Engineering Analysis of He Retention

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Engineering Analysis of He Retention Release
Experiments to Determine Desirable Engineered W
Armor Microstructure

• A. René Raffray
• UCSD
• With Contribution from S. ODell
• PPI
• HAPL Meeting
• University of Rochester's Laboratory for Laser
  Energetics
• Rochester, NY
• November 7-8, 2005

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He Implantation and Behavior in W Armor Quite
Complex, Consisting of a Number of Mechanisms
Due to their high heat of solution, inert-gas
atoms are essentially insoluble in most
solids. This can then lead to gas-atom
precipitation, bubble formation and ultimately
to destruction of the material.
Helium atoms in a metal may occupy either
substitutional or interstitial sites. As
interstitials, they are very mobile, but they
will be trapped at lattice vacancies, impurities,
and vacancy-impurity complexes. The
following activation energies were estimated for
different He processes in tungsten
1,2 - Helium formation energy 5.47
eV - Helium migration energy 0.24 eV - He
vacancy binding energy 4.15 eV - He vacancy
dissociation energy 4.39 eV - From 3, D
(m2/s) D0 exp (-EDif/kT) D0 4.7 x 10-7
m2/s and EDif 0.28 eV
1. M. S. Abd El Keriem, D. P. van der Werf and F.
Pleiter, "Helium-vacancy interactions in
tungsten," Physical review B, Vol. 47, No. 22,
14771-14777, June 1993. 2. W. D. Wilson and R. A.
Johnson, in Interatomic Potentials and Simulation
of Lattice Defects, edited by P. C. Gehlen, J. R.
Beeler and R. I. Jaffee (Plenum New York, 1972),
p375. 3. A. Wagner and D. N. Seidman, Phys. Rev.
Letter 42, 515 (1979)
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IFE Relevant Experimental data on He Implantation
and Release in W
He retention in polycrystalline and single
crystal W samples as a function of He dose per
cycle for different number of pulses based on He
implantation and temperature anneals
From UNC/ORNL experimental results described in
past presentations from L. Snead, et al., e.g. at
the March 2005 HAPL meeting or at the US/Japan
Workshop on Laser IFE, General Atomics, San
Diego, CA, March 2005
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Engineering Way of Interpreting Results
Detailed modeling of all mechanisms useful and
should be pursued However, many unknown
parameters Effective diffusion analysis
conducted to characterize activation energy
associated with controlling mechanism in He
migration and trapping in W Parametric study of
experimental results to estimate effective
diffusion activation energy to reproduce He
retention for each experimental anneal case
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Effective diffusion activation energy required to
reproduce the experimental results for single
crystal and polycrystalline W
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Effective Diffusion Activation Energy (Eeff,diff)
as a Function of Dose per He Implantation(The
curve fit has been drawn to suggest a possible
variation of the activation energy with the He
dose or concentration)
Hypothesis In general, trapping increases
with He irradiation dose which creates sites
through dpa's and formation of vacancies
(followed by an anneal of the unoccupied trapped
sites during the ensuing temperature
transient). At very low dose, only a few
trapping sites are activated by the irradiation
and the helium transport should be governed by
bulk diffusion (with an activation energy of
0.24-0.28eV).
IS THIS REASONABLE?
As the dose per cycle increases, an increasing
number of trapping sites are formed or
activated and Eeff,diff increases. It seems
that there is a near- threshold of He dose at
which Eeff,diff increases rapidly to 3.3-3.6
eV and stays at this value over a significant
dose range. Above this range, Eeff,diff
increases rapidly to 4.2-4.8 eV, indicating
an increase in trapping perhaps due to He
build up in vacancies (the vacancy
dissociation energy is 4.4 eV).
IFE Case 5 x 1016 atoms/m2
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Simulation of IFE W Armor Case
History of He Concentration in W Armor assuming a
per-shot implanted He concentration of 3.2x1022
atoms/m3 (assuming 6.8x1019 He ions per shot
for a 350 MJ target in a 10.5 m chamber with an
average implantation depth of 1.5mm) Eeff,diff
3.52 eV for polycrystalline W case Results for
Different Porous Microstructure Dimensions are
Shown
W Armor Temperature History for 350 MJ Target and
10.75 m Chamber Radius
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Example Results from Modeling He Retention in
Porous W IFE Armor
He concentration for SC W (Eeff,diff3.38 eV)
55-60 that of PC W (Eeff,diff3.52 eV)
Key question what at. of He in W is
acceptable for acceptable armor lifetime?
- Previously, G. Lucas suggested 15 at. as
critical concentration for a blister to
exfoliate - This suggests that porous W
microstructure dimension could be between 0.1
and 1 mm - However, given uncertainties in
modeling, it seems prudent to maintain a
porous W microstructure 50-100nm until
shown otherwise by prototypical experimental
results (Recommendation to PPI) As
indicative of cases with lower He ion doses,
example results for Eeff,diff2.4 eV also shown
(for lt1016 ions/m2 per shot ). - He
retention much reduced, indicating benefit of
operation at ion doses below the assumed
threshold shown earlier - This threshold is
1016 ions/m2 based on these initial
experimental results. Future effort is needed
to confirm and better understand the material
form dependence of this threshold (a factor of
five increase would bring it very close to the
current IFE case).
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PPIs Progress in Manufacturing Porous W with
Nano Microstructure

• Plasma technology can produce tungsten nanometer
  powders.
• - When tungsten precursors are injected into the
  plasma flame, the materials are heated, melted,
  vaporized and the chemical reaction is induced
  in the vapor phase. The vapor phase is quenched
  rapidly to solid phase yielding the ultra pure
  nanosized W powder
• - Nano tungsten powders have been successfully
  produced by plasma technique and the product
  is ultra pure with an average particle size of
  20-30nm. Production rates of gt 10 kg/hr are
  feasible.
• Process applicable to molybdenum, rhenium,
  tungsten carbide, molybdenum carbide and other
  materials.
• The next step is to utilize such a powder in the
  Vacuum Plasma Spray process to manufacture porous
  W (10-20 porosity) with characteristic
  microstructure dimension of 50 nm .

TEM images of tungsten nanopowder, p/n S05-15.