Solid DT Studies James K' Hoffer, John D' Sheliak,

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Solid DT Studies James K. Hoffer, John D.
Sheliak, Drew A. Geller
LA-UR-02-7258

• presented at the
• High Average Power Laser Review
• sponsored by
• The Department of Energy Defense Programs
• hosted by the
• Naval Research Laboratory
• Washington DC, December 5-6, 2002

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Target Injection-1 Target Materials Response -
LANL
Overall Objective. Response of target
materials to injection stresses FY 02
Deliverables.. 1. Design of an experiment to
determine the effect of a rapid temperature
transient on a representative DT ice
layer. 2. Design apparatus to measure DT yield
strength and modulus. 3. Measure solid
DT surface spectrum following beta-layering
over a layer of foam. Relevance of
Deliverables X Energy Needed for
injection into hot chamber X
NIF Research on materials in NIF targets
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Progress report deliverable No. 1
The effect of a rapid temperature transient on a
solid DT layer
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A new beta-layering cell has been designed and
fabricated.

• We have added internal thermometry to the design.
• We are modeling this geometry to determine the
  amount of heat actually flowing into the solid
  DT.
• The heater insert has become substantially larger
  just to accommodate the thermometer
• Lakeshore Cernox bare chip (.75 mm x .4mm x 1
  mm long)

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The thermometers were too big to fit!

• We had been informed that the leads were attached
  to the long ends of the element. They were, but
  in the wrong orientation!
• We had the precision shop use a small end-mill to
  widen the hole in the heater sleeve.
• The thermometer will be potted in epoxy after the
  heater winding is added.

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The new camera is ready to follow the action at
speeds up to 250 frames/second. Here we show
the new cell being rotated.
Rapid heating cell shown with front lighting
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We are building a simple heater winding insert
to heat the solid DT layer.

• The shape of the winding influences the shape of
  the resulting beta-layer, so it must be highly
  symmetric.
• Priorities of the LANL HEDP program have
  seriously impeded our access to the high
  precision machine shop.

• Nevertheless, the mandrels have been machined and
  the first heater winding has been wound

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We have added a new colleague to expedite our
experiments. He has begun to carry out a
thermal analysis of the heater cell dynamics.
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Progress report deliverable No. 2
Measurement of the strength of solid DT
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Our plan is to first grow a solid DT specimen
with the aid of beta-layering
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Energen, Inc. has supplied a magnetostrictive
actuator customized to our specifications
0.182
1.767
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The design is essentially complete. A set of
check prints is being prepared
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Power dissipation will not be low, as we first
had been informed
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We are working out the last details of the
strength cell

• Power dissipation of the Energen, Inc. actuator
  at steady state will be too high to allow for
  adjustment of the gap.
• Hence we have designed a stage to permit the
  200-m gap between layering posts to be adjusted.

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Progress report deliverable No. 3
The effect of a foam shell on the surface
roughness of the DT layer
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There are several hypotheses concerning the
effects of an intermediate foam layer on the
inner solid DT layer

• Beneficial effects
• A smoother interior surface
• Because of the fine cell structure of the foam,
  freezing should begin with the formation of many
  small, randomly-oriented crystallites. These
  crystallites should propagate into the pure DT
  solid layer, hence there should be no tendency to
  form large crystalline facets at the solid-vapor
  boundary.
• Supercooling of the liquid should not occur
• With millions of nucleation sites presented by
  the foam, the liquid will not supercool as is
  observed in smooth plastic spheres without a foam
  layer.
• Detrimental effects
• If irregular, the overall shape of the foam may
  affect the shape of the solid DT.
• But Im guessing that the gross shape of the foam
  will not influence the shape of the solid DT
  layer, because the foam is a thermal insulator
  and will not disturb the isotherms defined by the
  isothermal boundary (i.e., the metallic cell
  boundary in my cylindrical experiments or the
  layering sphere utilized for spherical targets
  at Omega.)
• The polymeric foam material may be damaged by
  beta activity and decompose.
• DT voids in the foam cells may become trapped
• The solid DT is 12.5 denser than liquid. A void
  space (full of DT vapor) therefore develops
  whenever a cell full of liquid is frozen. (When
  symmetrized by beta-layering, the void in a
  single spherical shell will extend exactly
  half-way across the cell.) Voids first formed in
  the foam cells tend to propagate inwards to the
  central vapor space. If the inner edge of a foam
  cell is blocked by a cell wall (i.e., if the foam
  is not completely open-celled), then the void
  may get stuck. Stuck voids may not be too
  detrimental, because they are sub-micron in size.
  But a secondary effect might be a very slow
  approach to equilibrium wrt the DT layer
  thickness.

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A foam-lined torus will permit clear optical
observations of the DT layer
Empty torusside view(windows not shown)
Filled with foam to yield a 75 micron-thick layer
at the waist, then filled with liquid DT
Filled with DT and beta-layered to yield a solid
layer 100 microns thick.
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2 mm tori have been fabricated from pure Pt
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At Sandia, Diana Schroen co-workers added
70-m-thick foam layers to four tori
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The filled tori all have noticeable defects in
the foam layers
Foam Cell B
Foam Cell C
Foam Cell D
Back lighting
Front lighting
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We measured the foam thickness by subtracting
an image of the unfilled Pt torus. We then
chose cell C based on
overall symmetry, average foam layer
thickness, and relative lack of defects at
the toroidal waist
Cell B - average d 125µm
Cell C - average d 66µm
Cell D - average d 47µm
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Following beta-layering, we can show the pure DT
layer by subtracting the empty foam image.
Note that the solid DT ignores the defects in the
foam!!!
empty foam
75 µm DT layer (total)30 µm is pure, while
the rest resides in the foam
Empty foam DT Layer
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The equilibration time is somewhat longer than we
have experienced in foam-free cells
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But the solid DT surface is much smoother than we
have observed in this geometry
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The mode 2 amplitude is responsible for a large
fraction of the total rms roughness. Some of
this is due to the fact that we now cannot line
up the empty torus on axis.
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By plotting a reverse sum of modes, the modal
spectrum can be seen. The presence of the foam
is dramatically reducing the roughness power at
l-modes above 8.
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This compares the best of our previous results
with our best result in foam. Simply put, we
have never seen such a smooth beta-layer!!
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This compares the previous graph with the best of
the LLNL data in spheres, where the solid DT
layer is grown as a single crystal.
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As a function of time, we notice that l-modes 3
to 8 equilibrate in the first 6 hours, followed
by all higher modes (up to 100) during the next
12 hours.
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This disappearance of the mid and higher modes
is precisely what we do not observe when no
intermediate foam layer is present.
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During the course of the first month of
experiments, we noticed that the foam layer
thickness diminished. However, the foam now
appears to be stable.
Foam 100902, avg d 66 µm
Foam 101502, avg d 57 µm
Foam 102302, avg d 51 µm
Foam 102802, avg d 53 µm
Foam 110402, avg d 45 µm
Foam 110102, avg d 47 µm
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It is possible to measure the surface roughness
of DT beta-layered onto a true spherical
surface, using only flat optics