20th International Conference on Plasma Surface Interactions 2012

1. 20th International Conference on Plasma Surface Interactions 2012 Eurogress, Aachen, Germany, 21-25.05.2012 O-15, M. Miyamoto Shimane Univ., Japan Influence of Be seeding onmicrostructures of tungsten exposedto D-He mixture plasmas in PISCES and its impacts on retention properties M. Miyamotoa*, D. Nishijimab, M.J. Baldwinb, R.P. Doernerb, Y. Uedac, and A. Sagarad a)Shimane Univ., b)UCSD, c)Osaka Univ., d)NIFS

2. Background and objective PSI under Mixed Ions irradiation • W was chosen as a plasma facing material (PFM) in the ITER divertor region. • Inthe ITER DT phase, the burning plasma will expose W simultaneously to He and other trace impurities such as C and Be besides H isotopes. • Concerning H isotope retention and surface damage in W, • Effects of He and C have been actively examined. • However, effects of Be remain obscure due to its toxicity. Especially, available information from the view point of microstructure is limited. Objective in Present Study • The influence of Be on microstructures and D retention in W exposed to D-He-Be mixture plasmas were examined with SEM,TEM and TDS. J. Roth, J. Physics, 100(2008)062003

3. PISCES-B~divertor plasma simulator ~ • Investigation of ITER mixed materials PSI PISCES-B allows exposure of samples to Be seeded plasma PISCES-B is contained within an isolated safety enclosure to prevent the release of Be dust.

4. Effects of He seeding Previous work SEM, TEM& TDS • SR-W, 5x1025 D/m2, 573K, without Be TEM, Cross section Pure-D D+He(20%) • Nucl. Fusion 49(2009)065035, JNM 415(2011)S657 • Suppression of blister formation • Significant reduction in D retention. • Formation of high density nano-sized He bubbles. • In the present study, we investigated the effect of additional impurity beryllium on D retention and microstructures. D2 Partial pressure [torr] Temperature [K]

5. Experimental conditions • Wsample • Bulk-W (for SEM and TDS) • (f25 x t1.5 mm, A.L.M.T. Corp) • Pre-thinned W（for TEM） • (f3 x <t0.1 mm, installed with a specially-fabricated Mo holder) • Plasma exposure in PISCES-B (low energy and high flux) • Gi ~ 1022 ions/m2s, Fi ~ 5x1025 ions/m2, Tsample ~ 573K Mo holder for TEM samples Bulk-W

6. Main results • (Appearance) • Surface morphology • Internal microstructure • D retention properties Experiments 1

7. Appearance after plasma exposure • D+Be(~0.2%)+He(~10%), 5x1025D/m2 @~573K • Ei: ~10eV (deposition condition) • The surface apparently discolored; • some part looks white, • while the other looks black. • Tightly clamping the sample to the holder with Mo cap caused surface cracking. • It is expected that Ts was not uniform over the sample during the plasma exposure. • The black part should correspond to ~573 K. • The white part may be rather lower than 573 K. • Ei: ~60eV (erosion condition) • The surfaces remained in metallic luster over the whole surface after the exposures despite crack formations.

8. SEM Surface morphology varies with Ei and Ts • D+Be(~0.2%)+He(~10%), 5x1025D/m2 @~573K, Ei: ~10eV (depo.), ~60eV (ero.)

9. TEM Be seeding suppressed internal He bubbles • Pre-thinned W, 5x1025 D/m2, 573K • Cavities are seen as white contrast at this image condition (s>>1), if they are present. No cavities (but blisters on surface) High density He bubbles Fine and dense cavities (D bubbles) in Be deposition layer Cavities are hardly seen (Suppression of He bubble formation) No cavities Cavities are hardly seen (Suppression of He bubble formation)

10. Total retentionof D TDS • D+Be(~0.2%)+He(~10%), 5x1025D/m2 @~573K, Ei: ~10eV or ~60eV. • D+Be (deposition) ; The black and white parts were separately examined. • D+Be+He (deposition) ; They were examined together without being separated. w/o Be Be (deposition) Be (erosion) D/Be ~ 0.03 pure-D D+He blackwhite D+Be (depo.) D+Be+He (depo.) D+Be (erosion) D+Be+He (erosion)

11. Effect of mixed plasma exposure Discussion ●D, Be, He bubble D+He (without Be) D+Be(+He) (deposition) D+Be(+He) (erosion) Be deposition layer(~mm) D+ D+ D+ Be+ Be+ He+ He+ He+ surface surface surface • Rather thick co-deposition layer of Be and D are formed. • Retained deuterium can be trapped as D bubbles in the deposition layer. • D/Be~0.03 at 573K (dep. Ts). Be/W mixing layer (~a few nm) • D retention is suppressed. • The thin Be/W mixed layer may obstruct the diffusion of D into the bulk. • High density He bubbles are formed and interconnect. Although the mechanism is not clear, it is interesting that the formation of He bubbles is suppressed • They seem to act as diffusion path for D

12. Summary Be seeding has a more dominant influence on microstructures and D retention than He seeding. The effects of Be seeding to D+He mixture plasmas on microstructures and D retention in W have been examined in PISCES-B. • For a reliable prediction of the tritium retention in ITER, the temperature dependence of microstructures and retention properties should be investigated.

13. Backup

14. D retention properties TDS • TDS of D2 for W exposed to D+(~0.2%Be)+(~10%He) plasma (1) without Be (2) Be (deposition) (3) Be (erosion) (a) Ei ~ 60 eV (b) Ei ~ 10 eV (c) Ei ~ 60 eV D+Be (white part) Pure-D D+Be (black part) D+Be+He D+Be D+He D+Be+He • (1) Due to He seeding to pure-D plasma, D desorption significantly decrease. • (2) A larger desorption occurred for each sample compared to the pure D plasma. He seeding doesn’t show significant change, but causes another desorption peak at higher temperature besides the low temperature peak. • (3) A tiny desorption peak appear at ~800 K. Similar to the deposition condition, He seeding leads to desorption at higher temperature.

15. Retained deuterium concentration in C, Be and W deposits under co-deposition conditions This study. (D+Be, deposition) • J. Roth et al., J. Nucl. Mater. 390-391 (2009) 1

16. Deuterium retention in various materials as a function of incident ion fluence. • 200 eV D ions, 300K D+Be (depo.) 10 eV, 573 K pure-D 60ev, 573 K D+Be (ero.) 60 eV, 573 K • J. Roth, J. Physics, 100(2008)062003

17. Internal microstructure TEM • Pre-thinned W, 5x1025 D/m2, 573K

18. Summary The effects of Be seeding to the D+He mixture plasma have been examined in W using PISCES-B plasmas. • Be seeding to D+He plasma results in the suppression of high density He nano-bubbles at both conditions of deposition and erosion. • In contrast, deuterium bubbles form in the Be deposition layer for the sample exposed to D+Be plasmas at the deposition condition. • Due to the formation of the co-deposition layer of Be and D, D retention increase for the sample exposed to D+Be plasmas at the deposition condition, and high ratio of D/Be (~0.03) is obtained. • D retention is suppressed for the sample exposed to D+Be plasma at erosion condition, regardless of He seeding. • The influence of He seeding on the retention is almost eliminated at both conditions of Be deposition and erosion. • For a reliable prediction of the tritium retention in ITER, the temperature dependence of microstructures and retention properties should be investigated.