Ion Beam Materials Analysis and Modifications Group

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Determining the Effect of Helium Injected into
HAPL Tungsten Armored Wall S. Gilliam a, S.
Gidcumb a, D. Doll a, N. Parikh a, J. Hunn b, L.
Snead b, R. Downing c a University of North
Carolina at Chapel Hill, Chapel Hill, NC
27599-3255, USA b Oak Ridge National Laboratory,
P.O. Box 2008, Oak Ridge, TN 37831-6138, USA c
National Institute of Standards and Technology,
Gaithersburg, MD 20899-3460, USA
Ion Beam Materials Analysis and Modifications
Group University of North Carolina at Chapel Hill
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Previous Work

• In previous work we have looked into the fate of
  tungsten armor implanted with helium. Variables
  explored have included
• Temperature (irradiation temperature and
  post-implant annealing)
• Microstructure
• Total integrated helium dose (concentration) and
  dose packets.

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Previous studies with monoenergetic helium
SEM of blistered tungsten

• Implanted helium is trapped and accumulates to
  form stable bubblesBubbles grow until the
  pressure blisters the surface
• 1.3 MeV 3He implanted at 850C to a dose of 2 x
  1021 He/m2 followed by a flash anneal at 2000C

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Less retention with cyclic implantation and
annealing

• Implanted 1019 3He/m2 at 850C followed by a
  flash anneal at 2000C
• Same total dose was implanted in 1, 10, 100, and
  1000 cycles of implantation and annealing

Relative 3He retention for single crystal and
polycrystalline tungsten with a total dose of
1019 He/m2. Percentage of retained 3He compared
to implanting and annealing in a single cycle.
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Deficiencies in Experiment and Understanding -
where do we go next? -

• Work to date has demonstrated that helium
  injected at high levels may not exfoliate
  tungsten as originally feared. The key to
  survival will be a combination of microstructure
  of near surface tungsten, small concentration of
  injected helium per pulse, and the
  time-at-temperature driving the helium diffusion.
• Insufficiencies in the current work.
• Generated data must be more accessible for
  modeling
• Time-at-temperature has not been IFE relevant due
  to the tools used (resistive
• heaters.) As to be discussed the kinetics of
  diffusion may be overly conservative
• in our experiments.

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How do we produce a helium threat spectrum?

• Degrade the monoenergetic beam by transmission
  through a thin Al foil
• Tilting a single foil provides a range of
  degraded energies by varying the path length d
  through the foil material where ? 0 is
  normal incidence

Foil
Tungsten
E0 He beam
E E0 ?Efoil
t

• Transmitted energy is approximated as a Gaussian
  centered at Ei (E0 ?Efoil)and broadened by
  the energy straggling through the foil ?

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Approximating the threat spectrum

• Helium threat spectrum is approximated as a
  function f(E)
• Approximate f(E) as a linear combination of the
  Gaussian degraded energieswhere f(Ej) is a
  point on the profile, wij is a weighting
  coefficient, Gi is the ith Gaussian contribution
  to the jth point on the profile f(E)

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Computing the solution

• Many of the matrix elements will be zero because
  Gaussians far away from Ei wont contribute to
  the point f(Ei)
• Weighting coefficient matrix elements correlate
  the Gaussians to each other
• Diagonalize W to find the weight for each
  individual Gaussian function so that the linear
  combination approximates the desired energy
  spectrum f(E)
• Weighting coefficients determine the dose to
  implant and each Gaussian has an associated tilt
  angle
• Assuming a constant beam current, then dose ?
  timeTherefore, we have tilt angle ? vs. time
• Apply a polynomial fit to this ? vs. t plot and
  use the time derivatives (i.e. angular velocity
  and acceleration) to program the tilt position
  motor

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Experimental progress of the project

• 1.8 MeV He beam transmitted through Al foils
  ranging 1.5 to 5.5 microns thickDegraded
  energies 1400 100 keVAl stopping power 300
  keV/micron
• Compare theoretical and experimental values of
  ?Efoil and ? through foils
• Implanted tungsten samples with 1.8 MeV 3He
  energy degraded by various foil thicknesses
  listed below

Foil thickness (?m) Tilt angle ? (degrees) Effectivethicknesst / cos ? (?m)
1.5 0 1.5
41 2.0
3.0 0 3.0
31 3.5
41 4.0
4.5 0 4.5
26 5.0
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?Efoil and ? from Neutron Depth Profiling

• NDP uses 3He(n, p)T reaction to measure the
  helium depth profileNumber of protons is
  proportional to helium concentrationDetected
  proton energy converted to depth scale by energy
  loss
• Projected range Rp and the longitudinal straggle
  ?Rp related to ?Efoil and ?

Tungsten
neutrons
?Rp
Helium
protons
Rp
Helium depth profile for tungsten implanted with
1.3 MeV 3He to a dose of 1020 He/m2
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?Efoil and ? from Rutherford backscattering

• Backscattering used to measure energy straggling
  through foils for comparison to theoretical
  predictions such as the Bohr model
• The key is a heavy energy marker such as Au on
  each side of the target foil

System resolution is ?E1 (?EDet2 ?EBeam2)1/2
25 keV Measured straggle of the transmitted
beam is ?E42 ?E2 ?EDet2 ?EBeam2 46
keV Energy straggling due to the degrader foil
alone ?E (?E42 ?E12)1/2 39 keV
D beam
1.5 ?m Al
Al E1
E4
?E1 ?E4
Au
1.7 MeV deuterium backscattering spectrum for 1.5
?m Al foil target with Au energy markers
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Where we are now with the helium threat spectrum

• Programming required for all calculations and
  foil tilt motion is near completion
• Samples implanted with foil degraded energies
  have been sent to National Institute of Standards
  and Technology (NIST) for NDP analysis
• Continuing energy straggling measurements via
  Rutherford backscattering with helium
• After we successfully produce the IFE helium
  threat spectrum
• 1) Implant tungsten samples with the helium
  threat spectrum to study surface blistering and
  retention characteristics
• 2) Introduce implantation at 850C and flash
  annealing at 2000C as we did with monoenergetic
  helium implantation

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TDS study of helium implanted tungsten
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TDS Data unimplanted polycrystalline tungsten

• Unimplanted polycrystalline tungsten sample
  ramped from RT to 2200C
• Background partial pressure level of 3He remained
  constant (5x10-12 Torr)
• Mass 2 is always present in mass spectrometery
  scans
• We have conducted TDS on 3He and 4He implanted W
  samples to determine if the tail of the mass 2
  peak affects the mass 3 peak value
• So far we conclude that the mass 2 peak tail is
  not a great concern.

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TDS Data Poly W implanted with 3x1020 4He/m2 at
RT
Time (s) Temp. (C)
RT 600 2000 2200

• Ramped sample temperature from RT to 2200C
• Small pulses of desorbed He around 600C
• Observed significant He desorption above 2000C
  which correlates to simultaneous blistering of
  the sample surface
• Surface was blistered after completing the TDS
  experiment

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TDS Data Poly W implanted with 5x1020 3He/m2 at
RT
Time (s) Temp. (C)
RT 600 2000 2200

• Ramped sample temperature from RT to 2200C
• Small pulses of desorbed He around 600 and 2000C
• Significant He desorption above 2000C correlates
  to surface blistering
• Higher partial pressure of 3He detected due to
  higher dose of 3He