We undertook an experimental campaign to study the effects of dopants on the radiation physics of ablators in indirect-drive hohlraum environments as part of the NLUF program on the OMEGA laser several years ago. Our primary goal was to demonstrate the

1
Tracer Absorption Spectroscopy of ICF Ablator
Materials An OMEGA Experimental Campaign and
Associated Modeling
David Cohen, Swarthmore College with David
Conners ('03) and Kate Penrose ('04), Swarthmore
College and Joseph MacFarlane, Prism
Computational Sciences Donald Haynes, University
of Wisconsin Paul Jaanimagi, University of
Rochester/LLE Otto Landen, Livermore National
Laboratory
We undertook an experimental campaign to study
the effects of dopants on the radiation physics
of ablators in indirect-drive hohlraum
environments as part of the NLUF program on the
OMEGA laser several years ago. Our primary goal
was to demonstrate the delay in the Marshak wave
propagation time to a specified depth in the
interior of plastic ablator materials caused by
the addition of Germanium dopant. The
experimental campaign proved to be quite
ambitious, necessitating the development of
target fabrication techniques, new diagnostics
(backlit tracer abstorption spectroscopy
employing the LXS at OMEGA) used simultaneously
with traditional ones, and detailed modeling of
the hohlraum radiation field as well as the
ablator physics in order to extract science
results. While we have demonstrated the
anticipated later turn-on times of the
spectroscopic signal in the doped ablator
samples, the results are muddied by the earlier
than expected tracer turn-on times in both
samples and the weaker than anticipated
absorption signals. During the course of the
campaign and with the associated computer
modeling, we learned several things that may be
of interest to the ablator physics and ICF
communities apart from the specific results of
our experiments.
2
OUTLINE

1. Scientific Context
2. Experiment Design
3. Overview of Results
4. Targets
5. Modeling
6. Data and Conclusions

3
Context Ablator dopants are used to control fuel
pre-heat, but they also affect the radiation
hydrodynamics of the interaction between the
hohlraum radiation field and the capsule.
4
Ablator dopants affect the opacity and density,
changing the manner in which energy is absorbed
by the ablator.
Controlling the process requires a means of
diagnosing the properties of doped and undoped
ablators in the hohlraum environment.
Traditionally, samples in and on hohlraums have
been evaluated spectroscopically via emission (in
e.g. gas-filled capsules and tracers in hohlraum
walls) and using shock break-out measurements.
Olson et al.
5
In a different context, Perry showed that
absorption spectroscopy in multi-layered targets
could diagnose radiation transport.
And Chenais-Popovics et al. showed that Cl Ka
absorption spectroscopy could diagnose material
properties. Laser-produced Bi plasma provided the
backlighter continuum source.
From Chenais-Popovics 1989
6
We put these ideas together, and building on a
previous effort to measure tracer emission
spectra from aluminum witness plates with Tina
Back, proposed an experimental campaign to use
backlit Cl Ka absorption spectroscopy to diagnose
radiation physics in the interior of ablator
samples. We proposed to do this by placing thin
tracer layers at specified depths in the
interiors of ablator samples mounted on
hohlraums. The spectroscopy monitors the
ionization conditions in that layer, effectively
diagnosing the time-dependent radiation field
properties at a specific location inside the
sample. A time-delay in the turn-on time of the
tracer signal between doped and undoped samples,
for example, would allow us to determine the
effects of dopants on the Marshak wave
propagation.
7
We undertook these experiments under the auspices
of the NLUF program at OMEGA in 1998. They
continued until 2000.
We encountered various challenges involving
experiment design, target fabrication, and the
interpretation of diagnostics.
I will discuss the series of experiments -- but
will focus on results from the April 2000
campaign in which we measured absorption spectra
in two different shots, one with a (undoped)
plastic witness plate and one with a 1.75 Ge
doped witness plate.
8
Our original plan was to make side-by-side
measurements on doped and undoped ablators
mounted on the same hohlraum
We mounted them first on the outside of
hohlraums, near the midplane one spectrometer
with two separate crystals was used
A halfraum with two samples (two tracers, two
backlighters) and two spectrometers
9

• We never were able to successfully measure a
  tracer spectral signal on these experiments
• Lower-than expected drive temperatures (tracers
  deeper than they ought to have been)
• Cross-talk between samples emission seen by
  spectrometers not coming from line-of-sight
  through samples?
• Problems with one spectrometer

Other ambitious plans included use of wedge
witness plates to make passive shock breakout
measurements (VISAR with J. Oertel)
simultaneously with tracer spectroscopy
10
Eventually, we settled on an approach involving
single samples mounted on the ends of halfraums
I will discuss mostly these experiments, carried
out in April 2000, as they have the most straight
forward target design and had the most positive
results.
11
Targets were fabricated at General Atomics by
Abbas Nikroo and assembled by Russ Wallace at LLNL
20 m
5 m
We will be comparing shots 19526 and 19528 from
April 2000 26 undoped, tracer depth 6.3m 28
doped, tracer depth 4.1m
halfraum radiation
NaCl tracer lt0.5m
Drive temperature history modeled and constrained
by DANTE
Ge-doped or undoped plastic (produced by GDP)
12
Experimental Set-up Including schematics of
diagnostic lines-of-sight
Note only one (blue) beam into the halfraum is
shown here,for simplicity. All shots were carried
out with 15 beams.
13
Quick Look at Main Result
Shot 26 undoped
Shot 28 doped
Some absorption signal, apparently, on a noisy
continuum Turns on later in the doped sample
(and tracer was even shallower in this sample)
See progression through ionization
states. But--in both cases--earlier turn-on than
models predict
14
Experimental Configuration
LEH facing P-7 (LXS in P-6) Gold Halfraums
L1200m, R800m
washer/aperture
positioning wires
Pb-doped plastic mount
Bi/Pb backligher foil
TVS-X view
15
Targets and Target Fabrication
SEM images (left) show that witness plates with
KF are bumpy (not a problem with NaCl tracers)
16
SEM images of finished witness plates Leakage of
KF tracer onto front of plate (right), but no
similar problem in witness plates with NaCl
tracer (left)
GA produced these witness plates by first making
the thicker plastic layer via glow-discharge
polymerization (GDP) The salt layer was
deposited on this plastic, and then the whole
assembly was put back in the GDP chamber and an
additional 5m of plastic was deposited on top.
17
SEM image of cross-section of plate
Note the salt crystalscould the weak (or
non-existent tracer signals be due to this?)
18
Same target as previous slide higher
magnification
19
And higher still
If there are large gaps in the tracer layer, no
amount of average areal mass will provide a
strong signal.
20
Witness plates were mounted on the ends of
halfraums backlighter foils hung 1.5mm from LEH
TVS-Y view
TVS-X view of plain foil
21
Target alignment in the chamber was non-trivial
and relied on the creation of reticles, some
keyed to the wires
TVS-Y view (theta 77.28, phi 19.96)
22
TVS-X view (theta 64.44, phi 270.00)
23
Modeling

1. Viewfactor modeling of hohlraum drive,
   constrained by DANTE (and using measured beam
   profiles as input)
2. Hydrodynamic calculations for time-dependent
   witness plate properties (1-D Lagrangian DCA and
   UTA atomic and EOS models short characteristics
   multi-group radiation transport)
3. CRE post-processing for spectral synthesis

We use codes written by Joe MacFarlane at Prism
Computational Sciences as well as some publicly
available codes written at the U. Wisconsin
Fusion Technology Institute.
24
VisRad Viewfactor Modeling

• 15 cone 2 and cone 3 beams into the halfraum
• 1 ns square pulse
• 3 beams onto the backlighter foil also 1 ns
  square, but staggered in time for more even
  backlighter source.

Note not all beams are shown.
25
Constraining the viewfactor modeling

• Beam powers and pointings are known
• Temperature dependent albedo is modeled
  (separately)
• X-ray conversion efficiency (of lasers) is a free
  parameter

Radiation flux monitored on element at DANTE
position
26
Shot 26
27
X-ray conversion efficiency was relatively low in
order to match DANTE data simulation on left
used a constant XCE0.55, while the one on the
right is the same one from the previous slide,
that reproduced the DANTE data.
28
DANTE temperature profiles (based on unfolds by
Bob Turner and new calibration, April 2000)
Are these temperatures low?
29
Calculated Drive Temperatures at Ablator

• View factor calculations were performed to
  compute the flux incident on the ablator and the
  wall temperature seen by Dante.
• The drive temperature at the ablator is slightly
  higher (up to 5) than the wall temperature
  seen by Dante.

• Dante sees laser hot spots due to several beams.
  Only 50 of the flux seen by Dante is due to
  wall reemission.

30
The Incident Flux at the Ablator Is Very Uniform
View of entire halfraum
1200 mm diameter ablator region
View from P7 direction

• The incident flux ( s TR4) at the ablator sample
  is 10 - 15 eV higher than throughout the rest
  of the halfraum.

• Using beams with uniform powers, the maximum
  change in flux (peak-to-valley) across the
  central 600 mm diameter of the ablator patch is
  only 1.4 (0.35 in TR).
• Because of aperturing, only central ablator
  region is seen.

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Effect of Beam Imbalance on Wall Temperature Seen
by Dante
Beams 25, 50, 45, 69 are in the Dante field of
view. Approximately 50 of the flux seen by
Dante is due to laser hot spots.
View from Dante (H16)
Figure (right) shows Shot 19526 beams energies
were relatively low for beams 25 and 69. When
taking into account actual beams powers --
the flux at Dante decreases by 8.7 --
inferred temperature decreases by 3 - 4 eV.
32
Effect of backlighter radiation on flux incident
on witness plate
33
Once the hohlraum radiation field is modeled
Hydro simulations of the witness plate
Electron Temperature
Note in Ge-doped sample, the peaks are narrower
-- shock wave and radiation wave move slower
34
Same simulations Radiation Temperature
Slower Marshak wave velocity in the doped sample
35
Spectral synthesis post-processing of hydro
results (undoped sample)
Note Shouldnt see Be-like until 0.6 ns.
36
Calculated tracer turn-on time vs. drive
temperature
37
CONCLUSIONS
Experiments were ambitious, but simpler single
patch design employed in April 2000 generated
time-resolved tracer absorption measurement in
doped and undoped samples. In both types of
targets, the signal turn-on time was earlier (by
factor 2) than expected, and lower ionization
stages were not seen). But, signal turned on
sooner in undoped sample than in doped sample.
We looked at many causes for the weak signals
and early turn-on times, including target
fabrication issues drive temperature may have
been underestimated, but this cannot account for
all of the discrepancy.
38
Extra slide following
39
In our VisRad modeling, we looked at three cases
(1) fixed XCE0.55 (2) Modeled XCE (from G.
Magelssen) (3) XCE adjusted to get match to
DANTE temperature.
Note The XCE needed to fit the DANTE data is
surprisingly low (above) and the witness plate
temperatures are generally lower than the DANTE
temperatures (right) in conflict with the slide
from the IFSA meeting, prepared by Joe.