Speaker
Matthias Willensdorfer
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/I2.107.pdf
Modification of the local edge stability by the plasma response to
non-axisymmetric magnetic perturbations in ASDEX Upgrade
M. Willensdorfer1 , T.B. Cote2 , C. Hegna2 , W. Suttrop1 , S.S. Denk1 , M. Dunne 1 , R. Fischer1 ,
L. Giannone1 , C.J. Ham3 , A. Kirk3 , F. Orain1 , D.A. Ryan3 , Strumberger1 ,
N. Wang4 , H. Zohm1 , the EUROfusion MST1 Team[*] and the ASDEX Upgrade Team
1 Max Planck Institute for Plasma Physics, 85748 Garching, Germany, 2 University of
Wisconsin-Madison, Madison, Wisconsin 53706, USA 3 CCFE, Culham Science Centre,
Abingdon, Oxon, OX14 3DB, UK, 4 AEET, SEEE, HUST, Wuhan 430074, P R China
The application of externally applied non-axisymmetric magnetic perturbation (MP)-fields
is a promising method to mitigate or even suppress the repetitive impulsive energy loss due to
edge localised modes (ELMs) which is expected to be intolerable in low-collisionality H-mode
plasmas of future fusion devices.
The mitigation of ELMs and the consequent reduction of the pedestal pressure (density
’pump-out’) are strongly related to the amplification of the externally applied MP-field by
marginally stable ideal kink modes at the edge. The 3D boundary displacement from these
kink modes is characterised by toroidally localised diagnostics with high radial resolution in
combination with toroidally rotating n=2 MP-fields. The important role of these kink modes
in the ELM mitigation is supported by the following findings [1]: 1. Same dependence of the
measured displacement and ELM behaviour on the applied poloidal mode spectrum, 2. Agree-
ment of the measured displacements with ideal 3D magnetohydrodynamic (MHD) code predic-
tions (e.g. MARS-F, VMEC), 3. Calculated displacements from the vacuum field approximation
clearly underestimate the experimental observations.
We also demonstrate experimentally that the induced 3D MHD geometry modifies the lo-
cal stability at the edge. An additional ideal MHD mode with ballooning structure in-between
ELMs is observed only at certain field-lines (helical position) within the 3D geometry in the
H-mode edge barrier region [2]. Infinite-n ballooning stability analysis using a realistic 3D
equilibrium from VMEC shows that the dominant reason for the local ballooning destabilisa-
tion is the 3D distortion of the local magnetic shear. Our investigations suggest that the observed
reduction of the edge pedestal pressure in H-mode due to the application of MPs results from
a change of the edge stability boundary introduced by the 3D perturbation of the local mag-
netic shear. Additionally, not only the observed ballooning mode before the ELM, but also the
dynamics of the following ELM crashes are influenced by the local lower stability.
[*] H. Meyer et al, Nucl. Fusion 57,102014 (2017)
[1] M. Willensdorfer et al, Nucl. Fusion 57, 116047 (2017)
[2] M. Willensdorfer et al, Phys. Rev. Lett. 119, 085002 (2017)
