1. PISCES R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 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. Ueda Graduate School of Engineering, Osaka University, Japan 1 Work performed as part of US-Japan TITAN Collaboration Presented at 49 th APS Meeting, Nov. 12-16, 2007

2. PISCES R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 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 T Wsurf < 1000 K. In the ITER ‘all W metal divertor’ option, T Wsurf > 1000 K. In DEMO, efficient power output also requires high W wall temperature. ITER remote handling - divertor cassette mock-up http://www.alca-schio.com/nuclear_fusion_plants.htm

3. PISCES R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 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 1600 K, D. Nishijima et. 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. Takamura et. 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. PISCES R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 6 PISCES-B: pure He plasma M. Baldwin & R Doerner, Nucl. Fusion (2008) T s = 1200 K, t = 4290 s, 2x10 26 He + /m 2, E i = 25 eV Nanoscopic morphology seems to be machine and material independent. NAGDIS-II: pure He plasma N. Ohno et al., in IAEA-TM, Vienna, 2006, TEM - Kyushu Univ T s = 1250 K, t = 36,000 s, 3.5x10 27 He + /m 2, E i = 11 eV W bulk (press/rolled W) 500 nm Nano mat. (SEM) Structures a few tens of nm wide Structures contain nano bubbles (AFM) (annealed W) 100 nm (VPS W on C) (TEM) LHD: pure He plasma M. Tokitani et al. J. Nucl. Mater. 337–339 (2005) T s = 1250 K, t = 1 s (1 shot), 10 22 He + /m 2, E i = 100-200 eV Nano morphology

5. PISCES R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 PISCES-B experiments study fusion relevent Plasma Materials Interaction (PMI).

6. PISCES R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 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 negligible weight loss/gain. – C and Mo impurities, (from PISCES-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 by nano-structures. – Hot W immersed in He gas does not form nanostructures. – Are nano-structures diffusion pathways into the bulk?

7. PISCES R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 The thickness of the nano-structured W layer increases with plasma exposure time. 300 s 2000 s 4300 s 9000 s 22000 s Consistent He plasma exposures: T = 1120 K,  He+ = 4–6 ×10 22 m –2 s –1, E ion ~ 60 eV SEM cross-sections of W targets exposed to PISCES-B pure He plasmas.

8. PISCES R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 The growth of the thickness of the nano- structured layer follows 1-D diffusion. t 1/2 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, D 1120 K = 6.6  0.4  10 –12 cm 2 s –1 D 1320 K = 2.0  0.5  10 –11 cm 2 s –1 Process is consistent with an activation energy of ~0.7 eV.

9. PISCES R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 7 (2) D 2 -He plasma E i = 60 eV n He+ /n e ~ 10 % t = 4200 s 10 25 He + /m 2 The He ion Flux / Fluence dependence is not as influential to nano-structure growth as ‘time’. (3) He plasma E i = 60 eV t = 420 s 10 25 He + /m 2 (1) He plasma, E i = 25 eV t = 4290 s 2x10 26 He + /m 2

10. PISCES R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 An incident beryllium flux in He plasma affects nano-structure morphology growth rate E i = 60 eV, T s = 1170 K, 5.4x10 26 He + m -2 He plasmaHe plasma with Be n Be+ /n e ~ 0.1 %, t = 9000 s Nano-structured layer ~ 4  m thick Nano-structured layer ~ 2  m thick, but morphology is similar. Surf. AES: 53% Be, 47% W t = 9000 s

11. PISCES R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 7 E i = 60 eV, T s = 1150 K, 10 25 He + /m 2 D 2 -He plasmaD 2 -He plasma with Be n He+ /n e ~ 10 %, n Be+ /n e ~ 0.2 %, t = 4200 s Similar slowed growth is also found in D 2 -10 % He plasmas with injected Be Nano-structured layer ~ 0.4  m thick Nano-structured layer ~ 0.1  m thick. Surf. AES: 88% Be, 12% W (Be 12 W ?) n He+ /n e ~ 10 %, t = 4200 s

12. PISCES R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 8 RN01312007 Be 12 W layer RN01292007 95% C layer Plasma deposited Be and C layers completely inhibit nano-morphology at ~1150 K. E i = 15 eV, T s = 1150 K, Fluence = 10 25 He + /m 2 D 2 -He plasma with Be n He+ /n e ~ 10 %, n Be+ /n e ~ 0.5 %,  t = 5000 s D 2 -He plasma with C n He+ /n e ~ 10 %, n C+ /n e < 0.1 %,  t = 3600 s Surface layer composition determined by x-ray microanalysis (WDS). At E i = 15 eV, Be and C deposited on W are not sputtered away.

13. PISCES R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 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.