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!