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


(Nano-scopic morphology) D2He. D2He Be. PISCES. PISCES ... of He induced nano-scopic morphology is found. ... He induced nano-scopic morphology is inhibited. ... – PowerPoint PPT presentation

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

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
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
  • 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
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.

PISCES-B experiments study fusion relevent Plasma
Materials Interaction (PMI).
D2-Be PMI experiments
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)
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

D ion fluence 1x1026 m-2
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
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
  • 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
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
  • 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
He PMI experiments
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.
  • 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.

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
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
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
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
  • Suggests growth from bulk.
  • But, W bulk is shielded from plasma by
  • Hot W immersed in He gasdoes not form
  • Do nano-structures provide He transport into
    the bulk?
  • What are the kinetics?(E.g. dependencies on
    temperature, exposure time, He ion flux)

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
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
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.

D2-He Experiments
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
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
  • Nano-structure growth may require surface
    saturation or mechanism that traps He.

ITER (Outer strike plate)A. Kukushkin, ITER
Report, ITER_D_27TKC6 2008
D2-He-Be Experiments
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
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
  • 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

Summary Implications
  • W exposed to D2 plasmas w/ Be at 10701320 K
    form Be-W alloy surfaces.
  • Alloying kinetics are optimal w/ Be
  • 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
  • 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
  • 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.