Speaker
Georg Friedrich Harrer
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P4.1030.pdf
Parameter dependence of small Edge Localized Modes
G.F. Harrer1 , E. Wolfrum2 , M.G. Dunne2 , P. Manz2 , M. Cavedon2 , P.T. Lang2 , T. Eich2 ,
B. Labit3 , F. Aumayr1 , the Eurofusion MST1 Team4 and the ASDEX Upgrade Team2
1 Institute of Applied Physics, TU Wien, Vienna, Austria
2 Max Planck Institute for Plasma Physics, Garching, Germany
3 Swiss Plasma Center, EPFL, Lausanne, Switzerland
4 see author list in H. Meyer et al. Nuclear Fusion 57 102014 (2017)
The development of small Edge Localized Mode (ELM) scenarios is important to reduce the
strain on plasma facing components. Such a scenario can be found at high density, close to
double-null plasmas as small ELMs or type-II ELMs in ASDEX Upgrade, which are character-
ized by a frequency fELM > 300 Hz and a low energy loss. Large type-I ELMs and small ELMs
can occur simultaneously. While type-I ELMs can be described by a global peeling-ballooning
model [1], the size and occurence of small ELMs is experimentally observed to depend for ex-
ample on the density around the separatrix [2].
Experiments changing the edge density via different fuelling mechanisms and varying the
plasma shape have been performed [3]. These experiments showed that small ELMs increase in
size (divertor current amplitude) with the density at the separatrix, but they are reduced when
the magnetic shear gets stronger. Both observations are in line with small ELMs being local
ballooning modes driven by the pressure gradient and stabilized by magnetic shear. They cause
transport and flatten the gradient region around the separatrix and thereby consequently nar-
row the effective pedestal width. Because a narrower pedestal is more stable against global PB
modes, the stability boundary is shifted towards higher pressure gradients and type-I ELMs do
not occur. As both, the local pressure gradient and the magnetic shear, strongly influence the
amplitude of small ELMs, a series of additional discharges at high triangularity and at high
separatrix density has been performed varying the toroidal magnetic field at constant plasma
current to vary the magnetic shear. The influence of the flux surface averaged and the local
magnetic shear on the amplitude and frequency of small ELMs will be shown.
References
[1] J.W. Connor et al. Physics of Plasmas 5 2687 (1998)
[2] H. Meyer et al. Nuclear Fusion 57 102014 (2017)
[3] G.F. Harrer et al. Nuclear Fusion to be submitted (2018)
