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LLE Review
Quarterly Report
Volume 25
LLE Review
Quarterly Report
Editor: A. Schrnid
(716) 275-3541
October-Decem ber 1985 UR
LLE
Laboratory for Laser Energetics
College of Engineering and Applied Science
University of Rochester
250 East River Road
Rochester, New York 14623-1299
This report was prepared as an account of work conducted by the
Laboratory for Laser Energetics and sponsored by Empire State Electric
Energy Research Corporation, General Electric Company, New York
State Energy Research and Development Authority, Ontario Hydro,
Southern California Edison Company, the University of Rochester, the
U.S. Department of Energy, and other United States government
agencies.
Neither the above named sponsors, nor any of their employees,
makes any warranty, expressed or implied, or assumes any legal liability
or responsibility for the accuracy, completeness, or usefulness of any
information, apparatus, product, or process disclosed, or represents that
its use would not infringe privately owned rights.
Reference herein to any specific commercial product, process, or
service by trade name, mark, manufacturer, or otherwise, does not
necessarily constitute or imply its endorsement, recommendation, or
favoring by the United States Government or any agency thereof or any
other sponsor.
Results reported in the LLE Review should not be taken as
necessarily final results as they represent active research. The views and
opinions of authors expressed herein do not necessarily state or reflect
those of any of the above sponsoring entities.
IN BRIEF
This volume of the LLE Review comprises reports on the performance
of the active-mirror-boostedglass development laser (GDL) single-beam
system; the implementation of multichannel, soft x-ray diagnostic
instrumentation; computer simulation of recent OMEGA laser implosion
experiments; materials and ultrafast technology developments in the LLE
advanced technology program; and the National Laser Users Facility
activities for October-December 1985.
The following are some highlights of the work described in this issue:
The active-mirror booster stage on GDL brings single-beam,
1054-nm output energy near the kilojoule level for 1-ns pulses.
Measurements of soft x-ray emission from 100 to 200 eV are now
possible with the help of a four-channel, GHz-bandwidth diode
spectrometer, which is fully integrated into a digital data acquisition
system.
Comparison of experimental data and predictions of one-
dimensional numerical simulations of OMEGA implosion
experiments reveal discrepancies that can be resolved by two-
dimensional' calculations. The consequences of irradiation
nonuniformities are well accounted for and required improvements
for future experiments are identified.
The technique of pulse chirping allows simple temporal stretching
and recompression operations to be carried out on picosecond
iii
LLE REVIEW. Volume 25
pulses, to amplify such pulses in compact systems and avoid the
dangers of optical nonlinearities due to high peak powers.
High-average-power glass lasers require glasses with improved
thermal-shock resistance. Recent gains in ion-exchange surface
treatments of a phosphate-composition glass have led to a fivefold
increase in thermal-shock resistance for that glass.
CONTENTS
Page
...
IN BRIEF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CONTENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Section 1 LASER SYSTEM REPORT . . . . . . . . . . . . . . . . . . . ..... .1
l.A GDL Facility Report . . . . . . . . . . . . . . . . . . . . . . . . ..... .1
1.B OMEGA Facility Report . . . . . . . . . . . . . . . . . . . . . ..... .2
1.C A Kilojoule-Scale Active Mirror System . . . . . . . . . ..... .2
Section 2 PROGRESS IN LASER FUSION . . . . . . . . . . . . . . . . . . . . 7
2.A Numerical Simulation of Recent, 24-Beam,
Blue (351-nm) OMEGA Implosion Experiments. . . . . . . . . 7
2.B Computerized, Wide-Bandwidth, Multichannel,
Soft X-Ray Diode Spectrometer for
High-Density-Plasma Diagnosis . . . . . . . . . . . . . . . . . . . .20
Section 3 ADVANCED TECHNOLOGY DEVELOPMENTS . . . . . . .33
3.A Ion-Exchange Strengthening of
Nd-Doped Phosphate Laser Glass . . . . . . . . . . . . . . . . .33
3.B Short-Pulse Amplification Using
Pulse-Compression Techniques . . . . . . . . . . . . . . . . . . . .42
Section 4 NATIONAL LASER USERS FACII-ITY NEWS . . . . . . . . .47
PUBLICATIONS AND CONFERENCE PRESENTATIONS
Donna Strickland, a graduate student in optics and a member of the Picosecond Research Group, is shown
aligning an optical fiber. The fiber is used to frequency chirp and stretch an optical pulse that can later be
amplified and compressed in order to achieve high-peak-power pulses.
Section 1
LASER SYSTEM REPORT
l.A GDL Facility Report
The glass development laser (GDL) system was used for LLE
interaction experiments and National Laser Users Facility experiments.
The system was also used for beam uniformity studies, the results of
which are now being implemented on the 24-beam OMEGA system.
In preparation of future experiments, GDL full system output tests at
pulse lengths shorter than 1 ns were carried out.
A summary of GDL operations this quarter follows:
System Test Shots 89
Target Shots 49
Pointing, Activation Shots -
14
TOTAL 152
ACKNOWLEDGMENT
This work was supported by the U.S. Department of Energy Office of Inertial Fusion
under agreement No. DE-FC08-85DP40200 and by the Laser Fusion Feasibility Project at
the Laboratory for Laser Energetics, which has the following sponsors: Empire State
Electric Energy Research Corporation, General Electric Company, New York State Energy
Research and Development Authority, Ontario Hydro, Southern California Edison Company,
and the University of Rochester. Such support does not imply endorsement of the content
by any of the above parties.
LLE REVIEW, Volume 25
1.B OMEGA Facility Report
During the first quarter of FY86 a number of improvements and
modifications were made to the OMEGA laser system. A
comprehensive upgrade of the laser front end took place. An actively
mode-locked, Q-switched (AMQ) YLF oscillator, preamplifier, and a
reconfigured predriver line were installed. Other activities included a
cw system alignment, characterization of a new zoom in-air spatial
filter, implementation of liquid-crystal-based polarization devices, and
frequency-conversion crystal tuning.
Preparations making GDL a 25th beam of OMEGA for x-ray
backlighting purposes are nearing completion. Optical as well as
software integration of the two facilities will be completed by the
beginning of the second quarter; the first, full-power shots of the GDL
beam into the OMEGA target chamber are scheduled to begin in
January 1986.
After completion of the system upgrade, OMEGA was used for two
experimental programs. The first program, comprising beam
uniformity studies, established a data base for beam intensity and
phase profiles on selected beam lines. The second, a National Laser
Users Facility program and a cooperative project with the Naval
Research Laboratory, involved target experiments aimed at vacuum
ultraviolet spectroscopy of selected elements.
A summary of OMEGA operations during this quarter follows:
Driver Test and Activation Shots 272
Beamline Test Shots 101
Target Shots -57
TOTAL 430
ACKNOWLEDGMENT
This work was supported by the U.S. Department of Energy Office of Inertial Fusion
under agreement No. DE-FC08-85DP40200and by the Laser Fusion Feasibility Project at
the Laboratory for Laser Energetics, which has the following sponsors: Empire State
Electric Energy Research Corporation, General Electric Company, New York State Energy
Research and Development Authority, Ontario Hydro, Southern California Edison Company,
and the University of Rochester. Such support does not imply endorsement of the content
by any of the above parties.
l.C A Kilojoule-Scale Active Mirror System
One year ago LLE revived its interest in the large-aperture, active
mirror laser amplifier as a booster for high-power neodymium laser
systems. Progress in hardware development involving new mounting
and sealing designs had resulted in the successful upgrade of GDL
with four 21-cm, clear-aperture, active mirror amplifiers.' Lacking the
proper front surface coatings on all four mirrors, multiple Fresnel-
reflection losses limited the peak energy to just above 300 J at 1-ns
pulse width for the single-pass case and precluded effective double-
LASER SYSTEM REPORT
pass operation of the active mirrors. Furthermore, focusability suffered
from the cumulative wave-front distortion caused by each of the
mirrors.
Recent progress in fabrication and coating technologies and
various assembly and testing techniques has resulted in the
successful upgrade of GDL with four fully coated and distortion-free
active mirror amplifiers, lnterferometric analysis, applied to every
Fig. 25.1 stage of fabrication, coating, assembly, and system operation, was
lnterferometric testing is an essential part of central to locating the sources of distortion and in defining effective
distortion-free mounting and sealing of the solutions. Figure 25.1 shows interferograms that represent the optical
large aperture, active-mirror amplifier disc. quality of an active mirror disc at two different steps of the procedure.
The straight, vertical fringes in (a) are The interference pattern in Fig. 25.l(a) is generated by a Fizeau
generated by a Fizeau interferometer and interferometer where the active mirror forms the end element in the
indicate that the unmounted active mirror
test arm. The straight vertical fringes show that the unmounted active
creates less than W6 wave-front distortion
mirror creates less than a sixth of a wavelength (M6) wave-front
(h = 633 nm) in its normal mode of double
pass reflection from the back surface. A
distortion (A = 633 nm) in its normal mode of double-pass reflection
Twyman-Green interferogram of the same from its back surface. A Twyman-Green interferogram [Fig. 25.l(b)] of
disc (b), when coated, mounted, and the same disc, following coating, mounting, and sealing, shows a
sealed, shows an accuracv-limited distortion of less than M4 (A = 1064 nm). This measurement is limited
distortion measurement of less than by the performance of the interferometer components. Interferometer
W4 (h = 1064 nm). normalization indeed shows that the distortion is kept below M4.
+21 cm- -21 cm*
Fizeau interferometry Twyman-Green interferometry
on a fabricated disk on a coated, mounted, and sealed
( A = 633 nm) disk ( A = 1064 nm)
During this GDL upgrade, three of the four active mirrors were
reworked to remove old coatings and to repolish surfaces found
inadequate because of excessive scratches and digs. Subsequently,
each laser disc was coated with an in-house-developed, recently
improved front-face dual-purpose coating. This coating simultaneously
acts as an antireflection coating at 1054 nm for the front surface, and
as a high reflector for the major neodymium pumpbands (Fig. 25.2).
This method of returning pump light back into the ampl~fierincreases
LLE REVIEW, Volume 25
the stored energy of a 25x3-cm, LHG-8 active mirror by more than
100/0. Coatings of such complex design usually exhibit low damage
threshold even at longer wavelengths (1054 nm). Recent tests
demonstrate, however, that improved coatings repeatedly withstand
irradiance levels higher than 5 GW/cm2.
This observation supports the belief that removal of subspecification
scratches and digs by fine polishing has increased coating adhesion
and strength over the entire disk surface.
Materials: Ta,05/Si02 Measured performance
Wavelength (nm)
G I 680
Fig. 25.2
Each active mirror amplifier is equipped In an effort to achieve unprecedented high powers in GDL without
with an improved front-surface coating the risk of damaging optical components, a dedicated effort was
designed to maximize the transmission of made to obtain a uniform-intensity distribution for propagation through
the 1053-nm laser beam at the front the active mirror amplifiers. A thorough investigation of the new
surface, while simultaneously reflecting the oscillator and predriver,2 as well as of the driver line and remaining
major neodymium pumpbands back into
rod amplifiers, resulted in smooth spatial intensity profiles, as
the amplifier. This in-house coating is
capable of increasing the stored energy of
illustrated by the contour plots of Figs. 25.3 and 25.4. A smooth,
a 25x3-cm, LHG-8 active mirror by more Gaussian spatial profile with a slight, 45O orientation characterizes the
than 10%. new predriver in GDL (Fig. 25.3). After several rod amplification
stages, which amplify the Gaussian-beam wings most prominently by
the characteristic edge-gain profile, the energy distribution at the input
to the active mirror booster remains smooth, center peaked, and at a
slight, 45O orientation [Fig. 25.4(a)].
Accurate alignment techniques implemented throughout the laser
chain have resulted in highly uniform active mirror intensity
distributions [Fig. 25.4(b)] at very high-peak-power levels. Figure 25.5
shows the 500-ps performance of the single-passed active mirrors in
GDL. The experiniental results exceeded theoretical predictions by
LASER SYSTEM REPORT
Fig. 25.3
Uniform spatial intensity distributions are
essential for damage-free, high-power
' Distance (mm)
scaling of LLE laser systems. A smooth G1681
Gaussian spatial profile with a slight, 4 5 O
orientation character~zesthe new prednver
output ln GDL
Distance (cm) Distance (cm)
(a) Active Mirror Input Near Field (b) Active Mirror Output Near Field
GI 682
Fig. 25.4
After several rod amplification stages, the energy distribution at the input to the active mirror booster
remains smooth, center peaked, and at a slight, 45O orientation (a). Accurate alignment techniques
used throughout the laser chain result in highly uniform active mirror outputs (b).
LLE REVIEW, Volume 25
I I I I I I I
- experimental
-theoretical
/:
- /'* -
-
I I I I I I
Fig. 25.5
Performance of a four-unit, single-passed
active mirror booster to the glass
development laser (GDL) at 500 ps.
Experimental results exceeded theoretical
predictions to produce more than 1 7W of
focusable power.
the code RAINBOW and yielded more than 1 TW of focusable power.
Higher powers can only be obtained in a double-pass configuration,
since the system is clearly driver limited at this point.
The active-mirror-boosted GDL facility is currently scheduled for
more extensive beam characterization, as it delivers 100-ps pulses for
short-pulse frequency tripling in x-ray backlighting experiments and
500-ps to 1-ns pulses for frequency-doubled interaction experiments.
ACKNOWLEDGMENT
This work was supported by the U.S. Department of Energy Office of Inertial Fusion
under agreement No. DE-FC08-85DP40200 and by the Laser Fusion Feasibility Project at
the Laboratory for Laser Energetics, which has the following sponsors: Empire State
Electric Energy Research Corporation, General Electric Company, New York State Energy
Research and Development Authority. Ontario Hydro, Southern California Edison Company,
and the University of Rochester. Such support does not imply endorsement of the content
by any of the above parties.
REFERENCES
1. LLE Review 21, 3-7 (1984).
2. LLE Review 20, 143-149 (1984).
Section 2
PROGRESS IN LASER FUSION
2.A Numerical Simulation of Recent, 24-Beam, Blue
(351-nm) OMEGA lmplosior~Experiments
LLE is presently involved in a series of experiments 'intended to
demonstrate the ability of direct-drive laser fusion to ablatively compress
microencapsulated deuterium-tritium (DT) fuel to 200 times its liquid
density. This campaign has been structured into three parts. The first
part was initiated in February 1985 and involved the implosion of glass
microballoons filled with various pressures of DT gas. Compression of
these initial targets attained maximum fuel densities of 6.0 to 6.5 glcm3,
or -30 times liquid DT density (30 XLD) and maximum neutron yields
in the range of 1.5 to 2.0 x 10ll. The experiments also served to test
recently added diagnostics on the OMEGA system which will be
necessary to evaluate the final 200-XLD targets. These diagnostics
include knock-on fuel ion spectrometry for measuring fuel areal density
(),I radiation chemistry techniques (RAD-CHEM) for measuring
shell areal densities (),2 neutron time-of-flight equipment for
determining core temperatures,3 and Kirkpatrick-Baez (K-8)
microscopes, which give spatially resolved x-ray emission information
from the imploding shell.4
The second phase of the 200-XLD campaign is to begin in January
1986. Because of the unavailabil~ty cryogenic targets, this phase will
of
involve the use of targets similar to those in the first phase, and high-
pressure (100- to 150-atm), DT-filled glass microballoons. One-
dimensional simulations show that the implosion characteristics (e.g.,
convergence ratios) of small-diameter, high-pressure targets resemble
those of targets using moderately thick layers of cryogenic fuel. As such,
LLE REVIEW, Volume 25
these "surrogate cryogenic" targets will provide preliminary information
concerning the implosion of cryogenic targets. Furthermore, because
the small diameters of these targets will result in high illumination
intensities (i.e., >2 x 1015 Wlcm2), these targets might provide exper-
imental data reflecting the presence of certain plasma physics processes
(e.g., 2 , self-focusing, etc.) in the underdense corona. Finally, should
w
the fabrication of bare plastic shells containing thick layers of cryogenic
DT fuel prove difficult, these high-pressure, glass-shell targets will be
frozen and used in.the final phase of the 200-XLD campaign.
The third and final stage of the 200-XLD campaign, expected to be
initiated in the summer of 1986, involves the use of cryogenic DT fuel
overcoated with plastic shells. It is through the compression of these
cryogenic targets that the expected goal of fuel densities in excess of
200 XLD is to be achieved.
This article presents comparisons between numerical simulations (both
one- and two-dimensional) and recent experimental results obtained
from the initial phase of the 200-XLD campaign on the OMEGA system.
The numerical simulations were carried out using the one-dimensional
hydrodynamics code, LILAC,5 and the two-dimensional hydrodynamics
code, ORCHID. Both codes, which were developed at the University of
Rochester, are based on a one-fluid, two-temperature approximation
employing the use of Lagrangian hydrodynamics. The calculations
include (1) tabular equation of state (SESAMEG), (2) flux-limited electron
thermal transport,' (3) multifrequency-group radiation transport using
Fig. 25.6 LTE opacities reduced from the LANL Astrophysical Library,e and
2,O.kJ generic target designs used for (4) the Use of geometrical optics ray tracing9 for the deposition of hser
attaining (a) high yield and (b) high density. energy by inverse bremsstrahlung. Emphasis is placed on comparison
Both designs call for initial fuel pressures of between experiment and simulation in neutron yields, fuel and shell
10 atm. areal densities, and x-ray emission images.
PROGRESS IN LASER FUSION
Target Design Overview
Preliminary scoping studies10 were performed to identify candidate
target designs for use in the initial phase of the 200-XLD campaign. The
two basic designs that emerged from this work are shown in Fig. 25.6.
The first of these designs [Fig. 25.6(a)] is intended to maximize neutron
yield. Optimal performance of such targets requires that the target shell
be accelerated to high velocities that will, upon stagnation at the center
of the pellet, produce high core temperatures. Therefore, these targets
have large initial diameters that maximize the absorbed laser energy
and use thin (low-mass) shells to maximize the resulting shell velocity.
The simulated behavior is represented in Fig. 25.7.
Viewing the radius versus time (R-T) plot shown in Fig. 25.7(a), it
should first be noted that this target is being driven ablatively rather than
explosively. In an exploding pusher, the shell blows apart, with roughly
half the mass moving out and the other half moving toward the center.
In an R-T plot for such an implosion, the half-mass point within the shell
(approximately the shell midpoint) would remain relatively motionless
until after the imploding part of the shell rebounds. For ablatively driven
targets, the driving force is not generated at the shell midpoint but at the
ablation surface near the outer edge of the shell. As such, the ablation
process "peels away" the outer layers of the shell while accelerating the
remaining mass inward. Although most of the shell mass is initially
accelerated inward, less than half remains by the time the shell reaches
stagnation. Neither the mass of the imploding shell nor the position of
the shell midpoint remains constant during the implosion for ablatively
driven targets.
One-dimensional simulations show that the glass-fuel interface velocity,
illustrated in Fig. 25.7(b), easily exceeds 4 x l o 7 cmls, and in some
cases approaches 6 x 107 cmls. These high-velocity shells produce
high core temperatures as a result of shock heating and shell
stagnation. The simulated neutron energy spectrum is shown in Fig.
25.7(c). Applying the prescription put forth by Brysk,3 the neutron-
averaged ion temperature for this implosion was -5.0 keV. The neutron
spectrum was integrated and gave a neutron yield of - 1.60 x 1012
neutrons. Both of these results are consistent with results from the
overall simulation. Although such yields are quite encouraging, the
corresponding maximum densities are low. The temporal density profile
is displayed in Fig. 25.7(d), showing a maximum fuel density of only
0.89 gm/cm3, corresponding to - 4 XLD.
The second type of design [Fig. 25.6(b)] is intended to maximize the
compressed fuel density. This type of target requires that increases in
the fuel temperature due to nonisentropic processes (e.g., shock
preheat) be kept as low as possible, providing a relatively massive
inward-moving shell with minimal resistance in compressing the fuel to
high densities. These targets require the use of small diameters to limit
the distance over which the acceleration can act, and of relatively thick
shells to lower implosion velocities. The simulated behavior is shown in
Fig. 25.8.
LLE REVIEW, Volume 25
0.0 0.4 0.8 1.2 1.6
Time (ns) Time (ns)
x109
13.8 14.0 14.2
Energy (MeV) Time (ns)
Fig. 25.7 As before, the R-T plot [Fig. 25.8(a)] shows that this target is being
One-dimensional LILAC simulation of a ablatively driven. An additional feature is that a slight shock breaks out
high-yield target: of the interior surface of the glass shell because of the lower velocity
(a) R-T plot, achieved by these targets. The lower velocities [between 1 and 2 x l o 7
(b) interface velocity history, cm/s, shown in Fig. 25.8(b)] produce only modest temperatures (1-2
(c) neutron energy spectrum, and
keV), allowing the incoming shell to compress the fuel on a much lower
(d) fuel density history.
adiabat to high densities. The analysis of the predicted neutron energy
spectrum [Fig. 25.8(c)] illustrates the generation of only modest
temperatures. This spectrum shows the neutron-averaged temperature
to be - 1.6 keV and the neutron yield to be 1.OO x 10lO.Because of
the low temperatures, the maximum fuel density predicted [Fig. 25.8(d)]
is 45 g/cm3, or 210 XLD.
PROGRESS IN LASER FUSION
x lo7 1 1 1 1 1 1 1
0.4 - (b) -
0.0 . -
--.
V)
E -0.4 - -
-
0
5 - 0.8 - -
.-
0
-
0
-1.2- -
- 1.6 - -
- 2.0
0.0 0.4 0.8 1.2 1.6 0.0 0.4 0.8 1.2 1.6
Time (ns) Time (ns)
x 107
0.0 0.4 0.8 1.2 1.6
Energy (MeV) Time (ns)
TC1968
Fig. 25.8 In summary, the targets used for the initial phase of the 200-XLD
One-dimensional LILAC simulation of a campaign fell into two categories. The first, high yield, was necessary to
high-density target: provide neutrons for testing new diagnostics to be used in the analysis
(a) R-T plot, of the final 200-XLD cryogenic target implosions. The other category,
(b) interface velocity history,
high density, was designed to maximize fuel density and provides an
(c) neutron energy spectrum, and
initial setting from which the final goal of 200 XLD can be achieved.
(d) fuel density history.
Experimental Comparison
The initial sequence of experimental implosions on the 24-beam, blue
(351-nm), 2-kJ OMEGA laser system was completed in June 1985.11 In
conjunction, there was a significant numerical-simulationcampaign, in
both one and two dimensions, designed to provide additional insight
LLE REVIEW. Volume 25
into the interpretation of the experimental results. Particular interest has
been paid to the comparison of overall neutron yields, fuel and shell
areal densities, and x-ray microscope film response curves.
The comparison between experimental and predicted neutron yields
has traditionally been displayed as a function of the initial glass shell
thickness. When applied to the large number of significantly different
target designs involving different initial aspect ratios, initial fill pressures,
and the occasional use of plastic overcoats, however, this relationship
failed to provide any useful insight into this series of implosions. A more
meaningful display of the comparison between the experimental and
predicted neutron yield was found in terms of the one-dimensional
predicted convergence ratio. (Here the convergence ratio is defined as
the ratio of the initial fuel radius divided by the fuel radius of the
stagnated core.) This relationship was further simplified by normalizing
the experimental yields to the corresponding one-dimensional prediction.
Results are shown in Fig. 25.9.
Si02 shells A CH2- Si02 shells
I I I I
1
Total shots: 65
8 16 24 32 40
Fig. 25.9
Experimental neutron yields, normalized to Calculated Convergence Ratio
TC1847
the calculated one-dimensional yield, drawn
as a function of the calculated conver-
gence ratio.
In Fig. 25.9, the strong dependence of the normalized neutron yield
on the predicted convergence ratio is clearly evident. This graph shows
that experiments with targets requiring high predicted convergence
ratios (i.e., 15-30) performed very poorly in comparison with one-
dimensional simulations. The declining agreement with increased
predicted convergence ratio suggests the possible presence of
implosion nonuniformities.The presence of these nonuniformities would
be expected to disrupt the compressed core and prematurely terminate
the thermonuclear burn. As a result, the neutron-weighted diagnostics
(i.e., knock-on ion spectrometry and RAD-CHEM techniques) would give
PROGRESS IN LASER FUSION
physical information that no longer corresponds to the time of peak
compression but rather to an earlier time corresponding with the
termination of burn. In Fig. 25.10, the measured fuel and shell areal
densities are compared with the areal densities at those times in the
simulations when the neutron yields equal the total experimental yields.
At these points in the implosions, Fig. 25.10(a) shows agreement
between the experimental and simulated fuel values over the
range of predicted convergence ratios. However, the shell
comparison, illustrated in Fig. 25.10(b), shows that the experiments
achieve higher values than the simulations as the convergence ratio is
increased. The different behavior of the fuel and shell areal-density
comparisons can again be explained by the existence of nonuniformities
in the target. At this point in the implosion, the fuel is dominated
by volume changes in the core due to the imploding shell. Although the
shell is deformed, the difference in volumes between the perturbed and
unperturbed cases is small and, as such, the experimental and
simulated fuel values should be in agreement. The shell
values, however, are very sensitive to distortions in the shell.
Because deformations in the shell cause increased compression, the
measured shell values would be higher than those predicted
by the simulation. It is important to point out, therefore, that attempts to
infer core conditions (e.g., fuel density) from the measured shell areal
density can give incorrect results if care is not taken to analyze the
entire implosion.
Calculated Convergence Ratio Calculated Convergence Ratio
Fig. 25.10 Further evidence of the existence of nonuniformities is provided by the
Experimental fuel (a) and shell (b) areal analysis of K-6 microscope images taken from a series of implosion
densities, normalized to calculated one- experiments on comparable targets, each illuminated at a different focal
drawn as a Of position. Since the level of illumination nonuniformity increases with
the calculated convergence ratio. decreasing focal position,l2 experimental agreement with one-
dimensional predictions should become poorer as the focal position is
LLE REVIEW, Volume 25
decreased. Comparing the azimuthally averaged f~lm density profile of
a K-B microscope image and the corresponding time-integrated profile
predicted by LILAC, as shown in Fig. 25.1 1(a), it is possible to
determine an average core radius from both the experiment and the
simulation. The ratio of the measured core radius to the predicted core
radius, versus initial focal position for these targets, is illustrated in Fig.
25.11(b). It can be seen that the agreement between experiment and
simulation becomes poorer as the focal position is reduced from 8R
(good illumination uniformity) to 2R (poor illumination uniformity).
Fig. 25.17
X-ray micrograph profiles:
(a) emission comparison between experi-
ment and simulation for shot No. 7 7309,
(b) ratio of averaged core radii (experi-
mental divided by simulated) drawn as a
function of initial focal position.
Illumination Nonuniformities
In attempting to understand the effects of the nonuniformities on target
performance, it is first necessary to identify the source of the
nonuniformities on target. The analysis of the nonuniformities originating
from the laser illumination is complex and difficult. Contributions to the
illumination nonuniformities can come from several sources, including
(1) number of beams, (2) individual beam profile, (3) focal position,
(4) energy balance between beams, and (5) beam placement on target.
LLE REVIEW, Volume 25
P-mode
Fig. 25.13
Modal structure of illumination nonuniformi-
ty for current OMEGA system at 6R focus-
ing.
areal densities of 2.51 x g/cm2 and 2.75 x g/cm2,
respectively. Additional results from the ORCHID simulation are shown
in Fig. 25.14, with each frame showing the condition of the mesh at
different times during the implosion.
The last frame in Fig. 25.14 corresponds to the point in the simulation
when the neutron yield equals the total experimental yield. In Fig. 25.15,
this frame has been magnified and plots of material density contours,
areal density contours, and velocity vectors have been added. From
Fig. 25.15(a) it can be seen that the shell-fuel interface has undergone
serious deformation, exhibiting a 48% peak-to-valley perturbation.
Although the density contours in Fig. 25.15(b) show the same behavior,
it is encouraging that the entire fuel region has a density in the range of
0.1 to 0.5 g/cm3. Areal densities are determined by evaluating the
integral of pdR from the center of the pellet out into the corona, as
shown in Fig. 25.15(c). Averaging over the fuel region in this graph
gives a value of 1.0 to 1.5 x g/cm2 for the fuel cpR > . This value
is in good agreement with the measured value of 2 . 1 3 ~ 1 0 - ~
+
- 3.29 x g/cm2 and the one-dimensional result of 1.21 x I O - ~
g/cm2.
The implosion deformations have affected the shell region more
seriously. This is especially evident along the polar axis, where -
compared with the region along the equator, as shown in Fig. 25.15(b)
- significant thinning has occurred. Because of the large variations in
density, contributions to the shell show strong angular
dependence. As can be seen in the shell region of Fig. 25.15(c), the
integrated values of shell vary between 5.0 x and
3.0~10-~ g/cm2. The average value of 2 . 1 2 ~ 1 0 - 3g/cm2 is 60%
PROGRESS IN LASER FUSION
Imposed Illumination Nonuniformity
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