Speaker
Anna Medvedeva
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.1098.pdf
High-frequency edge coherent modes observation in ASDEX Upgrade
A. Medvedeva1,2 , C. Bottereau2 , F. Clairet2 , G. D. Conway1 , L. Gil3 , P. Manz1 ,
F. Mink1 , V. Nikolaeva3 , D. Prisiazhniuk1 , U. Stroth1,4 , ASDEX Upgrade Team1
and EUROfusion MST1 teama)
1 Max-Planck-Institut für Plasmaphysik, 85748 Garching, Germany
2 CEA, IRFM, 13108 St-Paul-Lez-Durance, France
3 Instituto de Plasmas e Fusão Nuclear, IST, Universidade Lisboa, 1049-001 Lisbon, Portugal
4 Physik-Department E28, Technische Universität München, 85747 Garching, Germany
a) For a list of members, see H. Meyer et al, Nucl. Fusion 57 102014 (2017)
The modes which can be observed in a confined magnetized plasma are related to instabilities
caused by different drives. The goal is to describe the mechanisms and to identify the basic
properties of such instabilities, which can be measured in the experiment. During the I-phase
of L-H confinement transitions the turbulence level, background and turbulent flows oscillate
in the kilohertz range creating limit-cycle oscillations, while the turbulence frequency spectra
significantly modify in the plasma edge. High-frequency edge coherent modes (ECMs) have
been detected in the reflectometer signals during the experiments dedicated to the L-H transition
studies on ASDEX Upgrade [1]. The role of the ECMs in the clamping of the pedestal pressure
has been examined.
The modes have been observed in the ultra-fast swept reflectometer (UFSR) signal [2] con-
tinuously during the I-phase, after the transition to the ELM-free phase of the H-mode and
in between ELMs [3]. The mode frequency is in the range of 40–200 kHz and often several
branches are observed simultaneously. The UFSR data allow to locate the modes in the plasma
pedestal region. Through a detailed analysis it is shown that the ECM frequency increases with
plasma edge electron pressure. ECMs have low toroidal mode numbers between n = −2 and
−11. From the UFSR and poloidal correlation reflectometer data it follows that the ECMs have
a small radial wavenumber kr < 2 cm−1 and a poloidal wavenumber k⊥ = 0.2 − 0.4 cm−1 . The
absence of the ballooning character, the propagation in the electron diamagnetic direction and
the localisation close to the pedestal top indicate that the ECMs might be microtearing modes.
References
[1] A. Medvedeva et al., Plasma Physics and Controlled Fusion 59.12, 125014 (2017)
[2] F. Clairet et al., Review of Scientific Instruments 88.11, 113506 (2017)
[3] A. F. Mink et al., Nuclear Fusion 58.2, 026011 (2017)
