REPOSITORY > RESULTS

Abstract

Plasma facing components (PFC) in fusion devices are exposed to plasma. Plasma-wall
interaction processes are important as they impose a series of constraints on the components.
The lifetime of the PFCs is dominantly determined by two processes: erosion
and fuel retention. At the first wall the erosion is caused by physical sputtering due to
bombardment with energetic particles (in keV range) originating from the core plasma. In
numerous studies in the past it was shown that surface roughness of the target and an
angle of incidence of the particle affects the rate of sputtering or sputter yield. However,
not many of studies have been done for fusion relevant conditions. To this end we built
Low Energy Ion Sputtering Apparatus - LEISA at Jožef Stefan Institute. In LEISA we
can produce particle fluxes up to 8 × 10[SUP]18 Dm−2s−1 with the energies up to 5 keV. The
samples inside LEISA can be rotated from 0◦ to 90◦, to study the angular dependence
of sputter yield. Graphite tiles with surface roughness ranging from Ra ≈ 5 nm up to
Ra ≈ 2-3 µm with 115 nm thick molybdenum coating were exposed to 1 keV D ions in
LEISA. They were exposed to D ion fluence ranging from 0.89 up to 3.19×1023 Dm−2s−1
at the impact angles of 0◦, 40◦, 60◦ and 70◦. The surface roughneses were measured with
atomic force microscope (AFM), confocal scanning laser microscope (CSLM) and scanning
electron microscope (SEM). The Mo layer thickness was measured by Rutherford backscattering
spectroscopy (RBS) before and after D ion exposure. From the measured eroded
Mo thickness and applied the D ion fluence we calculated sputter yields for all angles and
surface roughness. For all the surfaces a strong angular dependence of the sputter yield
is observed. For smooth and intermediate surface roughnesses, up to Ra ≈ 280 nm, an
increase of the sputter yield with the angle up to a factor of five compared to 0◦ is observed.
In contrast, at the highest surface roughness in the 2-3 µm range the sputtering
yield decreases with increasing impact angle.
Same types of samples were exposed in the divertor region of the ASDEX Upgrade-
AUG tokamak, using the DIM-II manipulator. Samples were exposed to eight 6 s long
L-MODE discharges in D plasma. The results obtained at the samples from AUG match
well with the results obtained at the LEISA. At the AUG samples, we observed a similar
trend, the decrease of the sputtering yield at high impact angles for the samples with higher
surface roughness.
Sputter yields from LEISA experiment were compared with the simulations performed
with the SDTrimSP 3D, a Monte Carlo simulation code taking as an input surface representation
data. We achieved a good qualitative agreement with experimental results.
At the end we studied the effects of surface roughness on the fuel and impurities retention
on the samples exposed in AUG. The investigation was performed with micro-beam
utilising PIXE and NRA analytical techniques. The results show an intense retention in
the samples with high surface rougness.
The lifetime of PFCs is an interplay of both the erosion and retention, and we demonstrated
the important role of these two processes by the surface roughness. We are leaving
the determination of the optimal surface roughness of the PFCs for the next studies.