Heavy Ion Target Physics and Design in the USA

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Heavy Ion Target Physics and Design in the USA

• D. A. Callahan, D. S. Clark, A. E. Koniges, M.
  Tabak
• Lawrence Livermore National Laboratory
• G. R. Bennett, M. E. Cuneo, R. A. Vesey
• Sandia National Laboratories
• A. Nikroo
• General Atomics

15th International Symposium on Heavy Ion
Inertial Fusion Princeton, NJ June 7-11, 2004
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Our emphasis has been on validating our
calculations with experiment and theory

• The hybrid target, which allows a large beam
  spot, has become the target of choice
• The large beam spot is easier for the accelerator
  may allow a modular driver with a small number of
  separate accelerators
• The hybrid target uses some new target design
  features that need validation such as shine
  shields and shims to correct asymmetries
• In a LLNL/SNL/GA collaboration, we did our first
  experimental test of shims in April 04
• We succeeded in reversing a P2 asymmetry!
• Our next experiments are scheduled for September
  04
• We are using theory and simulations to gain a
  better understanding of Rayleigh-Taylor
  instability and capsule performance

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The hybrid target has become the target of
choice because it allows a large beam spot
Hybrid Target
6.7 MJ -- Gain 60
3.8 x 5.4 mm
Distributed Radiator Target
5.9 MJ -- Gain 70
1.8 x 4.15 mm
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The hybrid target uses shine shields and shims to
control symmetry
HI hybrid target

• The distributed radiator target and the NIF point
  design use beam placement to control symmetry
• The hybrid target uses internal shields to
  control symmetry
• A shine shield controls P2
• A shim corrects the P4
• The hybrid target and the Z double-pinch target
  use similar methods for controlling symmetry
• This results in a natural area for collaboration

Z double-pinch target
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In a LLNL/SNL/GA collaboration, we did the first
three experiments to test shims in April 2004
General Atomics Capsule Fabrication
LLNL Theory and Design
SNL Experiment and Hohlraum Fabrication
This collaboration worked very well!
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Large thin-shell capsules in double z-pinch
hohlraums provide null cases for P4 shimming tests
Z double-pinch hohlraum
6.7 keV backlit images
P4 vs. length and capsule size
Capsule diameter 2.15 mm
3.3 mm
4.5 mm 4.7 mm
Secondary hohlraum Radius ? Rsec Length ? Lsec
These experiments had a P2 asymmetry in addition
to the P4 so the shim was designed to take out
this combination
8/2003 RAVesey
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GA fabricated the capsules by rotating them under
a coater with a mask to produce the layer profile
See Abbas Nikroos talk this afternoon for more
details on fabricating these capsules
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These targets were first-of-a-kind so there were
a few problems

• As this was the first attempt at putting the
  layer on a capsule, the layer ended up being
  thicker than we had asked for
• Translation from flat to sphere for the P4 mask
  was not as expected--this is certainly fix-able
• Recent calculations show that the layer we asked
  for was too thick anyway
• Our plan was to stalk mount the capsules in the
  hohlraum, using the stalk (200 micron diameter)
  that was used to hold the capsule under the
  coater
• Due to a miscommunication, the stalks were
  removed and left small (30-200 micron) holes in
  some of the capsules
• We did not see any ill effects from these holes!
• The capsules were then mounted with formvar
• Wrinkles in the formvar caused ripples in the
  images of the capsule -- this was also seen in
  previous SNL shots (Sept 03)

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The first shot gave the best data and shows that
we reversed the P2 asymmetry!
No Shim
With Shim
Roger Veseys preliminary analysis of the images
gives
No shim P2 - 6 (waist high) P4 - 6.9
With shim P2 15 (pole high) P4 - 3
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Since the image was caught early in time, we see
evidence of the shim layer in the radiograph
Simulation of shot 1276
Remnant of the shim layer
Approximate location of the fiducials
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A more detailed comparison of the calculation and
experiment shows good agreement
Assumes layer was 0.3 microns thick in the 20
degrees around the poles (no data from target
fab in this region)
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The second shot was a repeat of the first, but at
higher convergence ratio
Experiment

• The image is clearly pole high so we have
  certainly reversed P2
• Rippling, which seems to be due to wrinkles in
  the formvar, is evident
• There is an unexplained divot near the south
  pole
• The spot where the stalk was removed was at the
  north pole
• Simulated image agrees qualitatively

Simulated Expt
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The third shot used a shim with half the thickness
Full thickness
Half thickness

• This shot had two purposes
• A backup in case the backlighter contrast was
  poor in shots 1 and 2 due to the remnant of the
  shim layer
• A further demonstration of the change in symmetry
• The capsule was put into the coater for half the
  time
• The image was clipped, making it difficult to
  determine P2

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Shots in September will remove a P4 and explore
different hohlraum mounting techniques

• Based on SNL experiments in Feb/March, we should
  be able to tune out the P2 asymmetry using
  hohlraum and shine shield geometry
• First four shots in September will tune out P2
  and serve as a baseline for the shim shots
• Four shim shots will try to correct and reverse a
  P4 asymmetry
• Because of the problems with capsule mounting,
  the first four shots will use two different
  mounting techniques
• Formvar on a split frame to reduce wrinkles
• Beryllium stalk mount along the direction of the
  backlighter
• We will use these results to decide which
  mounting technique to use for the shim shots

These experiments give credibility to the heavy
ion hybrid target as well as exploring a new
symmetry technique for all indirect drive targets
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Gain curves for the hybrid target are useful for
system optimizations
Point design
Modular and conventional drivers may not
optimize at the same beam energy/spot size so we
need gain curves to do a fair comparison
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Understanding nonlinear Rayleigh-Taylor
instability growth may be important for HIF

• For a power plant, we want to push the capsule
  performance in order to minimize the constraints
  on other parts of the system
• Moving to lower drive temperature reduces the
  peak power needed from the accelerator
• Allowing a rougher ablator may allow cheaper
  target fabrication
• These mean less margin and more Rayleigh-Taylor
  growth
• A power plant is likely to optimize beyond the
  capsule performance needed for the baseline NIF
• NIF does not need high gain or 0.25 targets!
• Understanding nonlinear Rayleigh-Taylor
  instability growth may be needed
• This is a difficult computational problem and
  theory can help us understand the limits of our
  models
• Theory can also guide us to the right parts of
  design space

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The model includes the effect of convergence on
the nonlinear phase of growth

• Model uses
• a Layzer-like, nonlinear bubble model in
  spherical geometry
• combined with a self-similar Kidder implosion
  model
• Solve this by transforming to a coordinate system
  that converges with the implosion
  (quasi-Lagrangian solution)

See Dan Clarks poster on Wednesday (WP-01) for
more details!
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Bubble growth in converging geometry is faster
than predicted by Layzer formula
Bubble curvature at apex agrees with HYDRA
using ALE relaxation (model does not apply to
spike)
Model and 2-d HYDRA both predict faster bubble
growth
Initial bubble growth agrees with Layzer model
(linear with time) followed by late time
acceleration similar to that seen in HYDRA
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Our emphasis has been on validating our
calculations with experiment and theory

• The hybrid target, which allows a large beam
  spot, has become the target of choice
• The large beam spot is easier for the accelerator
  may allow a modular driver with a small number of
  separate accelerators
• The hybrid target uses some new target design
  features that need validation such as shine
  shields and shims to correct asymmetries
• In a LLNL/SNL/GA collaboration, we did our first
  experimental test of shims in April 04
• We succeeded in reversing a P2 asymmetry!
• Our next experiments are scheduled for September
  04
• We are using theory and simulations to gain a
  better understanding of Rayleigh-Taylor
  instability and capsule performance