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17–22 Oct 2016
Kyoto International Conference Center
Japan timezone

Plasma response of external magnetic perturbations at the edge: Comparisons between measurements and 3D MHD models

20 Oct 2016, 14:00
4h 45m
Kyoto International Conference Center

Kyoto International Conference Center

Takaragaike, Sakyo-ku, Kyoto 606-0001 Japan
Poster EXS - Magnetic Confinement Experiments: Stability Poster 6

Speaker

Dr Matthias Willensdorfer (Max Planck Institute for Plasma Physics)

Description

At ASDEX Upgrade ELM mitigation using external magnetic perturbations has been achieved at high plasma densities (n_e/n_GW > 0.65, corresponding to nu*>1.2) and, more recently, at low pedestal collisionality (nu*<0.4) accompanied with density pump-out. To investigate the interaction between the plasma response and ELM mitigation, comprehensive experiments using various plasma configurations have been conducted. These studies indicate that the optimum poloidal spectrum for ELM mitigation does not show a maximum of the magnetic field pitch-aligned component. Instead, it is aligned with the mode at the edge that is most strongly amplified by the plasma as calculated using magnetohydrodynamic (MHD) response codes. These experimental investigations in comparison with MARS-F are consistent with previous observations and underline the hypothesis that the plasma response around the X-point causes the ELM mitigation. In order to measure the plasma response, we combined rigid rotating MP fields and measurements from toroidally localized high resolution diagnostics. Electron cyclotron emission (ECE) diagnostics, among others, have been used to determine the amplitude, the penetration and the poloidal mode structure of the flux surface displacements. To interpret the ECE measurements accurately, forward modeling of the radiation transport has been extended with ray tracing. The measurements are compared to synthetic data generated by combining the said forward model and a 3D ideal MHD equilibrium calculated by VMEC. The measured penetration of the helical displacement is in good agreement with VMEC, whereas the measured amplitudes in the midplane are slightly larger. The measured amplitudes also exceed the vacuum field calculations, which indicates the presence of an amplified kink response at the edge. Although the calculated magnetic structure of this edge kink peaks at poloidal mode numbers larger than the resonant components |m|>|nq|, the displacement derived from ECE-imaging appears as mostly resonant. This is expected from ideal MHD in the proximity of rational surfaces. Both, VMEC and MARS-F calculations reproduce this experimental observation. Further rigid rotating field experiments using different poloidal spectra of the external MP suggests that the same least stable modes are excited by the MP- field.
Country or International Organization Germany
Paper Number EX/P6-25

Primary author

Dr Matthias Willensdorfer (Max Planck Institute for Plasma Physics)

Co-authors

Dr Andrew Kirk (Culham Centre for Fusion Energy) Mr Branka Vanovac (FOM-Institute DIFFER, Dutch Institute for Fundamental Energy Research) Dr David Ryan (York Plasma Institute, Department of Physics, University of York) Mr Dominik Brida (Max Planck Institute for Plasma Physics, Garching) Dr Eleonora Viezzer (Max-Planck-Institut fuer Plasmaphysik) Dr Erika Strumberger (Max Planck Institute for Plasma Physics) Mr Florian Laggner (Institute of Applied Physics, TU Wien, Fusion@ÖAW) Dr Francois Orain (Max Planck Institute for Plasma Physics) Prof. Hartmut Zohm (Max-Planck-Institut für Plasmaphysik) Dr Ivo Classen (FOM institute DIFFER) Mr Marco Cavedon (Max-Planck-Institute for Plasma Physics) Dr Mike Dunne (IPP-Garching) Dr Rainer Fischer (Max Planck Institute for Plasma Physics) Mr Severin Denk (Max Planck Institute for Plasma Physics) Dr Sina Fietz (Max Planck Institute for Plasma Physics) Mr Tomas Odstrcil (Max Planck Institute for Plasma Physics) Dr Wolfgang Suttrop (Max-Planck-Institut für Plasmaphysik) Dr Yueqiang Liu (CCFE Culham Science Centre)

Presentation materials