[Zurück]

K. Dobes, J. Pyszkowski, T. Schäfer, F. Aumayr:
"Sputtering of fusion relevant surfaces by seeding impurities";
Poster: 19th International Workshop on Inelastic Ion-Surface Collisions (IISC-19), Frauenchiemsee/Germany; 19.09.2012; in: "Book of Abstracts, 19th International Workshop on Inelastic Ion-Surface Collisions (IISC-19)", (2012), S. 69.

1. INTRODUCTION
The interaction of a fusion plasma with the walls of its
containing vessel will constitute one of the key challenges
in the successful realization of a future fusion power plant.
A profound knowledge about sputtering yields of plasma
facing components under the impact of fusion relevant ion
species is hence desired. Besides the evolution of plasma
facing materials like tungsten, beryllium and possibly also
carbon in the environment of a burning fusion plasma and
especially the mixing of materials is of considerable
interest. In addition also the retention of hydrogen isotopes
in plasma facing components is of major concern.
Particularly the build-up of a tritium inventory within the
plasma vessel imposes operational limits due to safety
considerations.
ASDEX Upgrade was the first fusion machine to operate with
a full tungsten wall [1]. Also in future fusion devices like
ITER tungsten is foreseen as a divertor target [2]. In order to
reduce the power load to the divertor target plates to
acceptable values, radiation cooling is essential [3].
Especially in all-metal fusion machines, where no carbon is
present in the vessel, radiation due to intrinsic impurities is
no longer sufficient and has to be replaced by seeding of
additional impurities [3]. At ASDEX Upgrade nitrogen
seeding with feedback control has meanwhile matured into a
standard operational scenario [1]. Using nitrogen, the
divertor target power flux could be mitigated by high
radiation to a technically acceptable level, and the
performance of the plasma was even increased compared to
discharges without impurity seeding [1]. At ITER heavier
seeding impurities like Ne and Ar will be necessary and
used for radiative cooling. It is hence of considerable
interest to study the interaction of different seed impurity
ions (N+, N2
+, Ne+, Ar+) with tungsten and other fusion
relevant surfaces.
2. EXPERIMENT & RESULTS
In our studies we are using a quartz crystal microbalance
(QCM) technique [4 - 7] that has been designed at Vienna
University of Technology. The experimental technique has
been optimized for extreme high sensitivity and accuracy. It
is capable of determining mass changes of as small as 2
amu/ion􀀁nA/mm2 or accordingly 3􀀁10-4 tungsten monolayers
per second. Total sputtering yields for N+, N2
+, Ne+ and Arq+
ions on tungsten have been investigated under controlled
laboratory conditions. The high sensitivity of our QCM [7]
allows us to study the change in surface composition
during seed impurity impact and its influence on the
sputtering yield in-situ and in real-time. The code TRIDYN
[8], which takes into account a change of the surface under
ion impact due to implantation and erosion, is used to
model and better understand the investigated sputtering
phenomena.
We will present the flux dependence of tungsten erosion by
different seed impurity ions at fusion relevant impact
energies. In addition sputtering of tungsten-nitride
surfaces, which are formed as a result of high flux nitrogen
bombardment of tungsten [9], were investigated.
Furthermore a comparison of sputtering yields under the
impact of singly and multiply charged ions at the same
impact velocities will be presented and discussed.
3. ACKNOWLEDGEMENT
This work, supported by the European Commission under
the Contract of Association between EURATOM and the
Austrian Academy of Sciences, was carried out within the
framework of the European Fusion Development Agreement.
The views and opinions expressed herein do not necessarily
reflect those of the European Commission