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The effects of high fluence mixed-species (D, He, Be)

plasma interactions with W

18th Int. Conf. on Plasma Surface Interactions, May 26–30, (2008) Toledo, SpainSession 13. Fuel Retention in Metallic PFCs (II) I–20

M.J. Baldwin, R.P. Doerner, D. NishijimaUniversity of California, San Diego, USA

K. Tokunaga Kyushu University, Fukuoka, Japan

Y. Ueda Graduate School of Engineering, Osaka University, Japan

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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

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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.

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

PISCES ITER (edge)

Ion flux (cm2s–1) 1017–1019 ~1019

Ion energy (eV) 20–300 (bias) 10–300 (thermal)

Te (eV) 4–40 1–100

ne (cm–3) 1012–1013 ~1013

Be Imp. fraction (%) Up to a few % 1–10 (ITER)

Pulse length (s) Steady state 1000

PSI materials C, W, Be C, W, Be ..

Plasma species H, D, He H, D, T, He

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D2−Be PMI experiments

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Ts (K)1000 1500

f Be+

10-4

10-3

10-2

10-1

PISCES-B

D+-0.1He+=2 x1022 m-2s-1

ITER

D+-0.1He+=2 x1023 m-2s-1

15 eV

60 eV

PISCES-BBe injection limit

ITERdivertor

(Upper limit)

BE LAYERS FORM

BE LAYERSDO NOT FORM

2

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)

rDBe

Bein 1 RfJ

[9] yieldssputter , , BeBeBeD YYfractions redep. , , ed RR

see

dDBeBeBe

dDBeDBeout

1

1

1

TR

RYf

RYJ

flux diffusion bulk ,sT

[9] coef refl. WBe ,r R

[10]flux evap. ,e

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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

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At high temperature Be-W alloying is a concern: Alloy melting points are closer to Be than W.

Atomic percent Tungsten

0 10 20 30 40 50 60 70 80 90 100

Tem

pera

ture

(o

C)

1000

2000

3000

Weight percent Tungsten

0 70 80 90 100

Be W

34

22

o C

LIQUID

Be

12W Be

2W

<1750o C

<2250o C

12

87

o C

Be

22W

~952100 50o C

Stable Be-W alloys

• 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?

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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

Binding energy (eV)

110115

N(E

) (

Arb

uni

ts)

050

010

0015

0020

00

Be 1s

Met

allic

Be

111

.80

eV

Be

oxid

e

3035

020

0040

00

W 4f

Ber

yllid

e 1

11.1

5 eV

W 3

1.0

eV

Beryllidedoubletshifts-0.25 eV

STD

1320 K

1020 K

850 K

W 17% Be 83%

W 15% Be 85%

W 23% Be 77%

D+ ion fluence ~ 1.2×1026 m-2

fBe ~ 0.001

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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 m 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

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He PMI experiments

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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., JNM 367–370 (2007) S. Nagata, et. al., JNM 307–311 (2002)

Sub-micron scale holes/bubbles due to He plasma >1600 K, E.g.

D. Nishijima et. al . JNM 313–316 (2003)

& recently, in the range 1250–1600 K, nanometer scale bubbles and morphology has been observed. E.g. S. Takamura et. al , Plasma and Fusion Research 51 (2006)M.J. Baldwin 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.

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PISCES-B: pure He plasma M.J. Baldwin and R.P. Doerner, NF 48 3 (2008) 035001Ts = 1200 K, t = 4290 s, 2x1026 He+/m2, Eion = 25 eV

Nanoscopic morphology seems to be machine and material independent.

NAGDIS-II: pure He plasmaN. 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

W bulk(press/rolled W)500 nm

Nanomat.(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. JNM 337–339 (2005)Ts = 1250 K, t = 1 s (1 shot), 1022 He+/m2, Eion = 100-200 eV

Nano morphology

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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)

Energy (keV)

0 5 10

Cou

nts

CO

W

WW

Mo

W

W

WW

W

15 kV

RN01222007

B C - O 6 %Mo - W 94 %

A C 6 %O 9 %Mo 3 %W 82 %

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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 m thick of nano material is formed for 1 h of He plasma exposure.

• At 1320 K the layer is ~4 m 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

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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, He+= 4–6×1022 m–2s–1, Eion ~ 60 eV

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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.

t1/2 (s1/2)

0 50 100 150

Laye

r th

ickn

ess

, d (

m)

05

Ts=1120 K

Ts=1320 K

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D2−He Experiments

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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 m.

• Layer thickness, ~2.0 m in D2−0.2He is comparable to pure He.

D+He= 4–6×1022 m–2s–1

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He+(m-2s-1)

1021 1022 1023

Laye

r th

ickn

ess

( m

)

0.01

0.1

110

Ts= 1120 K

t = 3600 s

He

D2-He

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

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D2−He−Be Experiments

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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. D+He= 3×1022 m–2s–1

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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. D+He= 3×1022 m–2s–1

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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.