Speaker
Frederic Clairet
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P4.1093.pdf
Shear effect on edge turbulence during the L-H transition in JET and
ASDEX Upgrade plasmas
F. Clairet1, A. Medvedeva2, C. Bottereau1, G. Dif-Pradalier1, X. Garbet1, U. Stroth2,3, L.
Meneses4, ASDEX Upgrade team2,a), JET contributorsb) and EUROfusion MST1 teamc)
1
CEA, IRFM, 13108 St-Paul-Lez-Durance, France
2
Max-Planck-Institut für Plasmaphysik, 85748 Garching, Germany
3
Physik-Department E28, Technische Universität München, 85747 Garching, Germany
4
IPFN, IST, Universidade de Lisboa, 1049-001 Lisboa, Portugal
a)
For a list of members, see Appendix of A. Kallenbach et al, Nucl. Fusion 57 102015 (2017)
b)
For a list of members, see Appendix of X. Litaudon et al Nucl. Fusion 57 102001 (2017)
c)
For a list of members, see Appendix of H. Meyer et al, Nucl. Fusion 57 102014 (2017)
It is widely accepted that the improvement of the confinement in an L to H-mode transition is
the result of the suppression of the edge turbulence and that the radial electric field (Er) plays
a key role in the explanation of transport. In fact, sheared poloidal flows can influence the
turbulence via shear decorrelation mechanisms and, as a consequence, modify the induced
transport. We have performed high radial resolution turbulence measurements using fast
frequency swept reflectometry which is especially well suited for the study of the narrow
pedestal region. Measurements during intermediate transitions, such as the I-phase in AUG
and the M-mode in JET plasmas, have been performed. These L to H transition phases allow
following the evolution of the turbulence and the mechanisms involved as they can provide
time for statistics. We observe that both I-phase and M-mode offer similar characteristics in
the modifications of their turbulence frequency spectra, changing from broadband to
coherent modes in the pedestal region along with the deepening of the diamagnetic
contribution of the radial electric field well. Moreover, a detailed analysis of the complex
reflected signal displays a single side band feature at low frequency around few kHz and
reverses of sign radially. This feature is discussed in terms of eddy tilting in changing ExB
shear flow. The result is consistent with previous observations [1] of eddy breaking and
tilting by edge sheared flows and could be the cause [2] for the observed particle transport
reduction.
[1] I. Shesterikov et al. Phys. Rev. Letter 111, 055006 (2013)
[2] P. Manz, Phys et al. Rev. Letter 103, 165004 (2009)
