Speaker
David Anthony Ryan
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P4.1059.pdf
ELM Suppression Characterisation by Plasma Response Computation on
ASDEX Upgrade
D A Ryan1, L Piron1, A Kirk1, Y Q Liu2, M Dunne3, L Li4, B Dudson5, W Suttrop3, the ASDEX Upgrade team 3 and
the EUROfusion MST1 team[1]
1] CCFE, Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, UK
2] General Atomics, P. O. Box 85608, San Diego, California 92186-5608, USA
3] Max Planck Institute for Plasma Physics, Garching, Germany
4] College of Science, Donghua University, Shanghai 201620, China
5] York Plasma Institute, Department of Physics, University of York, York, YO10 5DQ, UK
Edge Localised Modes (ELMs) in H-mode tokamak plasmas may be controlled or entirely
suppressed by applying 3D magnetic perturbations (MPs). The applied perturbation is amplified by
the plasma response, and it has previously been established that the size of the peeling component
of this response is a reliable indicator for expected ELM control on ASDEX Upgrade [2] and MAST
[3]. This motivates further studies of the connection between the peeling response and ELM
behaviour. Using the MARS-F linear MHD code [4], the global plasma response to applied n=2 MP
fields was computed at 33 points in time from 3 recent ELM suppression experiments on ASDEX
Upgrade. The amplified peeling response was characterised using a previously employed [5] set of
figures of merit, based on the total magnetic perturbation and plasma displacement. The computed
amplified peeling response for poor mitigation, good mitigation, and suppression were compared.
Good mitigation is defined here as an increase in ELM frequency over the natural frequency of more
than 50% (not referring to target plate heat load reduction). It is found that the values of the peeling
response of the ELM suppressed cases occupy a relatively small subspace of that occupied by the
ELM mitigated cases. Although the sample size is small, it appears that the peeling response may
take larger maximum values for ELM mitigated cases than suppressed cases, consistent with a
previous suggestion a sufficiently large peeling response may inhibit or reverse ELM suppression
[6]. However, the peeling response for suppressed cases does not appear to be systematically
higher or lower than for mitigated cases. It has been previously suggested [7], that the observation
that ELM suppression access is easier at high triangularity may be related to the larger pedestal
pressure gradient in these cases amplifying the peeling response. To investigate this, the pedestal
pressure gradient of a standard ASDEX Upgrade equilibrium is scanned from shallow to steep. The
plasma response to an applied MP is then computed for this scan. The results indicate that the
amplitude of the amplified peeling response is not strongly sensitive to the magnitude of the
pedestal pressure gradient. These observations appear inconsistent with the suggestion that a
larger pressure gradient leads to a more strongly amplified peeling response which eases the
transition from mitigation to suppression. These results are discussed in the context of the broader
pursuit of a predictive theory for ELM suppression access, and the outlook for this effort briefly
examined. Further investigation into the cause of the lower spread of peeling response values in
ELM suppressed relative to ELM mitigated cases is underway and will be reported.
[1] See authorlist of H Meyer et al, 2017, Nuclear Fusion, 57, 102014
[2] A Kirk et al, 2015, Nuclear Fusion, 55, 043011
[3] A Kirk et al, 2013, Plasma Phys. and Contr. Fusion, 55, 015006
[4] Y Q Liu et al, 2000, Physics of Plasmas, 7, 3681
[5] L Li et al, 2016, Nuclear Fusion, 56, 126007
[6] A Wingen et al, 2015, Plasma Phys. and Contr. Fusion, 57, 104006
[7] R Nazikian et al, 2016, First observation of ELM suppression by magnetic perturbations in ASDEX Upgrade
and comparison to DIII-D matched-shape plasmas. 26th IAEA Int. Conf. on Fusion Energy, Kyoto, Japan
