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R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 U C S DU niversity o f C a lifo rn ia S a n D iego

Mixed plasma species effects on Tungsten

M.J. Baldwin, R.P. Doerner, D. NishijimaUniversity 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 CollaborationPresented at 49th APS Meeting, Nov. 12-16, 2007

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R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 U C S DU niversity o f C a lifo rn ia S a n D iego

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

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R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 U C S DU niversity o f C a lifo rn ia S a n D iego

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. 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.

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R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 U C S DU niversity o f C a lifo rn ia S a n D iego 6

PISCES-B: pure He plasmaM. Baldwin & R Doerner, Nucl. Fusion (2008)Ts = 1200 K, t = 4290 s, 2x1026 He+/m2, Ei = 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 UnivTs = 1250 K, t = 36,000 s, 3.5x1027 He+/m2, Ei = 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. J. Nucl. Mater. 337–339 (2005)Ts = 1250 K, t = 1 s (1 shot), 1022 He+/m2, Ei = 100-200 eV

Nano morphology

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R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 U C S DU niversity o f C a lifo rn ia S a n D iego

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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R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 U C S DU niversity o f C a lifo rn ia S a n D iego

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?

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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R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 U C S DU niversity o f C a lifo rn ia S a n D iego

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 ×1022 m–2s–1, Eion ~ 60 eV

SEM cross-sections of W targets exposed to PISCES-B pure He plasmas.

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R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 U C S DU niversity o f C a lifo rn ia S a n D iego

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.

t1/2 (s1/2)

0 50 100 150

La

yer

thic

knes

s ( m

)

05

Ts = 1120 K

Ts = 1320 K

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R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 U C S DU niversity o f C a lifo rn ia S a n D iego 7

(2) D2-He plasmaEi = 60 eV

nHe+/ne ~ 10 %

t = 4200 s1025 He+/m2

The He ion Flux / Fluence dependence is not as influential to nano-structure growth as ‘time’.

(3) He plasmaEi = 60 eV

t = 420 s1025 He+/m2

(1) He plasma,Ei = 25 eV

t = 4290 s2x1026 He+/m2

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R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 U C S DU niversity o f C a lifo rn ia S a n D iego

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 BenBe+/ne ~ 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

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R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 U C S DU niversity o f C a lifo rn ia S a n D iego 7

Ei = 60 eV, Ts = 1150 K, 1025 He+/m2

D2-He plasma D2-He plasma with BenHe+/ne ~ 10 %, nBe+/ne ~ 0.2 %, t = 4200 s

Similar slowed growth is also found in D2-10 % 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 (Be12W ?)

nHe+/ne ~ 10 %,t = 4200 s

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R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 U C S DU niversity o f C a lifo rn ia S a n D iego 8

RN01312007

Be12Wlayer

RN01292007

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 BenHe+/ne ~ 10 %, nBe+/ne ~ 0.5 %, t = 5000 s

D2-He plasma with CnHe+/ne ~ 10 %, nC+/ne < 0.1 %, t = 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.

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R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 U C S DU niversity o f C a lifo rn ia S a n D iego

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.