Correction of small error fields at high BetaN in ASDEX Upgrade

Tokamak plasmas are able to amplify even very small error fields when operating close to the MHD limits. As a result of this process, ideal kink modes and/or a tearing modes can become unstable and limit the plasma performance. In our experiments, in ASDEX Upgrade, the plasma is close to the pressure limit. The plasma has ?N values around the "no-wall" boundary. In this situation, the ideal kink becomes the main limiting factor. Previous experiments demonstrate that the dominant error field comes from the feedthroughs for the poloidal field coils [1, 2]. Although the error field itself is very small (Berr Btor <= 10-5 / ), correction of the resonant components of the error field improves plasma performance. The required correction was calculated using the electromagnetic code CAFÉ, which takes into account all relevant currents and realistic geometry of the conductors [3]. It minimizes resonant components (2,1) and (3,1) at the position of the resonant surfaces. The plasma with correction archives ?N = 3.2 - 3.3. The reference case without correction has ?N = 3.0. The plasma rotation is also higher in the case with error field correction. It is also shown experimentally that the correction of small non-resonant components is not important and do not influence on the results. The non-resonant torque from these non-resonant components of the error fields is negligible in our case.