C. Björkas, K. Vörtler and K. Nordlund
Department of Physics, University of Helsinki
Joint TFE-SEWG - Material Migration and Material Mixing meeting
MD study of erosion of Be and WC under D bombardment
Joint TFE-SEWG Material Migration and Material Mixing meeting
- Carolina Björkas2
ITER
First wall: Beryllium
Divertor strike point: CFC
Divertor baffle and dome: Tungsten
Plasma: D + T
How does Be erode due
to plasma impact?
How does the mixed WC
erode?
Joint TFE-SEWG Material Migration and Material Mixing meeting
- Carolina Björkas3
Method
We bombarded beryllium and WC surfaces with D ions In the Be case, we used both initially rough and perfect
(0001), (1120) ones For WC, both C and W determined surfaces were used 3 - 100 eV for Be and 100 - 300 eV for WC Flux ~2·1028 m-2s-1
Cell of ~3000 atoms
Distance 5 Å
Fixed bottommostlayers, temperaturecontrol at borders
Joint TFE-SEWG Material Migration and Material Mixing meeting
- Carolina Björkas4
D irradiation of Be
Sputtering is seen at 7 eV
A large fraction of Be is eroded as BeD molecules at low energies
Exps. are from Nishijima et al. Plasma Phys.
Control. Fusion 50 (2008) 125007 at fluxes
1-3·1022 m-2s-1 and temperatures < 400 K
New experiments are coming soon
Chemical sputtering!
Joint TFE-SEWG Material Migration and Material Mixing meeting
- Carolina Björkas5
D irradiation of Be
Joint TFE-SEWG Material Migration and Material Mixing meeting
- Carolina Björkas6
D irradiation of Be
Joint TFE-SEWG Material Migration and Material Mixing meeting
- Carolina Björkas7
D irradiation of Be
The movie shows a swift chemical sputtering process Low-energetic D ions release Be atoms from the surface
by breaking Be-Be bonds The SCS was not believed to be present in metals
This is possible since a surface Be atom that has bonds with D, has a weakened binding to the surface
At low energies, Be atoms are likely to have D neighbours The figure shows
the initial neighbours
of the sputtered
Be atoms
Joint TFE-SEWG Material Migration and Material Mixing meeting
- Carolina Björkas8
D irradiation of WC
A clear preferential sputtering of C was seen in the WC irradiation
Agrees with experimental findings
Joint TFE-SEWG Material Migration and Material Mixing meeting
- Carolina Björkas9
D irradiation of WC
300 eV D on WC resulted in collapsing amorphization First ~1000 impacts: creation of defects in sample In the following: D2 accumulation, collapsing of structureC: gray
W: magentaD: light gray
Joint TFE-SEWG Material Migration and Material Mixing meeting
- Carolina Björkas10
D irradiation of WC
Flaking was alsoobserved
C: grayW: magentaD: light gray
Joint TFE-SEWG Material Migration and Material Mixing meeting
- Carolina Björkas11
Conclusions
The D bombardment of Be showed a clear chemical effect
The swift-chemical sputtering mechanism is present
WC get amorphous due to D bombardment Even flaking is seen in a few cases
Joint TFE-SEWG Material Migration and Material Mixing meeting
- Carolina Björkas12
Thank you!
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