The Role of Perturbative Fields in Wendelstein 7-X*

The application of perturbative and corrective magnetic fields in Wendelstein 7-X plays a central role in the operation and physics exploitation of the experiment.  In the initial limiter campaign, the set of five copper trim coils were utilized to confirm the small magnitude of the n=1 error field (S.A. Lazerson et al. Nuclear Fusion 56 106005, T.S. Pedersen et al. Nature Comm. 7).  The first divertor campaign allowed direct assessment of the n/m=1/1 error field through flux surface mapping, allowing for symmetrization of divertor heat loads.  Application of symmetrizing fields allowed achievement of pulse lengths in excess of 25 seconds and injected energies of 75 MJ.  The trim coil perturbative fields were also used to calibrate and verify the magnetic diagnostics on W7-X.  In-vessel control coils were used to modify the n/m=5/5 divertor island chain allowing for divertor strike line actuation and assessment of island width on divertor performance.  In this presentation, we review measurements made in previous campaigns and examine the possibility of future scientific exploitation of perturbative fields in W7-X.