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22-27 October 2018
Mahatma Mandir Conference Centre
Asia/Kolkata timezone
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Experimental conditions for suppressing Edge Localised Modes by magnetic perturbations in ASDEX Upgrade

26 Oct 2018, 09:10
20m
Mahatma Mandir Conference Centre

Mahatma Mandir Conference Centre

Gandhinagar (nearest Airport: Ahmedabad), India
Oral EXS - Magnetic Confinement Experiments: Stability EX/7-TH/5 Active ELM Control

Speaker

Dr Wolfgang Suttrop (Max-Planck-Institut für Plasmaphysik)

Description

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 leads to stronger amplification of the external MP by marginally stable, edge localised, kink-peeling modes. Recent 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 $q_{95} = 3.6-4.2$ showed a window $q_{95} = 3.66-3.91$ for ELM suppression with $n=2$ MP. In the ELM suppression scenario used so far, there is a clear maximum edge density ($3.5 \times 10^{19}$ m$^{-3}$) for ELM suppression, which can also be expressed as a collisionality limit. 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 $32\%$ 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 the absence or presence of electron flow across the magnetic field ($v_\mathrm{e,\perp}$) 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 spectroscopy on $B^{5+}$ impurities, varies widely, $v_\mathrm{tor} = 0 - 40$~km/s. There is also significant variation of $v_\mathrm{e,\perp}$, despite ELM suppression being maintained. This includes cases with zero-crossing in the pedestal region (weak shielding) and cases where $v_\mathrm{e,\perp}$ (in electron drift direction) in the entire pedestal region is sufficiently strong to shield the resonant plasma response everywhere.
Paper Number EX/7-3
Country or International Organization Germany

Primary author

Dr Wolfgang Suttrop (Max-Planck-Institut für Plasmaphysik)

Co-authors

Dr Andrew Kirk (Culham Centre for Fusion Energy) Dr Carlos Paz-Soldan (General Atomics) David Ryan (UKAEA) Dr Eleonora Viezzer (Max-Planck-Institut fuer Plasmaphysik) Dr Hendrik Meyer (UK Atomic Energy Authority) Dr Marco Cavedon (Max-Planck-Institut für Plasmaphysik) Dr Matthias Willensdorfer (IPP Garching) Dr Mike Dunne (IPP-Garching) Dr Rachael McDermott (Max Planck Institut für Plasmaphysik) Dr Raffi Nazikian (Princeton Plasma Physics Laboratory) Dr Volodymyr Bobkov (Max-Planck-Institute for Plasma Physics)

Presentation Materials

Paper