Speaker
Wolfgang Albert Suttrop
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/O2.107.pdf
Experimental conditions for suppressing Edge Localised Modes by
magnetic perturbations in ASDEX Upgrade
W Suttrop1 , A Kirk2 , V Bobkov1 , M Cavedon1 , M Dunne1 , R M McDermott1 , H Meyer2 ,
R Nazikian3 , C Paz-Soldan4 , D A Ryan2 , E Viezzer1,5 , M Willensdorfer1 ,
the ASDEX Upgrade∗ and MST1† Teams
1 Max Planck Institute for Plasma Physics, Boltzmannstrasse 2, 85748 Garching, Germany
2 CCFE Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK
3 Princeton Plasma Physics Laboratory, PO Box 451, Princeton, NJ 08543-0451, USA
4 General Atomics, PO Box 85608, San Diego, CA 92186-5608, USA
5 Dept. of Atomic, Molecular and Nuclear Physics, University of Seville, 41012 Seville, Spain
Full suppression of Edge Localised Modes (ELMs) by magnetic perturbations (MP) in high-
confinement mode (H-mode) plasmas has been obtained in ASDEX Upgrade (AUG) in a shape-
match experiment with DIII-D [Nazikian, IAEA FEC 2016]. In contrast to previous scenarios
where ELMs were mitigated by MP, full ELM suppression in AUG requires stronger shaping.
This finding has been attributed to larger pedestal plasma pressure, which in turn lead to stronger
amplification of the external MP by marginally stable, edge localised, kink-peeling modes. Re-
cent experiments in AUG aimed to identify critical parameters for accessing ELM suppression:
Safety factor, plasma rotation, plasma edge density and collisionality. Edge safety factor scans
in the range of q95 = 3.6 − 4.2 showed a window q95 = 3.57 − 3.95 for ELM suppression with
n = 2 MP.
In the ELM suppression scenario used so far, there is a clear maximum edge density (3.3 ×
1019 m−3 ) for ELM suppression, which can also be expressed as a collisionality limit at ν∗i,ped =
0.36. Our present data set is still too sparse to discriminate between these quantities. In H-
modes with ELM mitigation or ELM suppression, the pedestal pressure is typically 30% below
that of ELMy H-mode with MP switched off and still somewhat below that of phases with
MP-mitigated ELMs.
The resonant, field-aligned MP components near the top of the H-mode edge gradient region
are believed to be essential for ELM suppression [Wade, Nucl. Fus. 2015] and their strength in
turn depends (in two-fluid MHD) on absence or presence of electron flow across the magnetic
field (ve,⊥ ) which can induce helical currents that shield the MP. In our experiment we find
that the toroidal rotation at the pedestal top, measured by charge exchange recombination spec-
troscopy on B5+ impurities, varies widely, vB5+
tor = 0−40 km/s. There is also significant variation
of ve,⊥ , despite ELM suppression being maintained. This includes cases with zero-crossing in
the pedestal region (weak shielding) and cases where ve,⊥ (in electron drift direction) in the
entire pedestal region is sufficiently large to shield the resonant plasma response everywhere.
∗ See A Kallenbach et al, Nucl Fus 57 (2017) 102015
† See H Meyer et al, Nucl Fus 57 (2017) 102014
