Progress on HAPL Foam Shell Overcoat Fabrication Presented by Jared Hund1 N' Alexander1, J' Bousquet

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Progress on HAPL Foam Shell Overcoat
FabricationPresented by Jared Hund1N.
Alexander1, J. Bousquet1, Bob Cook1, D. Goodin1,
D. Jasion1, R. Paguio1, K. Saito11General
Atomics, Inertial Fusion Technology, San Diego,
CA 19th HAPL WorkshopUniversity of
WisconsinMadison WIOctober 22 - 23, 2008
IFT\P2008-088
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Since the last HAPL meeting we have made progress
on overcoats

• Improved surface defects on foam which has
  resulted in fewer defects on the overcoat surface
• Measured parameters important for fill
  cycle/tritium inventory
• permeation rate at elevated temp
• Built 2nd high capacity GDP(roto)coater for more
  experiments

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We are focusing on low yield steps of the foam
target fabrication process

• Foam fabrication cumulative yield (pre-coating)

90
DVB
Droplet survival
RF
DVB
Curing/Drying
RF
DVB
Sphericity
RF
DVB at 3 NC
65
Wall uniformity (NC)
DVB at 1 NC
RF at 3 NC
RF at 1 NC (zero yield)
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We are also focusing on the overcoat yield

• Capsule fabrication cumulative yield

65
DVB
Foam Fabrication
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RF
HAPL Target
DVB w/15 µm
0
Gas Retention
RF w/15 µm 8
Thin High Z coating
5-10 µm CH Overcoat
RF w/10 µm 0
Surface Finish
DVB 0
RF 0
Foam DT
DT
DT Vapor
2.3 mm rad
Foam layer 0.18 mm thickness Divinyl Benzene
(DVB) or Resorcinol-Formaldehyde (RF)
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We are working to reduce the overcoating
thickness to 5-10 µm

• We are continuing our parametric evaluation
• of GDP coating conditions on foam
• Background pressure
• Foam surface
• Reactant flow rates
• The essential function of the overcoating is to
  be permeable at RT yet hold the fuel at cryo
• Test permeation with mass spectrometer
• At 20C
• At cryo (LN2) temperature
• Surface roughness
• Measured with Interferometric Profiler

3 dia. chamber
GDP Rotocoater
MS Signal of Good Shell Cycled between 20C and
LN2 temperatures
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We can successfully make gas tight overcoatings
at 15 microns thickness
Improved parameters April 2008
Plot of gas retention Oct 2007
Green pass cryo permeation test Black fail
cryo permeation test

• The improvement came from tailoring the coating
  pressure ranges

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We have reduced the isolated defects appearing on
the overcoated shells
30 nm RMS RF Coated with GDP
60 nm RMS RF Coated with GDP
Both areas 115 nm RMS

• Reducing defects on the foam reduced defects in
  the overcoating
• But - the background roughness is the same

The foam surface roughness is now below the spec,
but coating parameters need to be improved
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A number of items need to be optimized for good
surface finish and gas retention

• Coating pressure
• Evaluated background pressures for smoothing
  effect
• Experiments of 50 500 mtorr showed 250 mtorr
  was smoother
• 250 mtorr background pressure for entire run
• Sticking was a problem we were able to overcome
• Stress in the coating is a problem
• Use variable pressure 250 then 50 mtorr
• Current experiments high pressure for more of
  the coating thickness (but not all)

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The tritium inventory required is set by the
coating characteristics

• The temperature of the fill can affect the
  properties of the coating (and fill rate)
• Buckle strength (can decrease with T)
• Permeation rate (increases with T)

Plot of tritium inventory as a function of
temperature for various Ea and reduction in
buckle strength
High temperature fill will be necessary to keep
the tritium inventory low
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The permeation rate through GDP was measured as
function of temperature

• The permeation rate through GDP was measured as
  function of temperature

Ea 15 kJ/mole

• 4x faster permeation at 100C
• 10x faster permeation rate at 130C

But the actual fill rate may be reduced because
of a reduction in buckle strength of the shell,
but that will no greatly affect tritium inventory
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Conclusion

• Foam yields are progressing with systematic,
  parametric studies
• We are evaluating the mechanisms to improve
  coatings parameters for thinner, gastight, smooth
  coatings
• We will evaluate the needed Tritium inventory due
  to fill cycle
• measure effect of temperature on buckle strength