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M. Willensdorfer, E. Strumberger, W. Suttrop, M. Dunne, R. Fischer, G. Birkenmeier, D. Brida, M. Cavedon, S. Denk, V. Igochine, L. Giannone, A. Kirk, J. Kirschner, A. Medvedeva, T. Odstrcil, D. Ryan, . ASDEX Upgrade Team, -. EUROfusion MST1 Team:
"Three dimensional boundary displacement due to stable ideal kink modes excited by external n=2 magnetic perturbations";
Nuclear Fusion, 57 (2017), 1160471 - 11604712.

In low-collisionality (ν⋆) scenarios exhibiting mitigation of edge localized mode (ELMs),
stable ideal kink modes at the edge are excited by externally applied magnetic perturbation
(MP)-fields. In ASDEX Upgrade these modes can cause three-dimensional (3D) boundary
displacements up to the centimeter range. These displacements have been measured using
toroidally localized high resolution diagnostics and rigidly rotating n = 2 MP-fields with
various applied poloidal mode spectra. These measurements are compared to non-linear
3D ideal magnetohydrodynamics (MHD) equilibria calculated by VMEC. Comprehensive
comparisons have been conducted, which consider for instance plasma movements due to
the position control system, attenuation due to internal conductors and changes in the edge
pressure profiles.
VMEC accurately reproduces the amplitude of the displacement and its dependencies on
the applied poloidal mode spectra. Quantitative agreement is found around the low field side
(LFS) midplane. The response at the plasma top is qualitatively compared. The measured and
predicted displacements at the plasma top maximize when the applied spectra is optimized for
ELM-mitigation. The predictions from the vacuum modeling generally fails to describe the
displacement at the LFS midplane as well as at the plasma top. When the applied mode spectra is
set to maximize the displacement, VMEC and the measurements clearly surpass the predictions
from the vacuum modeling by a factor of four. Minor disagreements between VMEC and the
measurements are discussed. This study underlines the importance of the stable ideal kink modes
at the edge for the 3D boundary displacement in scenarios relevant for ELM-mitigation.

ELM mitigation, RMPs, ideal MHD, 3D Tokamaks