Mixed plasma species effects on Tungsten

1. Mixed plasma species effects on Tungsten M.J. Baldwin, R.P. Doerner, D. Nishijima University of California, San Diego, La Jolla, CA 92093 USA Y. UedaGraduate School of Engineering, Osaka University, Japan Work performed as part of US-Japan TITAN Collaboration Presented at 49th APS Meeting, Nov. 12-16, 2007 1

2. Plasma-material interactions with W under reactor relevant conditions are needed . • ITER has decided to remove the C from its divertor during D/T operation. • The implications of this decision need to be better understood. • High-temperature, large-fluence PMI data is lacking. • Presently the ITER divertor-liner/dome are expected to operate with TWsurf < 1000 K. • In the ITER ‘all W metal divertor’ option, TWsurf > 1000 K. • In DEMO, efficient power output also requires high W wall temperature. ITER remote handling - divertor cassette mock-uphttp://www.alca-schio.com/nuclear_fusion_plants.htm

3. The use of W as a plasma facing material does have drawbacks. • Below the threshold for physical sputtering, H and He plasma can blister W <800 K, E.g.W.M. Shu, et. al., J. Nucl. Mater.367–370 (2007) ` S. Nagata, et. al., J. Nucl. Mater.307–311 (2002)Sub-micron scale holes/bubbles due to He plasma >1600 K, D. Nishijimaet. al . J. Nucl. Mater. 313–316 (2003)& recently, in the range 1150–1600 K, nanometer scale bubbles and morphology has been observed.E.g. • S. Takamuraet. al , Plasma and Fusion Research 51 (2006)M. J. Baldwin et. al , to be published Nucl. Fusion January(2008) • The mechanisms that underpin these phenomena are not well understood, but have largely been attributed to the accumulation of diffusing D and He in defects and vacancies.

4. Nanoscopic morphology seems to be machine and material independent. PISCES-B: pure He plasmaM. Baldwin & R Doerner, Nucl. Fusion (2008) Ts = 1200 K, t = 4290 s, 2x1026 He+/m2, Ei = 25 eV • Structures a few tens of nm wide • Structures contain nano bubbles W bulk(press/rolled W)500 nm Nanomat.(SEM) Nano morphology (AFM) (annealed W) 100 nm (VPS W on C) (TEM) LHD: pure He plasma M. Tokitani et al. J. Nucl. Mater. 337–339 (2005) Ts = 1250 K, t = 1 s (1 shot), 1022 He+/m2, Ei = 100-200 eV NAGDIS-II: pure He plasma N. Ohno et al., in IAEA-TM, Vienna, 2006, TEM - Kyushu Univ Ts = 1250 K, t = 36,000 s, 3.5x1027 He+/m2, Ei = 11 eV 6

5. PISCES-B experiments study fusion relevent Plasma Materials Interaction (PMI).

6. What are W nano-structures & what mechanisms cause them to form? • Target nano-structure surface is visually black and easily to remove. • Nano-structures are nearly pure W and not plasma deposited. Why? • W targets show negligibleweight loss/gain. • C and Mo impurities, (fromPISCES-B plasma) in ‘A’ but not ‘B’.O consistent with surface oxidation • Suggests growth from bulk. • How do they grow? • W bulk is plasma shielded bynano-structures. • Hot W immersed in He gasdoes not form nanostructures. • Are nano-structures diffusionpathways into the bulk?

7. The thickness of the nano-structured W layer increases with plasma exposure time. SEM cross-sections of W targets exposed to PISCES-B pure He plasmas. 300 s 2000 s 4300 s 9000 s 22000 s Consistent He plasma exposures: T = 1120 K, GHe+= 4–6 ×1022 m–2s–1, Eion ~ 60 eV

8. The growth of the thickness of the nano-structured layer follows 1-D diffusion. • t1/2 proportionality implies growth kinetics that are controlled by a diffusional process. • The thickness of the nanostructured layer, d, agrees well with • d =(4Dt)1/2, • with, • D1120 K = 6.6 0.4 10–12 cm2s–1 • D1320 K = 2.0 0.5 10–11 cm2s–1 •   Process is consistent with an activation energy of ~0.7 eV.

9. The He ion Flux / Fluence dependence is not as influential to nano-structure growth as ‘time’. (1) He plasma,Ei = 25 eV t = 4290 s2x1026 He+/m2 (2) D2-He plasmaEi = 60 eV nHe+/ne ~ 10 %t = 4200 s1025 He+/m2 (3) He plasmaEi = 60 eV t = 420 s1025 He+/m2 7

10. An incident beryllium flux in He plasma affects nano-structure morphology growth rate Ei = 60 eV, Ts = 1170 K, 5.4x1026 He+ m-2 He plasma He plasma with Be t = 9000 s nBe+/ne ~ 0.1 %, t = 9000 s Nano-structured layer ~ 4 mm thick Nano-structured layer ~ 2 mm thick,but morphology is similar. Surf. AES: 53% Be, 47% W

11. Similar slowed growth is also found in D2-10 % He plasmas with injected Be Ei = 60 eV, Ts = 1150 K, 1025 He+/m2 D2-He plasma D2-He plasma with Be nHe+/ne ~ 10 %, t = 4200 s nHe+/ne ~ 10 %, nBe+/ne ~ 0.2 %, t = 4200 s Nano-structured layer ~ 0.4 mm thick Nano-structured layer ~ 0.1 mm thick.Surf. AES: 88% Be, 12% W (Be12W ?) 7

12. RN01312007 RN01292007 Be12Wlayer 95% Clayer Plasma deposited Be and C layers completely inhibit nano-morphology at ~1150 K. Ei = 15 eV, Ts = 1150 K, Fluence = 1025 He+/m2 D2-He plasma with Be D2-He plasma with C nHe+/ne ~ 10 %, nBe+/ne ~ 0.5 %, Dt = 5000 s nHe+/ne ~ 10 %, nC+/ne < 0.1 %, Dt = 3600 s Surface layer composition determined by x-ray microanalysis (WDS). At Ei = 15 eV, Be and C deposited on W are not sputtered away. 8

13. Summary • ITER will have significant levels of SOL Be impurities and diverted plasma will involve mixed species (D, Be, He) PMI with W PFC’s. • W surface morphology will evolve due to PSI. • - at low T (<800K) blisters may develop • - above 1600K bubbles and pits may occur • - between 1100-1600K, bubbles and nanostructures form • W nanostructure develops slightly slower during mixed-species (90% D, 10% He) plasma bombardment of W as compared to pure He plasma. • Small amounts of condensable impurities (Be, C) within the incident plasma do not prevent nanostructrue growth. • Sufficient impurity flux to coat W surface prevents nanostructrue growth, but may then result in W-Be alloy formation. • Nano-morphology issues (dust, retention, thermal conductivity, response to transient power loads) need to be investigated.