The effects of high fluence mixedspecies D, He, Be plasma interactions with W

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18th Int. Conf. on Plasma Surface Interactions,
May 2630, (2008) Toledo, SpainSession 13. Fuel
Retention in Metallic PFCs (II) I20
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. 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 lt 1000 K.
• Should ITER explore the all W metal divertor
  option. TWsurf gt 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_p
lants.htm
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W surfaces will interact with mixed speciesD,
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).
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D2-Be PMI experiments
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A simple particle transport calculation can be
used to predict the Be layer formation.
Values taken fromW. Eckstein, IPP Report 9/17,
(1998)D. R. Lide, CRC Handbook of Chem. Phys.,
Internet Version (2005)
2
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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)
  337339, 887.
• At 550 K a blistered surface is prevalent after
  exposure to D2 plasma.
• A thin layer of Be as little as a few 10s 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.

• Stable Be-W inter-metallics are
• 2200C (Be2W)
• 1500C (Be12W)
• 1300C (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
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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
  literatureE.g. Wiltner Linsmeier,JNM 337339
  (2005)
• However, at 850 K reaction rates and alloy growth
  is veryslow E.g. M. J. Baldwin, et al, JNM
  (2007) 363365 1179

D ion fluence 1.21026 m-2fBe 0.001
2
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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 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 1.21026 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 lt800 K, E.g. W.M.
  Shu, et. al., JNM 367370 (2007) S. Nagata, et.
  al., JNM 307311 (2002) Sub-micron scale
  holes/bubbles due to He plasma gt1600 K, E.g.
• D. Nishijima et. al . JNM 313316 (2003)
  recently, in the range 12501600 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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Nanoscopic morphology seems to be machine and
material independent.
PISCES-B pure He plasma M.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)

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
LHD pure He plasma M. Tokitani et al. JNM
337339 (2005) Ts 1250 K, t 1 s (1 shot),
1022 He/m2, Eion 100-200 eV
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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)

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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 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 121026 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,
GHe 461022 m2s1, 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 ?1016 m2s1
• D1320 K 2.0 ?0.5 ?1015 m2s1
• Overall process is consistent with an
  activation energy of 0.7 eV.

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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 mm.
• Layer thickness, 2.0 mm in D2-0.2He is
  comparable to pure He.

GDHe 461022 m2s1
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Nano-morphology growth rate depends on He flux
below 71021 m-2s-1.

• Two regions of interest
• Layer growth rate increases exponentially for
  He fluxes up to 71021 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.

GDHe 31022 m2s1
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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.

GDHe 31022 m2s1
8
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Summary Implications

• W exposed to D2 plasmas w/ Be at 10701320 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 71021 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.