Speaker
Luís Gil
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P5.1094.pdf
Edge instabilities across the L-H transition and in H-mode of ASDEX
Upgrade
L. Gil1, C. Silva1, T. Happel2, G. Birkenmeier2, G.D. Conway2, L. Guimarãis1, P. Hennequin3,
V. Nikolaeva1, F. Mink2, D. Prisiazhniuk2, T. Pütterich2, J. Santos1, E. Seliunin1, A. Silva1, U.
Stroth2, J. Vicente1, E. Wolfrum2, the ASDEX Upgrade Team and the EUROfusion MST1
Teama
1
Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade Lisboa, PT
2
Max-Planck-Institut für Plasmaphysik, Boltzmannstr. 2, 85748 Garching, Germany
3
Laboratoire de Physique des Plasmas, École Polytechnique, 91128 Palaiseau, France
The H-mode is currently the preferable operational regime for a fusion reactor but the physics
of the L-H transition and confinement enhancement is not yet fully understood. Turbulence
suppression is responsible for the formation of the edge pedestal, whose growth is believed to
be limited by the onset of instabilities. The L-H transition in ASDEX Upgrade (AUG) is often
accompanied by the appearance of edge coherent [1] or quasi-coherent modes (QCMs) [2] in
density fluctuations. QCMs have also been detected in other devices [e.g. 3-4], including
during ELM cycles. The underlying instabilities of these modes are mostly an open question,
despite their relevance for understanding edge transport and pedestal physics.
Slow power ramp shots at different densities have been conducted in AUG to study edge
instabilities across the L-H transition and in H-mode, including the intermediate I-phase.
Reflectometers are the main diagnostics used in this work, with emphasis on the frequency
modulated continuous wave reflectometer, which has the unique capability of providing
simultaneous measurements on the high-field side (HFS) and low-field side (LFS). It was
operated either in fixed frequency to measure density fluctuations at several radial positions
or in broadband sweeping mode to measure density profiles.
The existence of edge coherent and quasi-coherent modes with frequencies ranging from ~40
to 140 kHz and a complex time evolution after the L-H transition is observed. Their type and
behavior is different for the low and high density branches of the L-H power threshold. At
low density, the modes are coherent and have an up-chirping frequency with a multi-peak
structure. They are observed at the LFS and HFS and also feature a radial magnetic field
component, with dominant toroidal mode numbers from -3 to -9, where the negative sign
indicates propagation in the electron diamagnetic direction in the lab frame. In the high
density shots, the mode is quasi-coherent, with a down-chirping frequency and a broad peak
structure. It is observed in the density fluctuations but does not appear in the magnetic coil
signals. At medium density, near the minimum L-H power threshold, both types of modes are
observed: first the one with magnetic component and then the one without. There is a period
of alternation between the two types, which suggests different instabilities come into play.
These modes may play an important role in the H-mode pedestal structure, stability and
confinement. Their characteristics and relation to the inter-ELM modes will be discussed.
[1] A. Medvedeva et al., PPCF 59, 125014 (2017) [3] H.Q. Wang et al, PRL 112, 185004 (2014)
[2] S. da Graça et al, EPS Plasma Physics (2013) [4] A. Diallo et al, NF 55, 053003 (2015)
a
See the author list of Meyer et al 2017 Nucl. Fusion 57 102014.
