1. 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

2. Joint TFE-SEWG Material Migration and Material Mixing meeting - Carolina Björkas 2 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?

3. Joint TFE-SEWG Material Migration and Material Mixing meeting - Carolina Björkas 3 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 ~2·10 28 m -2 s -1 Cell of ~3000 atoms Distance 5 Å Fixed bottommost layers, temperature control at borders

4. Joint TFE-SEWG Material Migration and Material Mixing meeting - Carolina Björkas 4 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 1-3·10 22 m -2 s -1 and temperatures < 400 K New experiments are coming soon Chemical sputtering!

5. Joint TFE-SEWG Material Migration and Material Mixing meeting - Carolina Björkas 5 D irradiation of Be

6. Joint TFE-SEWG Material Migration and Material Mixing meeting - Carolina Björkas 6 D irradiation of Be

7. Joint TFE-SEWG Material Migration and Material Mixing meeting - Carolina Björkas 7 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

8. Joint TFE-SEWG Material Migration and Material Mixing meeting - Carolina Björkas 8 D irradiation of WC A clear preferential sputtering of C was seen in the WC irradiation Agrees with experimental findings

9. Joint TFE-SEWG Material Migration and Material Mixing meeting - Carolina Björkas 9 D irradiation of WC 300 eV D on WC resulted in collapsing amorphization First ~1000 impacts: creation of defects in sample In the following: D 2 accumulation, collapsing of structure C: gray W: magenta D: light gray

10. Joint TFE-SEWG Material Migration and Material Mixing meeting - Carolina Björkas 10 D irradiation of WC Flaking was also observed C: gray W: magenta D: light gray

11. Joint TFE-SEWG Material Migration and Material Mixing meeting - Carolina Björkas 11 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

12. Joint TFE-SEWG Material Migration and Material Mixing meeting - Carolina Björkas 12 Thank you!