux surface mapping diagnostic. In this configuration, a vacuum flux surface with rotational transform of n/m  =  1/2 is created at the mid-radius of the vacuum flux surfaces. If no error fields are present a vanishingly 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 m intrinsic island chain with a 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. [ABSTRACT FROM AUTHOR]