Speaker
Michael Komm
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/O4.105.pdf
Physics of power loading on the gap edges of castellated plasma-facing
components in fusion reactors
M. Komm1, J.P. Gunn2, R. Dejarnac1, R. Panek1, R.A. Pitts3, A. Podolnik1,4, S. Ratynskaia5
and P. Tolias5
1
Institute of Plasma Physics of the CAS, Za Slovankou 3, 182 00 Prague 8, Czech Republic
2
CEA, IRFM, F-13108 Saint-Paul-Lez-Durance, France
3
ITER Organization, Route de Vinon-sur-Verdon, CS 90 046, 13067 St. Paul Lez Durance Cedex, France
4
Fac. Math & Phys., Charles University, V Holešovičkách 2, 180 00 Prague 8, Czech Republic
5
KTH Royal Institute of Technology, Space & Plasma Physics, SE-10044 Stockholm, Sweden
In order to manage thermal stresses, actively cooled tokamak plasma-facing components (PFCs) in
high heat flux areas must typically be divided or “castellated” into small units separated by gaps both
in toroidal and poloidal directions. Heating of the edges introduced by this castellation is a serious,
and, until recently, little studied complication for component power handling [1]. It will be critically
important on ITER and imposes careful PFC shaping design to mitigate the consequences. In support
of the ITER tungsten divertor target design and as an interpretive tool for experiments on several
devices designed to study this problem, plasma interaction with castellated PFCs and the related role
of sheath electric fields has been investigated by means of the SPICE 2D and 3D particle-in-cell
codes. In addition to providing predictions of detailed ITER divertor power loading, this work
substantially improves the general understanding of the processes occurring in the magnetized sheath
which forms in the vicinity of the PFCs and is a key quantitative tool for validation of simpler ion
orbit approaches which neglect the sheath electric field [1], but which are less computationally
intensive when deployed for PFC design. These studies are being augmented by the investigation of
thermionic emission from tungsten surfaces, including 3D effects relevant to localized hotspots, and
providing in addition important constraints for the modelling of melt motion on PFCs subject to high
energy transients.
A number of interesting physics phenomena in the magnetized sheath affect the heat load distribution
on gap edges. Inside toroidal gaps, depending on the magnetic field orientation, electrons and ions
can either strike the same side (with the magnetically wetted side receiving all the heat load entering
the gap), or opposite sides (with the heat load shared between the two sides of the gap) due to ion
Larmor gyration and radial EB drift in the sheath. Dedicated PIC simulations have confirmed that
the former is the dominant mechanism of the toroidal gap power loading for ITER plasma conditions,
providing further confidence that ballistic ion orbit simulations can be used as a good approximation
in the study of gap edge loading for component design.
[1] J.P. Gunn et al., Nuclear Fusion 57 (2017) 046025
