1. H and He cluster formation in W Krister Henriksson Accelerator laboratory

2. 1. Why H, He, W ? Overall motivation: Investigate possibly harmful irradiation effects in ITER’s metallic PFC - specifically: the divertor - specifically: the divertor - W is a candidate material for the divertor - W is a candidate material for the divertor * low sputtering yield * low sputtering yield * high melting point * high melting point - irradiation: H and He ions (5-100 eV) - irradiation: H and He ions (5-100 eV) How can low-energy H/He ions create damage in W ?

3. (1): H/He ions can get trapped in W. (2): Enough ions can form clusters. (3): Continous ion supply: clusters => bubbles, blisters. (4): Large blisters may rupture => H/He/W ejection. Radiation losses scale with Z!

4. 2. He clusters in W Formation of He bubbles in W experimentally verified at RT, even for non-damaging irradiation. MDS setup: - 50, 100, 200 eV He at 0 and 300 K - 50, 100, 200 eV He at 0 and 300 K - high flux: 10 27 -10 28 He/(m 2 s) - high flux: 10 27 -10 28 He/(m 2 s) - high fluence: 10 20 He/m 2 - high fluence: 10 20 He/m 2 - SC target, 100% pure - SC target, 100% pure

5. Results: - clusters close to the surface - clusters close to the surface - rupture and leakage - rupture and leakage - insignificant W erosion - insignificant W erosion - no ’blistering’ - no ’blistering’

6. 3. Experimental differences: H and He in W Depth of blisters vastly different. - H: at micrometer depths - H: at micrometer depths - He: close to R p (<100 Å) - He: close to R p (<100 Å) Diffusivities seem to be roughly similar. Idea: Could this be due to different trapping behavior ?

7. 4. Energetics Different trapping tested by MDS. - H-H and He-He relaxed in W at 0 K - H-H and He-He relaxed in W at 0 K - H: - H:

8. - He: - He:

9. Conclusions: - strong self-trapping for He - strong self-trapping for He - weak self-trapping for H ! - weak self-trapping for H !

10. 5. Results from KMCS - KMCS using experimental parameters. - KMCS using experimental parameters. - Self trapping: H and He clusters close to the surface. - Self trapping: H and He clusters close to the surface. - Defect trapping: clusters deeper down. - Defect trapping: clusters deeper down.

11. 6. Summary Different self trapping behavior of H and He can explain the different blister depths in experiments. Different self trapping behavior of H and He can explain the different blister depths in experiments. He escapes from irradiated W, but W erosion is minimal. He escapes from irradiated W, but W erosion is minimal.