Speaker
Christian Perez von Thun
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/I2.108.pdf
Closing the gap between experiment and modelling to understand the
stability of the edge transport barrier at JET
C. Perez von Thun1 , L. Frassinetti2 , L. Horvath3 , S. Saarelma4 , E. de la Luna5 ,
M. Beurskens6 , J. Flanagan4 , C.F. Maggi4, S.J.P. Pamela4 , and JET Contributors∗
1 Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung —
Plasmaphysik, 52425 Jülich, Germany.
2 Royal Institute of Technology KTH, Stockholm, Sweden.
3 York Plasma Institute, University of York, Heslington, York, YO10 5DD, UK.
4 Culham Centre for Fusion Energy, Abingdon, OX14 3DB, UK.
5 Laboratorio Nacional de Fusión, CIEMAT, E-28040, Madrid, Spain.
6 Max-Planck-Institut für Plasmaphysik, Wendelsteinstr. 1, D-17491 Greifswald, Germany.
The most widely accepted physics model to explain the occurrence of Edge Localised Modes
(ELMs) in tokamaks is the peeling-ballooning (PB) model, in which ELMs are triggered by the
excitation of coupled PB modes. The validity of the model has been investigated experimentally
mainly with detailed pedestal profile measurements, and theoretically with help of MHD stabil-
ity codes. However, the non-ambiguous experimental identification of the PB modes themselves
has so far been missing. The work presented here closes this gap by characterising of macro-
scopic pre-ELM fluctuation measurements of the pedestal on JET for a wide operational range,
making use of a combination of improved edge profile and fluctuation diagnostics, and by car-
rying out a systematic comparison of the results with stability modelling predictions. We have
characterised the existence domain of modes, the measured toroidal mode numbers up to n = 16
and the dependency on the pedestal pressure gradient and current density, and the mode num-
bers are consistent with stability calculations. Together with earlier findings, the properties of
these fluctuations allow their identification as coupled PB modes. Effectively, this extends and
generalises to higher toroidal mode numbers (up to n ≤ 16) the identification [G.T.A. Huysmans
et al Nucl. Fusion 38 179 (1998)] of the lowest n = 1 modes (also termed ‘outer modes ’) as
pure external kink (peeling) modes. The observation of these modes opens up a new avenue to
test existing ELM models. We will show how these modes can be used to diagnose what regions
of plasma boundary stability space are being accessed in the various operating scenarios on JET,
and explore in how far the differences in pedestal behaviour between the previous CFC-based
wall and the current Be/W-based first wall can be explained in terms of PB stability.
∗ See the author list of ”X. Litaudon et al 2017 Nucl. Fusion 57 102001”
