Target Fabrication Experiences at OMEGA Applied to IFE Target Fabrication Issues

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Target Fabrication Experiences at OMEGA Applied
to IFE Target Fabrication Issues
D. R. Harding, R. Gram, M. Wittman, M. Bonino,
and Chi Hwa Wu Laboratory for Laser
EnergeticsUniversity of Rochester andA.
Nikroo and D. Czechowitz General Atomics
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Overview

• Permeability of metal overcoats on shells
• Buckle strength and permeability of foam shells
• Layering deuterium ice in non-foam shells
• Accuracy of the shadowgraphy technique
• Layering deuterium ice in foam shells

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An imploded foam target yielded a higher ion
temperature and lower rr than non-foam targets
Shot 33220 Y1n 1.74E11 Y2n 3.52E08 Tion 5.2
keV Burn 195 ps Offset 40 mm
Comparison with expected results depend on the
actual foam density.
GMXI core image
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Even the most permeable aluminum overcoat (50-nm)
significantly increases the fill time
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Metals other than Pd may be acceptable overcoats,
gas transport is probably more dependent on
microstructure than chemical composition

• Comparison of bulk permeability suggests Pd is
  the best candidate metal
• Pd 4x10-11 mol/m.s
• Fe 3x10-14 mol/m.s
• CH 6x10-15 mol/m.s
• (at 1 Pa DP and 300K)
• Caution
• Pd hydrides easily
• interstitial lattice sites become saturated with
  hydrogen this causes volumetric expansion and
  brittle failure
• Permeation is then substituted by diffusion
  through cracks
• Typically Pd is alloyed with 23 at. Ag to limit
  hydriding

Extrapolating bulk permeability to very thin
films is highly questionable because of
solubility and grain boundary effects need data.
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A foam foundation provides a small (50)
increasein the rupture strength of thin-wall
targets
L
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A parametric study of fill time versus allowable
pressure differential suggests an extended fill
cycle
IFE targets scaled from measurements of OMEGA
foam targets

• Fill time considerations
• Pressurization rate DPmax/t
• (DPmaxE.(w/dia)2 tw.dia)
• Fill time Final_pressure/Pressurization rate
• (Fill time Pmax . dia3 /E . w)
• For OMEGA target timemin 0.35 days
• Assumptions for the IFE target (3.90mm-dia)
• Permeability of the plastic is the same as GDP
• Only affect of the foam is to increase the
  elastic modulus of the overcoat by 50
• Metal overcoat (50-nm Pd) has no affect
• Permeation occurs at 300K

Wall thickness(mm) DPmax (atm) Fill time (days)
1 0.014 34
5 0.35 6.9
10 1.4 3.4
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LLE is investigating the feasibility of making
vapor deposited carbon foam

• Composition
• Hybrid carbon nanotube structure mostly pure
  carbon
• Advantages
• Strength
• Control density, wall uniformity
• Future work
• Properties of structure formed at low
  temperature required for capsule manufacture

Candidate method for providing a foam overcoat ?
durable foam to survive processing injection.
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The best ice layer possessed a 2-D rms roughness
of 1.1 to 2.1 mm for each of 20 rotational views
of the layer

• The rms roughness reported is for all modes
  (including 1) in each 2-D power spectrum.

T1852
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Combining multiple 2-D views of individual great
circles reveals 3-D features

• 20 great circles (15? intervals) rms 1.1 to 2.1
  mm

T1853
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Shadowgraphic analysis reports a higher roughness
than does AFM analysis
T1856
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Lowering the targets temperature 1 K below the
temperature where it is formed affected the layer

• Ice layers formed at 18.7 K.
• Temperature was lowered at 0.1 K/h.

T1854
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The ice layer recovers part of its original
smoothness when left to anneal at the lower
temperature
T1855
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Initiate crystal growth by decreasing the
temperature of the liquid below the triple point

• Too large a temperature decrease (gt50 mK) caused
• rapid crystal growth and poor resulting layer
  quality.

T1857
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Delamination between the ice and plastic produces
variable ice thickness while preserving an
isothermal inner ice surface
T1859
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Foam targets containing liquid deuterium become
opaque well above the triple point.
T 19K Rapid cool

• T gt 26K

T 19K Slow cool
Slowly cooling the target starting at 26K
improves the transparency.
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Foam opacity attributed to voids (gas bubbles)
developing in the foam pores as the liquid
densifies and surface tension prevents
back-filling
Foam pore
Liquid D2 densifies as temperature decreases
Surface tension and viscosity affect liquid
mobility.
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To date, it has not been possible to
characterizing the ice thickness in a foam target
Liq filled foam
Ice in a foam target
Ice filled foam

• The issue is the scattering effect of the
  liquid/ice in the foam pores, not the
  transparency of the foam alone future plans
  call for longer wavelength illumination (1.3 or
  1.8 mm) to reduce scattering losses.