Speaker
Anastasia Dvornova
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.1052.pdf
Modelling of TAE mode excitation with an antenna in X-point geometry
A. Dvornova1,2,3, G.T.A. Huijsmans2,3, S. Sharapov4, M. Hoelzl5, J. Artola1, S. Pamela4
1
Aix-Marseille Universite, CNRS, PIIM UMR 7345, 13397 Marseille, France
2
Eindhoven University of Technology, Eindhoven, The Netherlands
3
CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France
4
CCFE, Culham Science Centre, Abingdon, OX14 3DB, UK
5
Max-Planck-Institut fur Plasmaphysik, 85748 Garching, Germany
In magnetic fusion devices, excitation of Toroidal Alfven Eigenmodes (TAEs) can be
caused through wave-particle resonance by fusion-born alpha-particles or fast ions generated
by ion cyclotron resonance and neutral beam heating. TAEs may affect fast particle
confinement, reduce heating and current drive efficiency, cause damage to the first wall, and
decrease overall plasma performance. In the absence of fast ions, TAEs can be investigated
by launching electromagnetic waves by an external antenna and sweeping the antenna
frequency across the TAE frequency range in order to detect a high-quality peak in the
plasma response marking the weakly-damped TAE resonance. Excitation of TAE modes
with an external antenna has been very successful [1]. It was found, however, that TAEs,
probed with an external antenna in the limiter phase of the discharges, disappear when the
X-point forms in the magnetic configuration. This effect was thought to be likely due to an
increase in the TAE damping rates. More detailed studies [2] show that the damping rates
increase significantly with elongation and ellipticity.
The aim of the present work is to investigate in detail the effect of the X-point geometry
on the efficiency of the TAE excitation with an external antenna and on the TAE damping
rate. An equilibrium from a JET discharge with a clear effect of the X-point on the TAE
(pulse #42870) just before and after the X-point phase was analysed with the CASTOR linear
resistive MHD code including the external TAE antenna [3]. As the plasma boundary of the
simulation domain approaches the separatrix from the core, the amplitude of the excited TAE
mode is strongly reduced, in agreement with observations. Damping rates of 0.5 to 10% are
found, consistent with the previous results [1]. However, in the castor code the X-point
geometry can be only closely approximated. The JOREK-STARWALL nonlinear MHD
code has been extended to include the active TAE coils [4]. This does allow the simulation of
the excitation of TAE modes with an external antennas in full X-point geometry, including
the scrape-off layer. The simulations of antenna excitation of TAE modes is challenging due
to the low dissipation (i.e. resistivity, viscosity) that is required to avoid a strong damping.
Results in a limiter plasma with the time evolution code JOREK-STARWALL are in good
agreement with the steady state solution from CASTOR. Simulations of the JET case in
x-point geometry are compared to the linear MHD results to identify the cause for the
absence of antenna excited TAE mode in this configuration, as observed in the experiments.
[1] A. Fasoli et al 2010 Plasma Phys. Control. Fusion 52 075015
[2] D. Testa et al 2001 Nucl. Fusion 41 809
[3] G.T.A. Huysmans et al Physics of Plasmas 2, 1605 (1995)
[4] M. Hoelzl et al 2012 J. Phys.: Conf. Ser. 401 012010
