The effects of high fluence mixed-species (D, He, Be) plasma interactions with W

1. 18th Int. Conf. on Plasma Surface Interactions, May 26–30, (2008) Toledo, SpainSession 13. Fuel Retention in Metallic PFCs (II) I–20 The effects of high fluence mixed-species (D, He, Be) plasma interactions with W M.J. Baldwin, R.P. Doerner, D. Nishijima University of California, San Diego, USA K. Tokunaga Kyushu University, Fukuoka, Japan Y. UedaGraduate School of Engineering, Osaka University, Japan

2. W is believed to be one of the most important materials for next generation fusion reactors. • In the divertor of a burning plasma device only C and W are cable of withstanding the intense heat fluxes. • C is comparably better, but … • C use is limited to strike points in ITER divertor due to T retention and neutron damage issues. • ITER divertor-liner/dome are to be fabricated from W. TWsurf < 1000 K. • Should ITER explore the ‘all W metal divertor’ option. TWsurf > 1000 K. • A W DEMO, for efficient power output, also requires such high W temperature. ITER remote handling - divertor cassette mock-uphttp://www.alca-schio.com/nuclear_fusion_plants.htm

3. W surfaces will interact with mixed species:D, T, He, Be. • The desire to operate W surfaces in increased temperature and mixed species plasma regimes reveals a wide-ranging PMI parameter space that is essentially unexplored. • The UCSD PISCES program, through collaborations, (US-EU) and (US-JAPAN), are exploring effects on W (above 1000 K) in plasma regimes that support ITER and advanced reactor PMI development needs. • Experiments to be discussed: PMI effects on W surface properties in • D2−Be. (Be−W alloying) • He. (Nano-scopic morphology) • D2−He. • D2−He −Be.

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

5. D2−Be PMI experiments

6. A simple particle transport calculation can be used to predict the Be layer formation. Values taken from:W. Eckstein, IPP Report 9/17, (1998)D. R. Lide, CRC Handbook of Chem. & Phys., Internet Version (2005) 2

7. From 300-700 K, thin and thick layers of Be suppresses blister formation. • Blistering & exfoliation of blister caps is a concern for certain varieties of W. • Increased retention is associated with the trapping of hydrogen in blisters. • E.g. K Tokunaga et al. J. Nucl. Mater. (2004) 337–339, 887. • At 550 K a blistered surface is prevalent after exposure to D2 plasma. • A thin layer of Be as little as a few 10’s of nm, or thicker, is found to suppress blister formation. D+ ion fluence ~1x1026 m-2

8. At high temperature Be-W alloying is a concern: Alloy melting points are closer to Be than W. • Stable Be-W inter-metallics are: • ~2200°C (Be2W) • ~1500°C (Be12W) • ~1300°C (Be22W) • What will happen if Be transport into the W bulk is rapid enough that alloy formation is not limited to the near surface? Stable Be-W alloys 2

9. XPS confirms Be-W alloy formation on W target surfaces exposed in range 850-1320 K. • Be-W alloy line shifts are consistent with literature:E.g. Wiltner & Linsmeier,JNM 337–339 (2005) • However, at 850 K reaction rates and alloy growth is veryslow E.g.M. J. Baldwin, et al, JNM (2007) 363–365 1179 D+ ion fluence ~ 1.2×1026 m-2fBe ~ 0.001 2

10. Be availability drives alloying reaction: But PMI conditions can reduce Be availability. • Net Be deposits due to minimal re-erosion and minimal Be evaporation. A ~0.3 mm Be12W layer at W-Be interface. • Be deposits are re-eroded. Sparse ~Be12W surface nucleation over W rich surface. No Be sub-surface. • Minimal re-erosion, but increased Be evaporation leads to surface composition below stoichiometry for Be2W. No Be sub-surface D+ ion fluence ~ 1.2×1026 m-2

11. He PMI experiments

12. The effects of He ions on W produces destuctive surface effects. • Below the threshold for physical sputtering, H and He plasma can blister W <800 K, E.g.W.M. Shu, et. al., JNM367–370 (2007) S. Nagata, et. al., JNM307–311 (2002)Sub-micron scale holes/bubbles due to He plasma >1600 K, E.g. • D. Nishijimaet. al . JNM 313–316 (2003)& recently, in the range 1250–1600 K, nanometer scale bubbles and morphology has been observed. E.g. • S. Takamuraet. al , Plasma and Fusion Research 51 (2006)M.J. Baldwin and R.P. Doerner, NF 48 3 (2008) 035001 • 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. • Here we focus on He induced nano-morphology.

13. Nanoscopic morphology seems to be machine and material independent. PISCES-B: pure He plasmaM.J. Baldwin and R.P. Doerner, NF 48 3 (2008) 035001 Ts = 1200 K, t = 4290 s, 2x1026 He+/m2, Eion = 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. JNM 337–339 (2005) Ts = 1250 K, t = 1 s (1 shot), 1022 He+/m2, Eion = 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, Eion = 11 eV 6

14. Simple observations lead to speculation and questions about how W nano-structures grow. • Target nano-structure surface is visually black and easily to remove. • Nano-structures are near 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. • But, W bulk is shielded from plasma by nano-structures. • Hot W immersed in He gasdoes not form nanostructures. • Do nano-structures provide He transport into the bulk? • What are the kinetics?(E.g. dependencies on temperature, exposure time, He ion flux)

15. Nano-morphology is not observed below 900 K. Above, growth is temperature dependent. • No observed morphology after 1 h of He plasma exposure at 900 K. • At 1120 K, a ~2 mm thick of nano material is formed for 1 h of He plasma exposure. • At 1320 K the layer is ~4 mm thick for a little over 1 h of He plasma exposure. • Nano-morphology formed at 1120 K and 1320 K is seemingly identical. He+ ion fluence ~ 1–2×1026 m-2

16. At 1120 K, nano-structured layer thickness increases with He plasma exposure time. 300 s 2000 s 4300 s 9000 s 22000 s Consistent He plasma exposures: Ts = 1120 K, GHe+= 4–6×1022 m–2s–1, Eion ~ 60 eV

17. Layer growth follows kinetics that are controlled by a diffusion like process. • Observed t1/2 proportionality. • The thickness of the nano-structured layer, d, agrees well with • d=(2Dt)1/2, • with, • D1120 K = 6.6 0.4 10–16 m2s–1 • D1320 K = 2.0 0.5 10–15 m2s–1 • Overall process is consistent with an activation energy of ~0.7 eV.

18. D2−He Experiments

19. In D2−He plasmas, nano-morphology persists, but growth rate depends on He+ flux. • The presence of D2 does not appear to affect nano-morphology structure. • But growth rate can be affected. • After a little more than 1 h of He plasma exposure in D2−0.1He, layer thickness is only ~0.5 mm. • Layer thickness, ~2.0 mm in D2−0.2He is comparable to pure He. GD+He= 4–6×1022 m–2s–1

20. Nano-morphology growth rate depends on He+ flux below 7×1021 m-2s-1. • Two regions of interest: • Layer growth rate increases exponentially for He+ fluxes up to ~7×1021 m-2s-1. • Layer growth rate is optimal for He+ fluxes above this. • D2 does not likely affect nano-structured layer growth rate. • Lowest He+ flux data point (pure He) fits trend. • Nano-structure growth may require surface saturation or mechanism that traps He. ITER (Outer strike plate)A. Kukushkin, ITER Report, [ITER_D_27TKC6] 2008

21. D2−He−Be Experiments

22. PMI conditions determine surface properties. Strong Be re-erosion favors nano-morphology. • At 60 eV, plasma sputters away Be deposits. Little affect on the growth of He induced nano-scopic morphology is found. • WDS indicates minimal Be penetration within the nano-structured layer. • AES (surface only) indicates high Be near-surface concentration. GD+He= 3×1022 m–2s–1

23. A thick Be or C layer inhibits nano-morphology. • At ~ 15 eV, PMI conditions favor net Be or C deposition. He induced nano-scopic morphology is inhibited. • A ~Be12W alloy layer is observed on W in a D2−0.1He plasma w/ Be injection. • A C rich layer forms on W in a D2−0.1He plasma w/ CD4 injection. • At ~15 eV, the stopping range for both D+ and He+ is under 1 nm in Be or C. GD+He= 3×1022 m–2s–1 8

24. Summary & Implications • W exposed to D2 plasmas w/ Be at 1070–1320 K form Be-W alloy surfaces. • Alloying kinetics are optimal w/ Be availability. • Re-erosion and/or evaporation inhibits reaction kinetics w/ PMI. • W in He plasmas at 1070−1320 K develops a nano-structed surface layer. • Growth kinetics rate limited by a diffusive process. • Impact on reactor performance not fully clear. • – Issues include: high-Z dust, retention, erosion, thermal conduction. • In D2−He plasmas: D2 does not appear to influence W nanoscopoic morphology. • Optimal growth at 1120 K is observed for He+ flux above 7×1021 m-2s-1. • In D2−0.1He plasmas, small Be or C fractions can impact observed morphology. • Concerning ITER all metal divertor: • Liner and dome (T below 900 K). • – Minimal Be−W alloy & He induced nano-morphology • – Be may alleviate W blistering. • W metal strike-points (T above 1000 K). • – Almost certain to encounter Be−W alloy and/or He induced nano- morphology.