The goal of the first hohlraum energetics experiments on the National Ignition Facility (NIF) [G. H. Miller et al , Optical Eng. 43, 2841 (2004)] is to select the hohlraum design for the first ignition experiments. Sub-scale hohlraums heated by 96 of the 192 laser beams on the NIF are used to emulate the laser-plasma interaction behavior of ignition hohlraums. These ''plasma emulator'' targets are 70% scale versions of the 1.05 MJ, 300 eV ignition hohlraum and have the same energy-density as the full-scale ignition designs. Radiation-hydrodynamics simulations show that the sub-scale target is a good emulator of plasma conditions inside the ignition hohlraum, reproducing density ne within 10% and temperature Te within 15% along a laser beam path. Linear backscatter gain analysis shows the backscatter risk to be comparable to that of the ignition target. A successful energetics campaign will allow the National Ignition Campaign to focus its efforts on optimizing ignition hohlraums with efficient laser coupling

Glenzer, S H

Meezan, N B

Suter, L J

UNT Digital Library

UCRL-CONF-234387
Target designs for energetics
experiments on the National
Ignition Facility
N. B. Meezan, S. H. Glenzer, L. J. Suter
September 7, 2007
Fifth International Conference on Inertial Fusion Sciences and
Applications
Kobe, Japan
September 9, 2007 through September 14, 2007
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Target designs for energetics experiments on the
National Ignition Facility
N. B. Meezan, S. H. Glenzer, and L. J. Suter
Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA, U.S.A., 94551-0808
E-mail: meezan1@llnl.gov
Abstract. The goal of the first hohlraum energetics experiments on the National Ignition
Facility (NIF) [G. H. Miller et al , Optical Eng. 43, 2841 (2004)] is to select the hohlraum design
for the first ignition experiments. Sub-scale hohlraums heated by 96 of the 192 laser beams on
the NIF are used to emulate the laser-plasma interaction behavior of ignition hohlraums. These
“plasma emulator” targets are 70% scale versions of the 1.05 MJ, 300 eV ignition hohlraum
and have the same energy-density as the full-scale ignition designs. Radiation-hydrodynamics
simulations show that the sub-scale target is a good emulator of plasma conditions inside the
ignition hohlraum, reproducing density ne within 10% and temperature Te within 15% along a
laser beam path. Linear backscatter gain analysis shows the backscatter risk to be comparable
to that of the ignition target. A successful energetics campaign will allow the National Ignition
Campaign to focus its efforts on optimizing ignition hohlraums with efficient laser coupling.
1. Introduction
The National Ignition Facility (NIF) [1] is designed to achieve fusion ignition using the
inertial confinement fusion (ICF) scheme [2]. The design of an optimal ignition hohlraum is
a compromise between capsule hydrodynamic robustness and laser-plasma instabilities (LPI)–
especially stimulated Raman backscatter (SRS) and Brillouin backscatter (SBS). Reducing the
hohlraum operating temperature decreases the LPI risk because both the peak laser power and
the plasma ablation into the hohlraum are reduced; however, decreasing the ablation rate of the
capsule increases the risk of hydrodynamic instability, requiring more laser energy to maintain
capsule robustness.
The National Ignition Campaign has chosen hohlraum designs that span a wide range of
backscatter risk, as described by Callahan et al in the preceding paper [3]. The primary goal of
the first large-scale energetics campaign on the NIF, which will utilize 96 of NIF’s 192 laser beams
in the 30◦ and 50◦ cones, is to select the ignition hohlraum from the candidate designs. Three of
these ignition designs drive a capsule with a beryllium ablator at peak radiation temperatures
(TRAD) of 300 eV, 285 eV, and 270 eV. The fourth design drives a capsule with a plastic (CH)
ablator at 300 eV. The 300 eV beryllium design requires 1.05 MJ of laser energy, while the others
require 1.3 MJ. The 96-beam energetics campaign will accomplish this goal by measuring the
energetics performance of sub-scale hohlraum targets that emulate the plasma conditions and
LPI risk of ignition hohlraums.
2. Campaign goals
An ignition hohlraum must satisfy three energetics requirements:
R 0.7 mm
∅3.57 mm ∅2.55 mm
Gold wall with Au/B liner
60 µm Be on 
30 µm CH shell
25 µm CH liner
Figure 1. Schematic of the sub-scale target
for 96-beam energetics experiments. The
emulator has all of the features of the ignition
target that are relevant to LPI: A Au/B-lined
hohlraum wall, a cryogenic helium/hydrogen
gas-fill, a CH-lined laser-entrance hole (LEH)
with a polyimide window, and a surrogate
capsule with a beryllium (or CH) ablator.
(i) Reaches the desired peak radiation temperature TRAD
(ii) Low backscattered power on the inner (23.5◦ and 30◦) and outer (44.5◦ and 50◦) cones
(iii) Low levels of DT-fuel pre-heat due to hot-electron generation
Energetics performance is tested experimentally by measuring the net laser power delivered
to the hohlraum and the x-ray flux leaving the laser entrance hole (LEH). The NIF’s laser
diagnostics measure the delivered laser power. Backscatter diagnostics measure the reflected
SRS and SBS power on one 30◦ quad and one 50◦ quad. Previous experiments using one NIF
quad showed that some of the backscattered power is not reflected back into the beam apertures,
so the diagnostics also measure light scattered around the apertures [4]. The DANTE filtered
x-ray diode array [5] measures the absolute radiation flux at an angle of 37.5◦ from the hohlraum
axis. Hot electron generation is quantified by measuring the hard x-ray bremsstrahlung produced
by hot electrons with the FFLEX multi-channel filter fluorescer [6].
3. Target design
The design goal for the 96-beam energetics targets is to produce ignition hohlraum-like plasma
conditions. It is critical that the experiment address LPI in the correct physical regimes–the
laser beams must sample the same materials as in an ignition hohlraum at similar electron
density ne, electron temperature Te, and ion temperature Ti. To accomplish this, we directly
scale down the target from the ignition hohlraum geometry by a factor s. Length scales as
L96 = sL192, so area scales as s2, and mass (or volume) scales as s3. To preserve energy density,
we scale the laser pulse time by s and the total power by s2, P (t)96 = s
2P (st)192.
We choose a “plasma emulator” hohlraum that is 70% scale (s = 0.7) of the 300 eV, 1.05
MJ ignition design with a beryllium capsule (Fig. 1). The emulator has all of the features
of the ignition target that are relevant to LPI, including a thin (≈ 0.2µm) liner of 80 % gold
and 20 % boron co-sputtered onto the hohlraum inner wall. Boron ions in the blowoff plasma
strongly Landau damp SBS. The LEH is not scaled, i.e., RLEH,96 = RLEH,192, because the
full-size (s = 1) ignition phase plates are used. We deliver the directly scaled power to the
hohlraum with half of NIF’s 192 beams, so the maximum power per beam is just below the
ignition value: Pbeam,96 = 2× (0.7)2 Pbeam,192 = 0.98Pbeam,192. We use one target to emulate all
three ignition designs with beryllium ablators–only the laser pulse is changed. Note that direct
scaling preserves the peak intensity of each beam for all the emulator hohlraums. Figure 2 shows
the pulse-shapes for the beryllium capsule emulator designs. We can think of each laser pulse
as a 2-part experiment: the first three bumps (the “foot”) form the plasma, while the fourth
(peak) bump is the interaction pulse. In this context, Fig. 2 shows clearly why backscatter risk
decreases rapidly as TRAD is reduced: The plasma-forming pulse is reduced in duration and the
interaction pulse is reduced in intensity.
In order to achieve ignition-like plasma conditions, the capsule ablator must eject the correct
mass into the hohlraum before the interaction pulse begins. This requires matching the radiation
drive in the foot to the ignition drive. Due to the time-dependence of the hohlraum wall albedo
2.0
1.5
1.0
0.5
0.0
P
o
w
e
r/
b
e
a
m
 (
T
W
)
86420
Time (ns)
300 eV
285 eV
270 eV
solid=inner
dash=outer
Figure 2. Laser pulses for plasma emulators
of ignition designs with beryllium capsules.
300
250
200
150
100
50
0
T
R
A
D
 (
e
V
)
1086420
Time (ns)
0.8
0.6
0.4
0.2
0.0
B
e
 m
a
ss a
b
la
te
d
 (m
g
)
Emulator, directly scaled
Emulator, raised foot
Ignition
Figure 3. Radiation drive TRAD seen by
the capsule and mass ablated by the capsule
(dash) in hydra simulations of the 300 eV
beryllium design and its sub-scale emulator.
(see [2] §4) and to some target features that are not scaled (e.g., the thickness of the window), the
emulator foot-pulse must be increased to 20-40% above the directly-scaled power P96 = s2P192.
Figure 3 shows the TRAD seen by the capsule in 2-D radiation hydrodynamics simulations with
the code hydra [7]. The emulator with a directly-scaled pulse never reaches the ignition drive
in the foot. When the foot-pulse is increased, the drive matches the ignition drive. Note that the
emulator is not expected to reach the ignition peak TRAD. The mass ablated by the beryllium
capsule is also shown in Fig. 3. The emulator target with a raised foot-pulse matches the
(scaled) ablated mass of the ignition target up to the time of peak laser power. The x-axis
(time) is scaled by s = 0.7 for the ignition design curves in Fig. 3.
4. Target performance
The first requirement for an emulator target is that it produces ignition hohlraum-like plasma
conditions in the laser beam paths. Figure 4 shows the ray-averaged electron density ne/nc and
electron temperature Te extracted from hydra’s laser ray-trace package at 8.8 ns, just after the
peak of the laser pulse. Note that the x-axis (distance) is scaled by s = 0.7 for the ignition
target. We see that the emulator design with a directly-scaled pulse and the emulator with and
ignition-like foot drive bracket the ne/nc of the ignition design. This demonstrates that the
electron density inside the hohlraum can be tuned by adjusting the foot-pulse. The electron
temperatures Te for the emulators are 10-15 % below the ignition value.
We use linear gain analysis to compare the quantitative backscatter risk of the emulator and
ignition targets. The gain exponents for SRS and SBS are proportional to laser intensity and
to plasma length. Intensity is preserved, so an emulator with plasma conditions identical to
the ignition hohlraum should have a lower gain exponent G96 = sG192. Figure 5 shows the
maximum SRS and SBS backscatter gains Gmax (t). While s = 0.7 for this design, the emulator
SRS gain fluctuates between G96 ≈ 0.5G192 and G96 ≈ 1.1G192. On average, G96 ≈ 0.75G192
for SRS, but for SBS, G96 ≈ G192.
In order to reach ignition-relevant gains and to find the limits for acceptable backscatter, we
use s = 0.7 phase plates on the diagnosed 30◦ and 50◦ quads. These interaction phase plates
produce elliptical laser spots 49% smaller in area than the ignition phase plates, allowing the
emulator experiments to access intensities I96 ≤ 2I192. By varying the foot-pulse level and the
peak interaction intensity for a given emulator target, we can span the uncertainty in emulator
0.30
0.25
0.20
0.15
0.10
0.05
0.00
n
e
/
n
c
0.30.20.10.0-0.1-0.2
Distance from focus (cm)
6
5
4
3
2
1
0
T
e  (k
e
V
)
Emulator, directly scaled
Emulator, raised foot
Ignition
Figure 4. Ray-averaged electron density
ne/nc and temperature Te (dash) along a 30◦
quad from hydra simulations of the 300 eV
beryllium design and its sub-scale emulators.
3in
30
25
20
15
10
5
0
S
R
S
 g
a
in
 G
 i
n
 h
e
liu
m
 
9.59.08.58.07.5
Distance from focus (cm)
20
15
10
5
0
S
B
S
 g
a
in
 G
 in
 b
e
ry
lliu
mEmulator, raised foot
Ignition
Figure 5. Peak SRS and SBS (dash)
intensity gain exponents G (t) for a 30◦
quad from hydra simulations of the 300 eV
beryllium design and its sub-scale emulator.
quality and confidently find an energetically robust ignition hohlraum design.
5. Conclusions
The first large-scale energetics experiments on NIF directly test the energetics performance of
the candidate ignition hohlraums. Sub-scale emulator targets driven by 96 NIF beams provide
ignition-like plasma conditions to address backscatter risk in the correct physical regimes.
With a single target, adjustments to the laser pulse can be used tune the emulator plasma
conditions and the quantitative backscatter risk. These experiments provide an opportunity to
put hohlraum energetics issues to rest and move forward with optimizing the performance of
ignition hohlraums.
Acknowledgments
This work was performed under the auspices of the U.S. Department of Energy by the University
of California Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48.
References
[1] Miller G H, Moses E I and Wuest C R 2004 Optical Engineering 43 2841–2853
[2] Lindl J D, Amendt P, Berger R L, Glendinning S G, Glenzer S H, Haan S W, Kauffman
R L, Landen O L and Suter L J 2004 Physics of Plasmas 11 339–491
[3] Callahan D A et al. Optimization of the NIF ignition point design hohlraum , these
proceedings
[4] Glenzer S H et al. 2007 Nature Physics Advance online publication URL
http://dx.doi.org/10.1038/nphys709
[5] Dewald E L et al. 2004 Review of Scientific Instruments 75 3759–3761
[6] McDonald J W, Kauffman R L, Celeste J R, Rhodes M A, Lee F D, Suter L J, Lee A P,
Foster J M and Slark G 2004 Review of Scientific Instruments 75 3753–3755
[7] Marinak M M, Kerbel G D, Gentile N A, Jones O, Munro D, Pollaine S, Dittrich T R and
Haan S W 2001 Physics of Plasmas 8 2275–2280


Target designs for energetics experiments on the National Ignition Facility

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