Speaker
Allah Rakha
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P4.1004.pdf
Modelling of Alfvén cascades in NBI heated stellarator plasmas
Allah Rakha1, M.J. Mantsinen1, 2, A.V.Melnikov3, 4, S.E. Sharapov5, D.A.Spong6, A.
López-Fraguas7, F. Castejón7
1
Barcelona Supercomputing Center (BSC), 08034, Barcelona, Spain, 2ICREA, Pg. Lluís
Companys 23, 08010 Barcelona, Spain, 3National Research Center 'Kurchatov Institute',
123182, Moscow, Russia, 4National Research Nuclear University MEPhI, 115409, Moscow,
Russia, 5CCFE, Culham Science Centre, OX14 3DB, UK, 6Oak Ridge National Laboratory,
TN,37831,USA, 7Fusion National Laboratory, CIEMAT, 28040, Madrid, Spain
In magnetic fusion, machines with a Reversed magnetic shear (RS) with an off-axis local
minimum in the rotational transform are associated with internal transport barriers [1, 2]. In a
RS configuration, Alfvén eigenmodes (AEs) called reverse shear Alfvén eigenmodes (RSAE)
or Alfvén cascades (ACs) [2] can be excited by energetic ions enhancing fast-ion re-distribution
and losses, which are of major concern for plasma scenarios with low or reversed magnetic
shear [3]. In this paper, we study ACs observed [4] in the NBI heated discharge 27804 in the
TJ-II stellarator (B0 = 0.95 T, R = 1.5 m, a = 0.22
m, Nfp = 4, PNBI ≤ 1.0 MW, ENBI ≤ 32 keV, PECRH ≤
0.6 MW) using numerical calculations. In this
plasma with an increasing rotational transform ι in
time, an AC with fast frequency sweeping was
observed, with the range of the frequency
Figure: Frequency sweeping Alfvén Eigenmodes
(AE) in discharge 27804 in TJ-II stellarator. We sweeping in excess of 100 kHz and a minimum
focus on the AC observed at t = 1170 ms with fast
frequency in the range of ̴ 25 kHz as shown in
frequency sweeping and a large frequency range.
Figure [4].
We have modelled discharge 27804 based on a reduced MHD model [5, 6] using the
experimental density profile and the reversed ι profile reconstructed with VMEC [7]. Our
modelling shows a wide spectrum of modes covering a frequency range from ̴ 50 to ̴ 200 kHz
and located at a normalised minor radius of 0.3-0.7, roughly consistent with the position of the
iota extremum point and the experimental findings. If we use a monotonic ι profile instead, our
simulations show a smaller number of modes located at larger minor radii with frequencies
covering a narrower range than that observed in the experiment.
References
[1] Austin M. E. et al., 2006 Phys. Plasmas 13 082502. [5] Spong D. A.,et al., 2003 Phys. Plasmas 10 3217.
[2] Sharapov S.E. et al., 2002 Phys. Plasmas 9, 2027. [6] Spong D. A. et al., 2010 Phys. Plasmas 17 022106
[3] Fasoli, A. et al., 2007 Nucl. Fusion 47 S264. [7] Hirshman, S. P. and Whitson, J. C. 1983 Phys.
[4] Melnikov A.V., et. al. 2014 Nucl. Fusion 54 Fluids 26, 3553.
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