Speaker
Stephan Ertmer
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P5.1024.pdf
Ground state population of sputtered tungsten atoms by peak emission
analysis in PSI-2 argon plasmas
S. Ertmer1 , O. Marchuk1 , A. Pospieszczyk1 , A. Kreter1 , S. Brezinsek1
1 Forschungszentrum Jülich GmbH - Institut für Energie- und Klimaforschung - Plasmaphysik,
Partner of the Trilateral Euregio Cluster (TEC), 52425 Jülich, Germany
Tungsten (W) is one of the most promising materials for plasma-facing components (PFCs) in
future fusion reactor [1]. The lifetime of W divertor PFCs will finally be determined by the
erosion. The initial level population of sputtered W atoms from PFCs is of big interested for
interpretation of spectroscopic data used for the estimation of gross erosion rates. Monoener-
getic ion beam experiments with different metals (e.g. Fe) suggest a ground level population
of released atoms of over 95 % [2] - Sputtering experiments in the tokamak TEXTOR with W
PFCs exposed to a hot edge plasma (Te > 30 eV) lead to the assumption of a local thermal equi-
librium in the fivefold ground term 5 D and the 7 D3 level with an effective temperature TW of
0.1 to 0.3 eV of physically sputtered W atoms by energetic Carbon ions at surface temperatu-
res (Tsurf ) of more than 720 K [3].
To investigate the level population in more detail, we exposed a W sample (1.3×1.3 cm2 ;
Tsurf = 300 K) to an argon plasma in the linear plasma device PSI-2 (Te ≈ 2 eV). We measu-
red with an imaging spectrometer the line emission of several neutral tungsten (W I) transitions
with a high spatial resolution of 50 µm over the first few mm penetration depth in front of
the target. The axial distance of the peak of emission d x from target surface is approximately
proportional to the velocity of the sputtered atoms vatom times the lifetime of the upper energy
level τ [4]. This lifetime is equal to the reciprocal sum of the Einstein coefficients A and vatom is
transfered due to the ion collisions during sputtering to the sputtered atoms.
The axial peak position of the investigated ground state transition at 498.26 nm (7 F1 →5 D0 ) is
consistent with the expected velocity and the Einstein coefficient. Whereas the lines at 484.38 nm
and 424.43 nm, whose upper levels are not mainly fed by the 5 D0 level, peak further away from
the target than expected. This experimental data shows that W is sputtered primarily in the
ground level by a monoenergetic Ar ion beam for an impact energies between 100 and 200 eV,
if the W sample is at room temperature. The other levels of the ground term are in this experi-
mental condition populated subsequently of the plasma.
References
[1] S. Brezinsek et al. J. Nucl. Mater. 55 (2015) 063021
[2] A. P. Yalin et al. Applied optics 44 (2005) 6496
[3] I Beigman et al. Plasma Phys. Control. Fusion 49 (2007) 1833
[4] O. Marchuk et al. J. Phys. B: At. Mol. Opt. Phys 51 (2018) 025702
