Inertial Fusion: Monojoules to NIF pdf

Transcription

Lawrence Livermore Laboratory Report UCRL-BOOK-218519
Lasers and Inertial Fusion Experiments at Livermore
John F. Holzrichter, Ph.D.
Lawrence Livermore National Laboratory
January 2006
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The images on the cover of this document show a progression of inertial fusion laser
systems (on left side) and corresponding target experiments (on right side), starting with:
• Top:
Janus I experiments using a 20 J laser to irradiate 50 micron capsules (1974),
• Middle: Nova experiments using 25kJ of 3! light directed into a hohlraum, producing
soft x-rays which ablated and compressed 500 micron capsules, (1990).
• Bottom: NIF laser in final construction phase, to produce 1.8 MJ of 3! light directed
into a hohlraum, in which a 2000 micron (i.e., 2 mm) diameter capsule is
ablated and compressed by soft x-rays (planned for the 2010 time frame).
This document was prepared as a chapter for a book titled:
"Inertial Confinement Nuclear Fusion: A Historical Approach by its Pioneers"
Edited by Prof. G. Velarde, Prof. of Nuclear Physics at the University of Madrid and to
be published in 2006. In accord with the title, it is a personal recollection of events in
which I participated, starting in 1972 when I joined the Livermore Laboratory and lasting
through the late 1990s. Hence the style, many of the included subjects and references are
organized to meet the requests of the editor.
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Lasers and ICF Experiments at Livermore
John F. Holzrichter
[email protected]
Lawrence Livermore National Laboratory, and
The Fannie and John Hertz Foundation
Abstract
In 1971, after almost a decade of experimentation and analysis with laser plasma production and
with relatively simple laser fusion related experiments, and after a decade of progress in
generating higher power, shorter-pulse laser beams, the Atomic Energy Commission (AEC, now
part of the US DoE) and the Lawrence Livermore National Laboratory (LLNL) formalized an
inertial confinement fusion program with the objective of demonstrating DT fusion ignition
(possibly using laser energy as low as 10 kJ). Then, if warranted, a mega-joule class laser would
be designed and constructed to produce significant fusion gain, using deuterium-tritium fueled
fusion capsules. Our first experiments under this program began at LLNL in 1974, using a 20J,
Nd: glass, 1.06 micron laser - called Janus. Target and laser experiments progressed rapidly using
increasingly powerful lasers, up to the 10,000J Shiva laser in the late 1970s. During this period,
we decided that the most expeditious means of obtaining uniform compression of the fuel capsule
would be to use an indirect-drive, hohlraum target. The principle of this target is an integrating
cylinder, inside of which spatially “noisy” laser beams could be converted to a uniform flux of
soft x-rays. These x-rays, scattering from all directions, smoothly ablate the fusion capsule
surface, thereupon compressing the contained fuel to fusion conditions. During this period, we
came to believe that direct-drive fusion, as first published, would be difficult to implement
because of single-laser-beam amplitude noise and multi-beam-intersection non-uniformities that
would cause unacceptable drive non-uniformities on the target capsule surface. In addition, we
suspected that the high laser power densities needed to compress the pellet would generate hotelectrons, causing excessive capsule and fuel preheat. It also became clear that a shorter
wavelength laser (probably with output at 0.35 microns) would be needed to reduce the extreme
levels of hot electrons being seen in 1.06 micron irradiated hohlraums, caused primarily by a
Raman laser-plasma instability. In the 1980s, new laser ideas based on full-beam harmonic
conversion and new low non-linear-refraction laser materials, as well as new target designs,
based on increasingly accurate simulation tools, validated by sophisticated diagnostics, enabled
the “control” of hohlraum conditions. By the end of the 1980s, hot electron production was
reduced to acceptable levels and then 100x fuel densities were achieved using ablative
compression. In the early 1990s target conditions similar to those needed for fusion-ignition and
gain experiments were demonstrated using the upgraded Nova laser. Simultaneously, a highenergy laser design – sufficiently economical and capable of generating mega-joule outputs—was
demonstrated in the “Beamlet” experiment. Carefully prepared arguments, bolstered by abundant
data, were presented formally to the DoE and to its review bodies to request permission to begin
design and construction of a nominal 2 MJ, 0.35 micron output laser driver called the National
Ignition Facility (NIF). It was granted in 1993. In a few years from today, ICF ignition and gain
will take place – 40 years after we started this remarkable “odyssey”. This article is my personal
account of these events.
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I. Introduction
In 1971, the Lawrence Livermore National Laboratory (LLNL) and the US
Atomic Energy Commission (now the US Department of Energy, i.e., DoE) made a
decision to start a program in inertial confinement fusion (ICF). Its objective was to use
(contemplated) high power, short-pulse laser beams to heat several milligrams of
deuterium-tritium fuel (DT), contained in small spherical capsules, to fusion conditions.
It was estimated (Nuckolls 1972) that ignition of such fuel might be accomplished using a
laser of about 104 J (10 kJ), with a pulse duration of about 1 ns, generating 1013 W of
laser light. More importantly, it was expected that efficient fuel burn up would take place
by delivering about 1 million-joules of laser energy (2x106 J) in a time-shaped, nominal
10 nsec light pulse, providing a peak power of about 2x1014 W. For a detailed review of
the target physics program see Lindl (1995) and the chapter by John Nuckolls in this
book. However, in the early 1970s, pulsed lasers, at the 0.1 nsec pulse duration, were
only capable of delivering about 1.0 J of laser light to a target surface (producing 109
watts). They had poor beam amplitude and focusing qualities, they caused significant
target preheat, and had poorly understood time-bandwidth structures. These problems
were not surprising since the laser had only been demonstrated10 years earlier by
Maiman (1960). Inventing and developing laser designs, laser technologies, target
fabrication technologies, understanding target physics, inventing simulation techniques,
and developing very advanced diagnostics to meet the ICF goal was without question, the
“grand challenge”.
This paper originated with a book chapter describing early work in Laser Fusion
by Velarde ( 2006). Its format is derived from the book and its content describes the
author’s working view of several significant events in which he had the privilege to
participate, and which have contributed to the 1993 decision to build a 1.8 MJ driver
(NIF) and to soon conduct an Inertial Fusion Gain experiment. It is written in a narrative
style, with several of the cited papers being those on which I was principal author,
accompanied by my immediate, working colleagues at that time. These cited papers are
used as part of this personal narrative because they accurately described the events of the
particular moments when they were written. In addition, I mention many individuals and
groups in this article, without formal referencing, because their contributions have left a
special impression in my (recallable) memory. Individuals, whose work is not cited here
but who contributed greatly, have been referenced in the LLNL laser program reports, in
the general literature including the cited papers, and in my more deeply buried-thandesired memory - to them I apologize. There is a very large bibliography of published
material on ICF, which should be referenced for historical accuracy and for detailed
technical descriptions of the many simultaneous and sequential challenges that have been
overcome.
IA. Introduction - Laser Drivers
In the 1970 time frame, it was anticipated by Emmett (Emmett et al 1974) that
needed high pulsed-power lasers could be built using a Nd:glass and Nd:crystal laser
system. (see Figure 1) The Nd3+:glass laser was discovered in the mid-1960s by Snitzer
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(1966). Shortly thereafter, a Q-switched laser oscillator and rod amplifier system was
built at Company General d’Electricite in France (1969). One of these systems was
purchased by the Naval Research Laboratory and then outfitted with a short-pulse
Nd:YAG oscillator and a disk laser amplifier (1970). The Neodymium (Nd3+) ion is a
“perfect” optical storage entity for high pulse power laser applications. It is capable of
holding energy in a metastable atomic state for many 100s of microseconds, thereby
allowing efficient stimulated-emission at or near the peak gain wavelength of 1.064
microns, in very short, < 1ns laser pulses. (Krupke 1974) . In particular, Nd3+ ions doped
into optical glass plates (i.e., Nd:glass herein) have a desirably low stimulated-emission
cross-section (typically 2 x 10-20 cm-2), with broad enough absorption bands to absorb
excitation light from relatively low-cost xenon flash-lamps, and with no absorption at the
laser wavelength. In addition, Nd in glass disks can be fabricated into large diameter
plates, which can be excited uniformly, by arrays of flashlamps to significant gain levels.
Many such plates (i.e., elliptical disks), when placed one after the other at Brewster’s
angle, inside of an array of flashlamps, make a disk-laser amplifier (e.g., McMahon et al
1973). A nominal 0.001J seed pulse, generated by a laser-oscillator using a Nd:crystal
(Nd:YAG or Nd:YLF) and preamplifier system, can be amplified to the kilojoules/beam
level by passing the initial beam through one to two meters of glass in several disk
amplifiers and through optical glass lenses, polarizers, and off of 1st surface mirrors (all
arranged to optimize the laser performance). Hence cumulative large-signal amplifier
gains, of > 106 (over and above passive and active losses) characterize typical fusion
laser designs. Such high gain must be controlled to prevent parasitic laser oscillations, to
stop feedback from the target, which is usually located near the retro-reflecting focal
point of the final focusing lens, and to prevent target damaging laser pre-pulses. Early
laser-target systems used a one-pass, expanding-area amplifier-chain architecture, while
the most recent design for the National Ignition Facility (NIF) laser system employs a
multi-pass, regenerative technique. Another advantage of using 1 micron Nd-ion based
laser technology, that turned out to be critical to the ICF laser program, was the rich
knowledge base generated during the 1960s regarding coherent, nonlinear-optical
physics, including harmonic conversion, longitudinal and transverse Raman and Brillouin
conversion, self-focusing, and damage phenomenology. Finally, most importantly, we
could use the many existing optical instruments sensitive to the 1.06 micron wavelength
to diagnose the laser beam and its interaction with the target.
In 1973 we were able to build a several hundred joule, short pulse laser system
(Cyclops) for laser R&D (Glaze 1973). Shortly thereafter, in 1974, Ralph Speck and I
built a 0.1 nsec, 20 J, Nd based, high-power single-beam laser-system (Janus) using
Cyclops component designs to conduct well controlled target irradiation experiments.
We were able to use commercial silicate optical glasses for the lenses, windows, and the
for the laser amplifier materials, existing electro-optic materials, as well as improved, but
still relatively conventional coatings for the lenses, mirror blanks, windows, and
polarizers. While in the 1970s we thought that these laser system elements could be
scaled to relatively large diameters, perhaps 20-30 cm producing 1 kJ per beam, it was
inconceivable, at least to me, that we would eventually build multi-megajoule lasers for
high gain fusion research by using Nd:glass as the laser medium. Continued laser R&D
on new laser glasses, control of laser beam limiting physics, system engineering,
reliability, cost management, excellent collaborations with industrial suppliers, and close
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working relationships with the target design and fabrication groups made this possible.
Today Nd:glass-plate amplifier-chains are being used in parallel clusters, 192 beams on
NIF, collectively generating >1.8 MJ of blue light (up to 5 MJ of 1.05 micron light) soon
to be directed into a hohlraum target. Each chain generates both very high energy, over
10 kJ, at very high power, > 1012 W, and at the 2nd or 3rd harmonics of the fundamental,
with flexible pulse shapes, and with sophisticated spectral and spatial on-target laser-light
qualities.
Aside from the Nd:glass laser, it was clear that other laser systems such as the
longer wavelength lasers CO2 (see work by Singer et al at Los Alamos and Manes, Haas
et al. at Livermore) and Iodine (R. Siegel at Garching), and the shorter wavelength KrF
laser might offer advantages for the fusion application (see Krupke at al 1978), depending
upon the program objective. However, we were concerned that long wavelength lasers
might generate excessive plasma-instabilities preventing light absorption and generating
too many hot electrons. On the other hand, the KrF laser could not easily generate shorttime pulses and its UV laser light was known to damage optical elements at low and
uncertain optical fluence levels. Hence after much study regarding upcoming ICF
program experimental needs, the Nd:glass system was chosen.
The scaling of the Nd:glass laser system to the mega-joule energy level, while at
the same time demonstrating the key concepts of Inertial Confinement Fusion at each
step in the process, was the audacious challenge that Emmett and his team (including this
author) accepted. (see Figure 1) The closely coupled target simulation, analysis, and
target fabrication program, was similarly challenged being led by John Nuckolls and his
team.
Figure: 1 From lower left, clockwise around the table, 1973: J.Glaze, J..L.Emmett (facing
viewer on left), J.F.Holzrichter (author who is facing viewer on right), and W. Krupke (far right)
discussing laser and ICF program strategy. J.B.Trenholme (not pictured) played a major role in describing
the physics and optimization of laser systems for ICF applications.
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IB. Introduction – Fusion Targets
In late 1959, Nuckolls and his colleagues were studying how DT filled capsules might be
compressed and might cause the contained DT fuel to undergo fusion efficiently
(Nuckolls et al 1959). Upon demonstration of the first laser by Maiman (1960), efforts to
use lasers to heat and compress targets continued through the 1960s. An important stage
of inertial fusion would occur when ignition of the central part of the compressed DT fuel
took place, see Nuckolls et al (1972). It was hoped, if everything worked perfectly, that a
10kJ short pulse, short wavelength laser could compress and ignite the central fuel. It
was further calculated that about 1 MJ of laser energy would be needed to cause the rest
of the fuel, compressed around the central “hot-spot”, to react yielding more fusion
energy than laser energy absorbed, i.e., Fusion Gain. To experimentally demonstrate
these astonishing ideas in a realistic research program, many physical phenomena
associated with the compression and fusion reaction had to operate as postulated. These
were:
1) A sufficiently smooth, spherical capsule containing a sufficient amount of DT
fuel could be fabricated, and then compressed such that the fuel would remain
spherical with little mixing of contaminants from the outer parts of the capsule,
the fuel could be compressed in as “cold” a state as possible (using a minimum
amount of driver energy to do the work of compression)); and finally after
compression, the center “hot spot” of the fuel could be shock-heated to a high
enough temperature that it would cause the surrounding fuel-layer to under go
fusion burn-up, before the compressed fuel structure “disassembled”.
2) A method could be devised to absorb and convey enough of the laser optical
energy to a capsule’s surface, at a rate > 1014 W/cm2, with sufficient uniformity,
and without excessive ablator and fuel preheat.
3) A progression of increasingly powerful, “target-matched” lasers could be devised
and constructed in a measured way, at a low enough cost, and used to conduct
fusion-capsule experiments sufficiently rapidly, that the sponsoring communities
and the working scientists-engineers would remain interested in the ICF
objectives. This was especially demanding since the needed laser energy scaled
approximately as the cube of the target dimensions.
4) Finally, appropriate targets could be fabricated and diagnostic systems could be
implemented in such a way that at each step of the R&D program, the expected
physical phenomena could be validated and understood.
The program strategy that was chosen by Livermore Laboratory management was to
concentrate most of its ICF resources on the demonstration of necessary conditions for
fusion reaction, at the relevant scale lengths. This approach enabled the key issues
impeding progress toward ignition and gain to be identified, and then resolved. In
particular, our successes in constructing large enough and well-enough understood lasers
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during the 1970s and early 1980s, enabled the design of key experiments with sufficient
diagnostic resolution using target scales up to 1 mm (i.e., at > 1000 wavelengths). These,
followed by careful analysis of the outcomes, led within about 10 years to an
understanding of the conditions needed to demonstrate laser driven ICF.
Significant ICF demonstrations, in approximate order of
accomplishment from 1972 to the present:
• Repeatable and high-resolution laser targeting
and diagnostics technologies,
• Thermal fusion conditions in compressed capsules,
• Indirect-drive hohlraum target concept and fabrication,
• Ablative compression using soft x-rays inside a hohlraum,
• 1 TW laser beam (Argus at 1 TW = 1012 W) and the
corresponding 20 beam, 20 TW Shiva laser,
• Understanding absorption & Raman scattering as the source of hot-electrons in
hohlraum targets, => 1 micron laser wavelength to be too long,
• 10 TW, 10kJ laser beam design for Nova (and Novette),
• Laying to rest questions concerning ICF ignition using Halite/Centurion
• Demonstrating control over the key parameters needed for
Laser-excited ICF target ignition and gain
• Laser architecture capable of being scaled to a
multi-megajoule, short wavelength laser,
• A 1.8 MJ laser system, NIF, designed and under construction
In the 1980s a 10kJ, 1.05 micron laser beam with efficient, large area 2!, 3!, & 4!
harmonic converters was demonstrated on the 2-beam Novette laser and the 10-beam
Nova laser was set-up for 3! target irradiation. The short wavelength light was absorbed
producing few hot electrons, and a100x fusion fuel compression was demonstrated.
Finally predictive analysis, precision laser irradiation, repeatable experiments, and high
temperature hohlraum designs enabled the demonstration of a highly compressed fusion
capsule, scalable to ignition and fusion gain. To accomplish the fusion gain experiment
with > 1 MJ of 3! light, new laser architecture was designed and high performance
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optical materials were developed. In the late 1990s, the construction of the NIF laser
began. In 2005, clusters of NIF beams validated the system’s design objectives of > 10kJ
of 3! light per beam, providing confidence that 1.8MJ of 3! laser light would be
delivered to a high gain target.
Effective total beam areas,
with circular area examples for
Cyclops, Shiva, and square beam
area for NIF
Figure 2: Plot of effective laser output diameter (cm) per laser system, as if the entire individual laser beam
output areas were coalesced into one collective aperture. System 9 shows one equivalent dimension (blue
bar) and the corresponding equivalent square area of all 192 square-beams of NIF. System 8 shows the
diameter of Nova’s equivalent circular target optics area; system 7 is Nova’s equivalent output-amplifier
diameter, system 6 is 2-beam Novette system; system 5 shows the 20-beam Shiva equivalent output area
circle and diameter as a blue bar. System 4 shows the 2-beam Argus20cm-amplifer output, system 3 is the
one beam 31cm-dia. Cyclops laser; system 2 is Janus-II with 2-beams at 8cm diameter each, system 1 is
Janus-I with one beam at 8cm diameter. (Note: the output energy of each laser is proportional to the
cumulative area). The dramatically increasing laser performance is a consequence of the energy needed to
match the approximate cubic increase in energy versus the target linear-scale dimension. This is because a
linear increase in a fusion target’s dimensions corresponds to a cubic increase in fusion target volume,
hence the need for corresponding cubic increase in laser energy for heating and compression. Conversely,
a reduction in laser output for a planned experiment sequence, e.g., for the purpose of reducing the rate of
damage on the laser’s output optics, would reduce the linear scale of the experiments by approximately the
cube root of the energy reduction. For example, a 40% reduction from peak laser performance to a safe,
low-maintenance-cost operating condition at 60% of peak power, would lead to a 15% reduction in linear
target scale, a relatively small consequence for most experiments. Because of the scaling of the irradiated
targets, we were driven to increase laser performance by almost 100-fold per decade from the mid 1970s to
the mid 2000s.
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II. 1970s: The exploration & concept validation period at Livermore
IIA. Solid state laser issues regarding fusion experiments:
My first job upon joining the Livermore Laboratory in 1972 was to participate in
the design and then validate the performance of a family of increasingly large diameter
laser disk amplifiers, providing needed information for the laser system designers and
builders. Using results from earlier work at the Naval Research Laboratory in 1969-72
(McMahon at al 1973), we built several different sizes of rod and disk amplifiers to
obtain design code normalization and “standardized” components. We also developed
time-bandwidth limited oscillators, Faraday isolators, special optical coatings, and many
other needed optical elements. These data included amplifier gain (and gain coefficients)
versus flashlamp power and versus Nd disk concentrations, studies of parasitic gain
control within the glass laser disks themselves (i.e., transverse amplified spontaneous
emission and parasitic gain limits), as well as understanding loss coefficients, gain
uniformity, and other engineering data. There were many problems with the early laser
amplifier experiments, ranging from flashlamp failures (see Fig 3) to particulate
contamination of the optical surfaces. The early laser amplifiers were optimized to
minimize laser glass optical path and meet desired gain levels (about 3X). To do this
high Nd ion doping and high flashlamp loading was utilized. In later designs, first on
Nova and then for NIF, other criteria were optimized taking advantage of longer pulse
durations, with less self focusing, but at the same time increased energy fluence.
Figure 3: Damaged amplifier due to a flashlamp explosion, 1976. The Shiva laser system used
2000 such flashlamps. NIF uses about 6000 lamps that are each about 10x the volume of a Shiva lamp.
Control of accidental electrical explosions has been a critical task, now largely accomplished.
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As soon as components were available, the one-beam Cyclops laser, was
constructed to demonstrate cumulative laser beam gain, propagation, isolation, as well as
to validate newly invented concepts, mainly high power spatial filtering and relay
imaging (Glaze et al 1973). This beam-line propagation knowledge was needed to reach
the TW (1012 W) per beam-line power goals, which are for example: 100J in 0.1 ns, or
1000J in 1 nsec, or 10,000J in 10 nsec.
Two major propagation issues stood in our way, and became apparent very
rapidly. The first was the well known appearance of small-scale self-focusing
phenomena in the laser-beam’s near-field profile (often leaving faint tracks in the bulk of
the laser glass, called “angel-hair”) at power densities nearing 1010 W/cm2, a level at
which we wished to operate our amplifiers. Particulate contamination on laser optical
surfaces was (and remains) the major source of this problem because it causes laser light
diffraction, which increases down-stream beam amplitude and phase noise and causes
damage inside and on surfaces of the most highly stressed “down stream” optical
elements (see Figure 4 below). The second problem was low aperture usage (i.e., “fillfactor”) of the propagating laser beam, which meant, for example, that we were
constrained to use a 5 cm diameter laser beam in an 8.5 cm amplifier aperture to prevent
“edge-clipping” leading to diffraction ring propagation and damage. This meant that
only 40% of the expensive gain-area could be used.
Figure 4: Beam diffraction rings, amplified by small scale self-focusing showed up in “near-field”
laser experiments as damage to optical surfaces and within the glass substrates.
The self-focusing problem (first discussed by Bespalov et al (1966) slowly
yielded to careful examination. Small transverse-scale, “hot spots”, in high-powerdensity laser-beam amplitude-profiles cause the index-of-refraction of an optical material,
where the hot spot propagates, to increase with power. (See Figure 4) The local
refraction-increase causes small virtual “lens-lets” to focus the beam’s hot-spots more
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strongly, which in turn cause the spot to become hotter, etc. Bliss et al (1976) performed
very careful measurements of the non-linear growth rates versus spatial frequency,
Trenholme and Glass (1977) described how the self-focused amplitude and phase grew
with propagation distance. Hunt et al (1976) described how to optically Fourier
Transform the spatial beam structure (at full laser power), to then filter the high spatial
frequencies away, and to then reconstruct a smoothed beam profile capable of subsequent
propagation. However, as described below, the far-field targeting impact of self-focusing
was puzzling until we understood how both amplitude noise and phase noise could add
together, intensifying the noise level near the far field even more strongly than in the near
field. The fill factor problem, and the down-stream noised induced damage problem was
partially solved by John Hunt’s invention (Hunt 1976) of image relaying. We modified
the spatial filters to become “astronomical” telescopes that could project a “cleaned” and
“frozen” high-fill-factor beam profile to a down-stream image location. This enabled
designers to use 80% optical aperture fill-factors for each amplification stage, and to then
expand the beam diameter (maintain high aperture fill-factor) for a next gain or beam
transport stage, enabling power and energy fluence control on and inside the laser optical
elements. In addition, extreme attention was paid to maintaining very clean, “low-noise”
optical surfaces, developing low non-linear index glasses, and other techniques to control
the exponential growth rates of self-focusing noise.
This understanding of propagation and new components enabled at least a
doubling of the laser output per aperture and a tripling of the accessible gain-operating
levels of individual laser chains. It made possible the 1 TW/beam, 0.5 kJ/beam Argus
laser test bed, upon which successful beam propagation experiments were completed.
These enabled the successful completion of the 20 beams, 20 TW Shiva laser system by
the end of the decade of the 1970s. Then, we were able to design the 10 kJ, 1TW per
chain output beams for the Nova laser system, which were first demonstrated on the
Nova prototype (called Novette) in 1983 and then on Nova starting in 1985. The data
obtained during the 1970s at LLNL and at other collaborating laboratories provided a
fundamental basis for solid state fusion laser systems, including the now-in-construction
NIF laser system.
IIB. Understanding target irradiation, symmetry, preheat, hot electrons.
Beginning in 1974, I started working with Hal Ahlstrom, Lamar Coleman, Erik
Storm, Ralph Speck, and many others to develop the Laser Plasma Interactions group.
Our first laser was a single beam target facility, called Janus, which was designed to
provide 20J in 0.1 nsec to match the time scale of the target implosions. It was LLNL’s
first set of laser-target-compression experiments (see Figure 5). They didn’t work very
well for many reasons. Preceding us, researchers at KMS fusion (Charatis et al 1975) ,
used a very sophisticated spherical illumination system to demonstrate the compression
of a DT filled glass sphere with attendant neutron production. We were strongly
motivated to demonstrate superior fusion conditions at Livermore, using our own
approaches based on ablative implosions, to understand the quality of the implosions, as
well as to validate the thermal nature of neutrons seen at KMS and those soon to be
generated by ourselves.
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Our first experiments, using fast planar diode optical detectors and pinhole x-ray
cameras, showed that the targets were often being heated and destroyed, about 10nsec
before the main drive pulse arrived. This was shown to be due to the laser oscillator’s
mode-locked pulse-train, 10-5 of which was leaking though the switch-out Pockels cell
switch. The optical switch selected one pulse, from a recurring train of (sometimes
incompletely suppressed) oscillator pulses spaced at about 10nsec intervals, for
amplification and direction to the target. These pre-pulses, as well as strong preheating
amplifier fluorescence noise at 1.06 micron, were soon controlled. A notable observation
from that period was that when a strong laser pulse interacted with a longer scale target
plasma, created 10nsec earlier by one of the oscillator leakage pulses striking the target,
an extremely intense hot electron signal was noted (i.e., a forerunner of soon to be seen
long plasma-scale hohlraum target problems). This “accidental” target condition was
accompanied by a very strong laser back-reflection (fortunately stopped by Faraday
isolators before it burned out laser components).
An important issued had to do with positioning the converging laser beam(s) on
the desired part of the target, and then obtaining an estimate of this partially-focused
beam’s amplitude vs. time as it interacted with the plasma. We knew that these lasers
were about 10x to 30x diffraction limited, which could lead to 100s of small hot spots at
the focus. However, the parts of the target actually irradiated were usually in the
“intermediate field” of the focusing laser beam, where the beam still preserved some
degree of smoothness associated with the laser output beam amplitude profile. The
important decision to concentrate on hohlraum targets, and not direct drive, enabled us to
make rapid progress without worrying too much about “perfect” beam phase and
amplitude control. However, as I discuss later, smoothing of the hot spots in the partially
focused and later, defocused beams became necessary to reduce peak laser power and
associated hot-electron production in those hot spot regions. Many very clever
techniques to do this were employed on the upgraded Nova experiments and built into the
base design of the NIF laser system (see work by Henesian, Dixit, et al).
A second unexpected, dynamic problem was noted when the targets were still not
performing as calculated (in fact were not responding at all). While the Janus laser beam,
measured in the near and far fields with still and streak-cameras, showed significant
amplitude noise, it didn’t seem like there was “enough” noise to reduce the target’s
performance. By using recently developed fast streak cameras in the far field (see
camera work by Coleman, Lerche, and Thomas), and using an unusual optical system
designed to project the image of the laser beam’s focal spot onto the camera slits, we saw
that at peak beam power, the laser intensity on target dropped to almost zero.
(Holzrichter and Speck, 1976). This was a consequence of the dynamic, non-linear
scattering of main-lobe (i.e., low-spatial frequency) laser-beam energy into smaller
dimensioned laser-beam hot-spots (i.e., high spatial beam frequencies), such that at peak
laser power, the beam was completely converted into high spatial frequencies, which
missed the laser target! These phenomena were correlated with near-field experiments
by Glaze et al 1974, and were formulated into the near and far-field “B-integral” limits
for high power laser beam propagation (see Trenholme et al).
Once these issues were understood on the Janus I targeting system, it was
reconfigured with higher gain amplifiers and a spatial filter to reduce the “B-integral” and
the noise. It then produced well understood target irradiation pulses. (see Holzrichter and
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Speck,1976). Next, working with Eric Storm, a 2nd beam was added creating the Janus II
laser, and much improved, more symmetrical target experiments took place. (See Fig. 5).
Many important diagnostics techniques were developed and added to the Janus
target experimental system, and many demonstrations of important target physics
occurred. These included high resolution target imaging, target temperature
characterization (including extremely hot x-rays under some conditions), laser-target
coupling, implosion symmetry , analysis of exploding pushers, and a demonstration of
thermonuclear conditions in the imploded capsule, using fusion-alpha-particle
spectroscopy. (See Figure 5 ).
Figure: 5 Janus laser system where LLNL’s first ICF implosion experiments took place in 1974.
Early experiments used 20 J from one beam, and then 40 J from two beams. Energy absorbed by the
targets was typically 6J and 12J respectively in 100 psec laser pulses. See Figure 6 for a single-beam target
example.
14
Poor Symmetry of
Irradiation
Figure: 6 One-sided exploding pusher target, irradiated using Janus I, generated soft x-ray
emission from the stagnating capsule, and from the original surrounding shell. Problems with this
implosion are the asymmetric heating of the back of the capsule and imperfect aiming of the laser light
(coming from the right sides of images), Shortly after these experiments, we measured thermonuclear alpha
particles from 2 beam targets of this type, which strongly supported the thermonuclear nature of the
measured neutrons and, together with many other diagnostics, enabled the analysts to understand these
types of implosions.
II-C: Understanding at Scale:
During the 1970s “soft” x-ray detectors were designed to measure the internal
hohlraum temperatures of the indirect drive targets (see Figure 6). High energy x-ray
detectors determined the presence of very hot-electron spectra, due to the relatively long
laser plasma-wave interaction lengths. In addition, using pin-hole and grazing-reflector
imaging techniques (later coupled to x-ray streak cameras), they measured the dynamics
of the fusion capsule implosion process. The first experiments were exploding-pushers
(see Figure 6) followed by capsule surface ablation targets (Figure 7). Hal Ahlstrom,
15
Lamar Coleman, Harry Kornblum, Ken Manes, Fred Seward, Bill Slivinsky, Erik Storm,
and many others developed exquisite diagnostics for these applications and conducted
remarkable experiments in very short periods of time. The suites of diagnostics
eventually included fast optical-, x-ray-, and neutron-streak cameras, a wide suite of xray detectors including imaging soft x-ray detectors, optical and x-ray backlighting
techniques, integrated and time resolved imaging spectroscopic techniques, and nuclear
time-of-flight and several nuclear-chemical sampling techniques.
The target construction group lead by Charles Hendricks, Bill Hatcher, and Bert
Weinstein was heavily tasked during this period to fabricate precision spheres and fill
them with high pressure DT gas, to then mount the spheres inside hohlraum cylinders
along with internal laser shine shields, and then to mount beam alignment and diagnostic
aides to the outside of the targets, and finally mounting the assemblage onto support
structures. The construction of targets to meet the demands of the designers and to then
validate that the assembly was of the material and material purity selected, the stated
thicknesses, shapes, fill pressures, and other criteria was very demanding. However, by
taking this degree of care, the experimenters and analysts had confidence that the
observed outcomes were a consequence of target physics (often incompletely
understood), and not due to target construction uncertainties.
Many other laboratories contributed to understanding the laser target coupling and
hot electron production mechanisms. In particular the work at Ecole Polytechnic by
Fabre and his coworkers demonstrated the effectiveness of short wavelength coupling,
and the U of Rochester group invented many laser harmonic conversion techniques and
investigated short wavelength coupling to targets.
The experimental program, started with 15J – 30J Janus experiments –
demonstrated the thermal nature of the neutrons being produced inside compressed
target-capsules. Then by using the 100J Cyclops laser one- and two-beam hohlraum
targets were used to demonstrate ablation-driven implosions. Next the 2-beam 1000J
Argus laser was configured for hohlraum target experiments, where higher density
ablation target compression was demonstrated, albeit in a higher electron-preheat
environment than desired. Finally, experiments up to10 kJ on the Shiva laser led to 100x
fuel compressions but with a great deal of ablator and fuel preheat. Nevertheless, the
long target interaction scale length experiments, first using the Argus and then the more
energetic Shiva laser, both using “Cairn-design” hohlraum targets, led to an
understanding of the major source of hot electrons. See Figure 7. These were shown to
come from a parametric (Raman) instability due to laser-light-wave coupling to a longscale-length, !-critical-density plasma inside the hohlraums (Lindl and Manes 1980).
The availability and accuracy of these data, with their careful simulation and analysis,
made possible an astonishing pace of laser development, target design and
experimentation, and fusion target understanding.
16
Figure: 7 The hohlraum target (e.g., a “Cairn” design here) made possible excellent laser beam absorption,
good conversion to soft x-rays, acceptable x-ray transport to the target, good uniformity of compression by
the x-rays, good x-ray temperature in the hohlraum, but the implosion was limited by very strong hotelectron production from 1micron laser-to-plasma-wave coupling over the mm length of plasma from the
entrance hole to the “first bounce”. These targets were relatively easy to diagnose by viewing the capsules
through side holes and by collecting radiochemical debris exiting the side ports. With the completion of
the short wavelength Nova laser system in the mid to late 1980s, these targets started performing almost
“perfectly”.
II-D: The Shiva Laser and 100KJ scale lasers:
In 1975, I moved from the laser plasma group to managing the solid state
laser program at the Livermore Laboratory, reporting to John Emmett and working
closely with John Nuckolls. The group of scientists and engineers working on the fusion
laser projects was terrific – Erlan Bliss, Jim Glaze, Bill Hagen, John Hunt, Dirk
Kuisenga, Bill Martin, Jim Murray, Bill Simmons, John Trenholme, and Ralph Speck.
Bob Godwin had recently joined the Emmett team, as the program manager for the Shiva
project. He introduced us to large-project management techniques, and was instrumental
in making Shiva a very successful laser project. My major role was to integrate the best
ideas from the Janus, Cyclops, and Argus target experiments, as well as those from the
solid state R&D laser group and the Shiva laser team to implement Shiva to be as
effective a laser-target system as possible (See Figure 8 below).
17
Figure 8: The author left, Carl Haussman middle, and Bob Godwin on right, next to one of the
Faraday Rotator-Isolator units on a Shiva laser beam, 1978. Shiva was the first of LLNL’s large laser
systems, incorporating most of the high power propagation and materials experiences from Cyclops and
Argus, as well as target experiences from the Janus, Cyclops, and Argus experiments. This laser generated
20 TW in a short pulse (100ps) and 10 kJ in a long pulse (3 nsec) at 1.06 microns (using silicate laser glass
and hence operating at 1.06 microns laser wavelength).
18
In 1977, after a great deal of work understanding high power laser science, with
the engineering of newly developed components (e.g., such as adding partial beam
relaying to Shiva), and with the inevitable issues of large-project management, the Shiva
was completed. It worked very well as a laser and as a targeting system, although the
wavelength at 1.06 microns proved to be too long for efficient implosions. Shiva was
completed in 1977 as planned, and as is normal for a large complex scientific system, it
took a year or so to bring it up to full performance. It enabled critical coupling
experiments to be conducted on long-scale-length laser-plasma interaction physics, at
1.06 micron wavelength, irradiating 1 to 2 mm scale hohlraums. With it, researchers
achieved up to 100x fuel compression in a Cairn Target (see Figure 7). However, the
experiments very clearly showed that DT ignition and gain required shorter wavelength
light and greater laser energy onto the target fuel capsule.
At the time Shiva was being completed, it became clear that a larger laser would
be needed to demonstrate hohlraum target conditions for ignition and gain after the Shiva
experiments. John Emmett, Tom Gilmartin, John Trenholme, and I (part time) began
designing a 200kJ, 20 beam laser, soon called Nova. This laser would need to cost
significantly less per joule than preceding lasers such as Shiva, otherwise it would not be
affordable. (See review by Holzrichter at al 1982) To accomplish this we maximized the
amount of power and energy available per individual beam line by adding extra
amplifiers and increasing the diameter of the output amplifiers to 46cm diameter and the
beam transport optics to 76 cm diameter. This optimization was done by John
Trenholme, and it accounted for the facts that the fixed costs of adding extra beam lines
was quite high, whereas the cost to extend a beam line to generate more energy was
moderate in cost. To handle the high performance from longer and larger diameter beam
lines, recently perfected relay imaging was used and new low non-linear index of
refraction laser-glasses were developed. Jack Campbell working closely with Bill
Krupke, Marv Weber, and John Emmett examined low index glass former-families such
as beryllium fluoride, fluoro-phosphate, and phosphate glasses. Each was formulated
with Nd doping, measured for laser properties such as cross-sections and energy
extraction capability, and prototyped to determine the best Nova laser-glass. Phosphatebased glass turned out to be the best candidate based on cost, performance, and
production ease, hence it was chosen for scaling to the Nova production quantities –
although, as we discovered later, it had a latent tendency to form small Pt precipitates.
The first Nova laser system design consisted of 20 beams, designed to generate 200 kJ of
1.05 micron light, at the 3-10 nsec pulse duration.
However, after the data from Argus and Shiva were analyzed, and based upon the
short wavelength laser plasma interaction work by Mike Campbell and his coworkers at
Livermore (Campbell 1982), data from the Ecole Polytechnique, U. of Rochester, and
KMS Fusion it became clear that shorter wavelength lasers were needed. The Foster ICF
review committee, meeting in 1979, confirmed this point of view. It then recommended
that Nova be recast as a 10 beam 100kJ laser, with harmonic conversion, wavelength
spreading, and other risk minimizing targeting systems.
19
III. 1980s: High Energy, 3rd harmonic lasers and controlled target conditions
III-A: New Master-Oscillator Power-Amplifier Laser - Nova:
The laser-hohlraum coupling and laser-plasma instability experiments, as well as
analyses of high power laser performance, provided the basis for a much improved laser
design and for the successful demonstrations of many fundamental ICF concepts during
the decade of the 1980s. After the formal review by the Foster Panel, it was
recommended that the next step in the ICF program should not concern itself with
ignition, but instead should provide a significant (i.e., 10-fold increase) in system target
performance. The next laser should operate at one or more of the harmonics of the Nd:
glass laser system, and should operate at the 100kJ level. It was suggested that this
should be done using 10 Nova beams, not 20 beams as first proposed. In addition, the
system should be constructed to be capable of great flexibility in providing variable pulse
shapes, spectral broadening, irradiation geometry, and other potentially useful attributes.
Also, we were directed to build a two beam prototype to demonstrate the new laser
technologies (named Novette) and to provide sufficient diagnostics to understand the
experiments involving the laser itself, target coupling, and target responses.
A harmonically converted 1.05 micron, 10-beam Nova laser was designed to
provide many pulse lengths, needed to emphasize either high power or high energy
necessary to accommodate single shell (shorter pulse) and double shell (longer pulse)
target designs. Major contributors were J.Emmett, E. Bliss, L.Coleman, D.Eimerl,
W.Gagnon, T. Gilmartin, R.Godwin, F.Holloway, C.Hurley, H. Lowdermilk, D. Milam,
John Murray R.Ozarski, H. Powell, W.Simmons, M. Summers, J.Trenholme,
P.Wallerstein, et al. (see Holzrichter 1985). The laser was designed to generate light with
sufficient flexibility to generate 10TW short pulses and 10kJ at 3 nsec, with typically
50% harmonic conversion efficiencies. The new laser designs made use of new materials
such as phosphate based laser glasses (with a 1.05 micron gain peak), high damage
threshold sol gel optical coatings by Ian Thomas (see UCRL LLNL Laser reports), long
pulse spatial filters which also served as relay imagers, a high-gain 2-plate mosaic-laser
disk for use in the 46 cm output amplifier (to enable high amplifier gain in a large
aperture without internal parasitics), a non-circular “split-disk” beam, an expanded
output-beam transport system at 74cm diameter to minimize target optics damage, and a
double plate, 3x3 harmonic crystal array using 2nd harmonic conversion in plate 1 to
green light and then by mixing red and green in plate two, generating 3rd harmonic
output at 0.35 microns (Craxton 1981). The physics of the laser harmonic conversion,
the crystal growth to very large sizes, the alignment, cutting, polishing, coating, and
mounting of the 74 cm diameter, the 3x3 harmonic converter arrays was a tour-de-force
led by Mark Summers, David Eimerl, and their precision machining colleagues at LLNL,
their industrial collaborators mainly at Cleveland Crystals Inc., and help from the
Rochester group, and other collaborating laboratories. In addition during this period we
evolved a very productive working relationship with the French group at CEA-Limeil.
20
New oscillator technologies (developed by Dirk Kuizenga and Jim Murray) were
installed that provided improved pulse shaping and at the same time, generated precision
timed target-backlighting pulses. Finally, the laser had advanced automatic beam
pointing/centering controls and extensive laser diagnostics for keeping track of multiple
beam colors, beam noise, and other needed information.
Figure 9: The 10 beam Nova laser (5 beams on each side structure). Nova was completed in 1985
and began 3rd harmonic experiments immediately. By 1989 it had been fitted-out with low-particle laserglass and improved polarization control to increase harmonic energy. It was then used for several
sequences of very successful 3! hohlraum coupling, ablation, symmetry, compression, and related
experiments. These experiments met the “contract requirements” set forth by the outside review
committees, upon which the process of design and review for a mega-joule laser began.
The first two beams of the Nova laser (called Novette) provided about 10KJ on
target at a target time-scale is about1.0 nsec. The 5X larger Nova was designed to provide
pulses mainly in the 1ns to 3 ns time scale (see Fig 9), but was also expected to provide
versatility in pulse shape from 0.1 ns to 5 ns. Hence the laser design process led to an
output area of about 1600 cm2 at the exit of each amplifier, which leads to 10 joules/cm2
energy-density at the amplifier output. After expansion to 74 cm diameter, the fluence
was about 5 J/cm2 at the target optical elements (at 1.06 micron light). These levels were
21
acceptable from a damage point of view, as long as there were no particles inside or on
the optical surfaces. The corresponding output system power-density was about 3 x 109
W/cm2, almost the same as with the earliest, very short pulse designs discussed earlier,
but with a much longer propagation paths in both the laser-glass and the focusing
elements (about 2.5 meters) and in the air path to the target (about 50 meters). Hence the
designers of the Nova laser (and its prototype Novette) had to control both energy density
to prevent surface and bulk optical damage and to control power density to prevent both
small-scale and large-scale self-focusing in the long glass path, and, in addition, they had
to deal with non-linear rotational-Raman conversion in the long air paths. The Novette
laser demonstrated the promised Nova laser beam-line performance in 1983, which
provided the needed confidence for the completion of the Nova laser in 1985.
III-B: Generation of Target Conditions Suitable for Fusion gain:
In 1981, I took over the management of the ICF and target laser program at
Livermore, an event that was both a great honor and a greater challenge. We had just
experienced a very detailed review of ICF program progress conducted by the Foster
committee. The result was that we had to redirect our efforts to first concentrate on the
validation of the basic tenants of indirect-drive ICF target performance. We took
advantage of the Nova laser project, then just starting, by borrowing two of its beams and
building them into a temporary laser configuration, called Novette. The purpose was to
test the laser ideas and to conduct confidence building, short-wavelength target
experiments. The Novette project was managed by G.Suski, K.Manes, and R. Speck and
borrowed members of the Nova team. It began operation in 1983, with the primary
wavelength at 1.05 micron and the first experiments being conducted at the 2nd harmonic
(also called 2!). Soon experiments at the 4th harmonic took place, and then, just before
shut down, we were able to use one of the 3! harmonic converters from Nova. Novette’s
job was to show that the physics of laser-light coupling to fusion targets, over reasonable
length scales, would become favorable when using shorter wavelength light. Both the
laser experiments and the target experiments worked well (Manes and Simmons 1985),
but with limited time before the system had to shut down so that the parts could be
returned to Nova.
One very interesting set of 100 psec experiments that we performed on Novette, used
cylindrical lenses to form a line focus. With it Matthews et al 1985 and Hagelstein 1983
were able to demonstrate the hereto shortest wavelength laser emitting, at 213
ångstroms. Since then, many stimulated soft x-ray transitions have been explored at many
places, for their scientific interest but also for potential use as a laser plasma diagnostic
probe.
While the laser target experiments were proceeding well, it was clear that it was
going to be a while before we could conduct ignition and gain experiments using lasers.
Hence we began the Halite-Centurion ( H/C) program, which made use of underground
nuclear experiments to better understand ICF conditions. Eric Storm, Hank Shay, and
DeLynn Clark led the Livermore team, with vital contributions from many others. These
experiments were successful and they lay to rest questions regarding the feasibility of
ICF ignition, and gave the community confidence that ICF would succeed at the
megajoule scale.
22
Shortly after the Novette experiments, Nova began its target irradiation program
(Campbell 1986). At this time we changed the management of the ICF program with
Erik Storm becoming ICF director, Mike Campbell in charge of ICF experiments, and
John Lindl in charge of ICF target design, with the purpose of validating hohlraum
conditions for the fusion objectives and defining the next laser system. Experiments at
3! demonstrated low-preheat, 225 eV hohlraums at a millimeter scale lengths, and with
sufficient diagnostic resolution to confirm calculations regarding expected ignition and
gain at the several Mega-Joule level. By the end of the decade of the 1980s, using 25 kJ
of 3rd harmonic light, the teams demonstrated a very symmetric 100x compression of DT
fuel, in a high temperature 300 eV hohlraum cavity (delivering a power density of almost
1015W/cm2 to the capsule), with little preheat. The diagnostics needed to accomplish this
feat are remarkable because they were designed to work on cantilevered structures inside
the 5m diameter Nova target chamber, and they had to provide 30 picosec time resolution
and 10 micron spatial resolution, as well as have the capability to sample small sections
of target material and to detect low neutron counts to validate the fusion reaction
qualities.
III-C: Design of Mega-joule Laser Systems:
With the success of the Nova experiments, it became clear that a 1.0-2.0 MJ size
laser would be needed for high gain demonstrations. I started dedicating my time to the
design of such a laser system. Later, in 1988, the DoE engaged the National Academy of
Science, which formed a review committee led by Prof. Steve Koonin, to review the
target and laser work conducted on Nova and with H/C in the 1988-90 time frames. They
commended the target work but stated that a lower cost, 1-2 Mega-joule laser designed to
demonstrate ignition and modest gain, 5-10 fold, should be the proper next step. If such a
laser were to be affordable, many improvements in performance/cost measures would be
need to be invented by the laser designers and materials scientists. Manes, Ozarski,
Trenholme, Storm, and I analyzed the costs of several new, unusual laser designs. The
general approach was to use very low cost laser materials. One example was to consider
laser glass manufactured almost like plate window glass, but of very high quality – with
very uniform index of refraction, low stress levels and low depolarization, low internal
particulate-count, and with no absorption at 1.05 microns. The laser beam lines
themselves were designed to be as “linear” as possible, so that they could be mounted
closely side by side, in a “log-pile-like” geometry. This would minimize costly building
space. Several other new laser ideas were explored including off-axis multi-passing in
large area laser cavities, large aperture EO switching, phase conjugation, harmonic beam
switching, adaptive phase correction, and many other ideas. A multipass regenerative
system was selected for optimization. It was made possible, in part, by Julius Goldhar’s
(1984) invention of a large-area electro-optic plasma-electrode technique, which after
several years of disbelief on our part, turned out to be astonishingly easy to fabricate and
very reliable. Many extremely clever ideas were invented, evolved, and reused from
prior lasers to make a Mega-joule laser system design plausible. There were studied
throughout the 2nd half of the 1980s and into the 1990s with the objective of making an
affordable, muli-megajoule laser driver.
23
IV. 1990s: Demonstrations of required fusion target performance and a new laser
architecture for ignition and fusion gain demonstrations.
In 1990 successful target experiments on the upgraded Nova laser enabled 100X
compression in a 300eV hohlraum, with only 1% target preheat. This result made it
possible to design a credible high gain ICF capsule that would achieve moderate gain
with about 2 MJ of 3! laser light. The increased Nova capability was made possible by
increasing the output energy per beam, and improving the control and diagnostics relative
to the original Nova laser, using improved, zero-impurity phosphate laser glass (J.H.
Campbell et al 2000), improving the beam profile smoothness, using time shaping and
spectral dispersing techniques, and increasing the irradiation symmetry of the target
capsules by very precisely directing the laser beams onto the inner walls of the
hohlraums. Similarly, target design and target fabrication continued so that a “hydrodynamically equivalent” hohlraum target could be irradiated by the much improved Nova
laser. This work was conducted under the management of Mike Campbell - ICF director,
John Lindl - target design and Joe Kilkenny – experiments director, and of course, it
involved enormous efforts by many dedicated individuals over the period from 1985
through the closure of Nova in 1995. In particular, Mike Campbell and John Lindl
worked tirelessly to develop needed support for the “next” programmatic steps needed to
demonstrate ICF.
Figure: 10 The left image shows an almost perfect implosion of DT fuel, exhibiting 1% symmetry, using
upgraded pointing and beam balance on Nova. The right image shows a purposeful 10% change in P2
irradiation symmetry and consequent, predicted asymmetry.
.
24
In particular, very well developed diagnostic techniques and associated
instruments were used on very carefully designed laser-hohlraum targets (about 2 mm
scale) to obtain needed time and spatial resolution to settle remaining uncertainties,
clearing the way for a decision to take the next step in ICF demonstrations. These
demonstrations showed significant reduction in hydrodynamic instability growth as
predicted using ablative stabilization, acceptable efficiencies of light-to-ablator energy,
control of laser-plasma hohlraum instabilities over 1 – 2 mm length interaction paths,
reaching acceptable implosion velocities, and obtaining needed convergence ratios of fuel
layers (Figure 10). These achievements, and others involving the measurement of a host
of relevant parameters satisfied the review committees.
At the same time that the “next steps” were being discussed, it was important to
demonstrate a basic “beam line” unit that could be replicated several hundred times to
build a multi-megajoule laser. A test laser called “Beamlet” was constructed and used to
demonstrate the many improvements in laser materials and in laser design that would
make an affordable, precision laser for demonstrating ICF gain at several mega-joules
(Van Wonterghern et al, 1997). The major purpose of this laser was to demonstrate the
unusual off-axis, regenerative drive laser-stage, and to demonstrate laser damage control.
This system had to operate with high output fluence, especially on the targeting optics,
since it has no expansion stage in the beam transport, as well as a long optical path. The
very sophisticated regenerative laser stage is first “filled” with a 10 joule beam
propagating backwards from the output booster amplifier, that is then injected into the
regenerative cavity, expanded to full aperture. The switching occurs at full beam-size,
using a 40 cm square-aperture Pockels-cell electro-optic switches. The pulse is then
trapped inside the cavity, and allowed to reflect back and forth 4 times within the
regenerative cavity, each time offsetting itself in the far field by several centimeters.
When it reaches the 4 kJ level, it is switched out of the cavity and then through an output
booster amplifier, reaching about 15kJ output per beam (at 1.05 microns) and extracting
about 50% of the very expensive stored amplifier energy (much more than the 10-20%
extraction of earlier laser designs). The beam is a 40cm x 40cm square shape, designed
for close packing with other beams (see Figures 12 and 13), enabling the use of more
efficient amplifiers, and it increases the beam fill-area by 33% compared to an equivalent
round beam. This aperture dimension is at an optimum given constraints due to needed
large signal gain, pulse shape fidelity, internal laser plate parasitics, and constraints of
economical production.
The advantage of the NIF laser design is that it is relatively compact in
longitudinal space, compared to an expanding beam power amplifier system like Nova,
and it’s one regenerative amplifier can amplify the beam during several passes (e.g., 4 on
NIF), saving the cost of many pre-amplifying stages and requiring no Faraday isolators.
However, great care has been taken to be certain that no prepulses (due to the laser pulse
cycling inside the cavity) escape and destroy the target in advance of the main target
pulse. The regenerative amplifier approach was also chosen to minimizes pulse
distortion, which is important because the fusion targets for gain demonstrations require a
complicated pulse that must be injected, with a pre-corrected shape over many decades of
intensity, to account for saturated gain induced pulse distortion. Finally, many laser
component and system studies commenced and continued into the next decade to
determine the optimal way of building components, assembling laser elements,
25
maintaining them under clean conditions, diagnosing problems and controlling the
system to meet the needs of the ICF target community. The optical performance of each
NIF laser chain is remarkable both for its 1.05 micron energy as high as 20 kJ, and its 3!
energy at 10.5 kJ, as well as for the low beam amplitude and phase noise level which
minimizes target-system optics damage and plasma instability growth within the target
itself. See Figure 13.
The beautifully conducted, precision “Precision Nova” experiments showed a
large measure of hydro-equivalency between demonstrated target performance and that
needed in targets designed for ignition and gain. This and the demonstrations of an
advanced, low cost laser design on the “Beamlet” laser test bed, led to formal acceptance
of the next phase of ICF. The National Academy and the DoE recommended proceeding
with a mega-joule class laser system whose purpose was to demonstrate ignition and
moderate gain from DT filled targets. The DoE approved the conceptual design of such a
laser called NIF in 1993.
26
V. 2000 to the present: The National Ignition Facility and new ideas
In 1993 a decision was made to begin the formal process by which a large
construction project is approved by the US Department of Energy. The NIF laser
(Figure 11) design was reviewed many times before construction began. See Figure 13,
for the present plan and the NIF website at http://www.llnl.gov/nif/project/index.html for
details. In 2000, a review showed that the budget was significantly short of what was
needed to complete the project and to meet the fusion milestones. The major issues were
the cost of installation, maintenance of a particle free environment in and around the
beam lines, and the complexities of bringing 192 beams onto a target from two sides. To
redirect the project, two very experienced technical managers, George Miller and Ed
Moses. (Miller et al 2004), took over the management. They regained the confidence of
the DoE and other “stake holders”, continued valuable innovation, and are now managing
the assembly of the 10,000s of parts. Most of the parts are purchased and are being
installed.. Individual beam lines on NIF have been demonstrated to meet and exceed the
energy and power objective at 3! output (see Fig 13 below).
Figure: 11 Layout of NIF Laser with the 2 laser bays extending up and to the left,
and the 192 beam target chamber in the middle right side of the image.
27
Figure: 12 Single NIF laser beam line, showing regenerative section on the left (with Main amplifier),
optical switch, and power output amplifier. It recently generated 19kJ of IR light / beam.
Figure: 13 NIF output beams at full performance, these are the highest quality, high power laser beams
ever produced. The left image shows a 2nd harmonic image, and the right shows a 3rd harmonic image,
the fundamental beam, at 1.05 microns, produced an energy level of about 13kJ.
28
At the same time that NIF is being readied for experiments to begin in the 201time frame, researchers are continuing to examine new ideas to reduce the drive energy
needed for reaching ignition and gain. Examples are the “fast igniter” (Tabak et al 1994).
These and other ideas appear to be reducing the thresholds for ignition and gain, reducing
the laser energy and hence reducing laser maintenance problems.
VI. Conclusion:
The scales at which the ICF demonstrations could be accomplished were limited
in turn by acquisition of knowledge, development of new materials and diagnostics,
design of experiments, and construction of each demonstration laser and target system. It
is interesting to note that, in retrospect, each decade saw the development of lasers,
instruments, and targets that were at about 100x more capable and more sophisticated
than those used during the preceding decade. Starting with capsules of about 0.08 mm in
diameter (on two-beam Janus) and scaling their energy to the now planned 4 mm
diameter NIF capsules, the 50-fold increase in diameter leads to a 50 cubed, =125,000,
increase in energy. Scaling the early 2-beam 30 J experiments by this increase of
125,000 gives an approximate 3.7 MJ energy requirement for the 2-input cone NIF
experiments, a value not too far from the “as designed” 5MJ NIF output at 1.05 micron,
and 1.8 MJ at the 3rd harmonic, 0.35 microns. Recalling that we started in 1971 with
approximately 1-Joule, short pulse fusion lasers, this million-fold increase in energy and
target performance has been a remarkable enterprise.
VII. Acknowledgements
There are many thousands of people who have contributed to the successes of the
national and international ICF program, among them are particular people to whom I am
especially grateful for working with me in those areas where I concentrated my energies.
The LLNL senior management in 1971, Carl Haussman (Associate Director of LLNL)
and Roger Batzel (Director of Livermore), both supported the ICF program continuously
and energetically. John Nuckolls created the ICF concept and many of his contributions
are discussed elsewhere, especially in the book (Velarde 2006) for which this article was
written. John Emmett arrived at LLNL in 1971 to become Director of the ICF program,
and with Bill Krupke – Deputy Director of the Laser Program, formed a unified ICF
program at LLNL and led it forward. The leaders mentioned above attracted many of us,
including this author, to Livermore to work on ICF. Emmett became Associate Director
at LLNL for Laser Programs in the late 1970s, and deserves enormous credit for
exceptional technical leadership, for effective management, and for strategic technical
planning—tirelessly leading the ICF program forward, through good times and very
difficult times.
I do thank my many colleagues, some of whom I have already mentioned in the
text and references, and others with whom I have had significant interactions such as Bill
Kruer, Ken Manes, Claire Max, Hank Shay, Larry Suter, and, a very special colleague
over the years, John Trenholme. Ken Manes and Erik Storm read this report, and John
Lindl and Bob Kaufman helped me recall past events. Other collaborators were K.Boyer,
29
J. Jansen and S. Singer at the Los Alamos Laboratory; R.McCrory, W.Seka, and J. Soures
at the University of Rochester; many colleagues at the Naval Research Laboratory where
many of us first began high pulsed power laser work, J.P. Vandevender and J. Yonas at
Sandia Laboratory; Prof. Robert Dautray and Dr. Michael Andre and their colleagues at
the CEA in France: Dr. Eduard Fabre at Ecole Polytechnique; Drs. R.Siegel, J.Meyer-terVehn, & S. Witkowski at Garching, with special thanks to Profs. T. Haensch and
H.Walther for their hospitality while I worked at Garching; Profs. N.Basov and
A.Prokhorov at the Lebedev and Kurchatov Institutes in Russia; Prof. C.Yamanaka at the
Osaka University group in Japan and H.Takuma at NEE; M. Key (also recently at LLNL)
and J. Weale at the Aldermaston Laboratories in England; and the very creative group at
KMS Fusion including K. Brueckner, B. Guscott, F. Mayer, R. Johnson, and others. All
of these colleagues, and their colleagues, have collaborated in bringing this remarkable
effort in fusion research to its present verge of major success.
The word has been supported by the US Department of Energy at the
Lawrence Livermore National Laboratory. This review is available from LLNL as
UCRL-BOOK-218519 .
30
References:
Bespalov,V.I. and Talanov V.I. “Filamentary Structure of Light Beams in Nonlinear
Liquids” JETP Lett. 3, 307 (1966)
Bliss,E.S., Hunt, J.T., Renard, P.A., Sommergren, G.E., Weaver, H.J., “Effects of
nonlinear propagation on laser focusing properties”, IEEE J. Quant. Electron QW-12, 402
(1976)
Campbell, E.M., Turner, R.E., Griffith, L.V., Kornblum, H., McCauley, E.W., Mead,
W.C., Lasinski, B.F., Phillion, D.W., and Pruett, B.L., “Argus scaling experiments” Laser
Program Annual Report, UCRL-50055-81/82, pp. 4-2 (1982)
Campbell, E.M., Hunt, J.T., Bliss, E.S., Speck D.R., and Drake R.P., “Nova experimental
facility” Rev. Sci. Instrum. 57, 2101 (1986)
Campbell, E.M., Kilkenny,J.D., Kania,D.R., Young,P.M., Lane,S.M., Glendinning,S.G.,
Phillion,D.W., Darrow,D.B., Kauffman,R.L., Lindl,J.D., Suter,L.J., Hatchett,S.P.,
Kruer,W.L., Munro,D.H., Haan,S.W., Wallace,R.J., Hatcher,C.W., Upadhye,R.S.,
Kyrazis, D.T., Hermes, G.L., Speck,D.R., Bibeau,C, Ehrlich,R.B, Matthews,D.L.,
MacGowan, B.J., Rosen,M.D., and Maxon,M.S., “Recent results from the Nova Program
at LLNL”, Laser and Particle Beams 9 (2), 209 (1991)
Campbell, J.H. , Suratwala, T.I. , Thorsness, C.B., Hayden, J.S.
Thorne, A.J., Cimino, J.M., Marker III, A.J., Takeuchi, K., Smolley, M.,
Ficini-Dorn, G.F. “Continuous melting of phosphate laser glass”, J. of Non-Crystalline
Solids, 263-264, 342 (2000) Elsevier
Charatis G, Downard J., Goforth R., Guscott B,. Henderson T., Hildum S., Johnson R.,
Moncur K., Leonard T., Mayer F., Segal S., Siebert L., Solomon D., and Tomas C.,
“Experimental study of laser driven compression of spherical glass shells” Proc. of the 5th
IAEA Plasma Fusion Conf. in Tokyo, Japan 1974, published in Plasma Physics and
Controlled Nuclear Fusion Research 1974, IAEA 2, 137 (1975) Vienna
Craxton, R.S. “High-Efficiency harmonic generation from 532 to 266 nm in ADP and
KD*P” Appl.Phys.Lett. 30, 91 (1977)
Emmett J.L., Nuckolls, J.H., and Wood L.L., “Fusion power by laser implosion”,
Scientific American, 230, 24-37 (June 1974)
Emmett, J.L., Krupke, W, and Davis, J.I., IEEE J.Quant. Electron. 20, 591 (1984)
Glaze, J. (1974) James Glaze was the Cyclops Laser project leader in 1973, working
with Erlan Bliss, John Hunt, Gil Leppelmeier, Bill Simmons, Bob Boyd, and others who
made major contributions to fusion laser design, especially propagation, and
implementation.
31
Goldhar, J. and Henesian, M. "Electro Optical Switches with Plasma Electrodes", Optics.
Lett. 8, 73, (1984)
Hagelstein, “Review of radiation pumped soft x-ray lasers” P.L. Plasma Phys. 25, 1345
(1983)
Holzrichter, J.F. and Speck, D.R., “ Laser Focusing Limitations from Non-Linear Beam
Instabilities," J. Appl. Phys. 47, 2459 (1976)
Holzrichter J.F., Ahlstrom, H.G., Speck, D.R., Storm, E., Swain, J.E., Colemen, L.W.,
Hendricks, C.D., Kornblum, H.N., Seward, F.D., Slivinsky, V.W., Pan, Y.L.,
Zimmerman, G.B., and Nuckolls, J.H. “Implosion Experiments with an Asymmetrically
Irradiated Laser Fusion Target” Plasma Physics 18, 675, 1976, Pergamon Press
Holzrichter J.F. Eimerl, D., George, E.V., Trenholme, J.B., Simmons, W.W., Hunt J.T.,
“High Power Pulses Lasers” , Journal of Fusion Energy, Vol. 2, 1 (1982)
Holzrichter, J.F., Campbell, E.M., Lindl, J.D., Storm, E., “Research with high-power
short-wavelength lasers”, Science 229, 1045 (1985)
Holzrichter, J.F. “High power solid-state lasers” Nature 316, 309 (1985)
Hunt, J.T., Glaze, J.A., Simmons, N.W., and Renard, P.A.,”Supression of self-focusing
through low-pass spatial filtering and relay imaging” Appl.Opt. 17, 2053 (1976)
Hunt J.T. and Speck D.R. “Present and future performance of the Nova laser system”
Opt. Eng. 28, 461(1989)
Krupke, W.F. “Induced-emission cross sections in neodymium laser glasses” IEEE
J.Quant.Electron.QE-10, 240(1974)
W.Krupke, V.George, J.Murray, D. Prosnitz, and their colleagues (1978) were key
players at Livermore working on advanced lasers. Their work is described in detail in the
LLNL Laser Program annual reports. The KrF laser system, as well as several other
excimer laser systems, were examined carefully for their applicability to ICF
experimentation, and also ultimately to fusion power applications. It was felt that the
excimer system technologies, potentially useful one day for ICF energy production, were
being supported strongly by the US DoD in the 1970s and into the 1980s. The properties
of these lasers were quite different that those needed for early ICF experimental
programs, hence they were not pursued. Excellent work on these systems is to be found
in papers by G.Canavan, J.Daugherty, J.J.Ewing, R.Hunter, J. Mangano, and many
others.
Lindl J., “Development of the Indirect-Drive approach to Inertial Confinement Fusion
and the Target Physics Basis for Ignition and Gain”, UCRL-JC-119015, L-19821-1, 1995
32
Lawrence Livermore National Laboratory Laser Program Reports: UCRL-50021-76-year
(from 1973-2005)
Maiman, T.H., Hoskins, R.H., D‘Haenens, I.J., Asawa, C.K., Evtuhov, V. “Optical and
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Manes, K.R. and Simmons, W.W. “Statistical optics applied to high power laser beams”
J. Opt. Soc. Am A2, 528 (1985)
Matthews, D. L., Hagelstein, P.L., Rosen, M.D., Eckart, M.J., Ceglio, N.M., Hazi, A.U.,
Medecki, H., MacGowan, B.J., Trebes, J.E., Whitten, B.L., Campbell, E.M., Hatcher,
C.W., Hawryluk, A.M., Kauffman, R.L., Pleasance, L.D., Rambach, G., Scofield, J.H.,
Stone, G, and Weaver, T.A., ” Demonstration of a Soft X-Ray Amplifier” Phys. Rev.
Lett. 54, 110 (1985)
McMahon J.M., Emmett J.L., Holzrichter J.F. and Trenholme J.B., "A Glass Disk Laser
Amplifier" IEEE J. Quant. Electron, OE-9, 992, October 1973
Miller G.H., Moses E.E., Wuest C.R. “The National Ignition Facility: enabling fusion
ignition for the 21st century” Nucl. Fusion 44, 228 (2004)
NIF (2006) laser facility, see the following home page for laser and experiment
information - http://www.llnl.gov/nif/project/index.html
Nuckolls J.H. et al (1959) LLNL internal memorandum
Nuckolls J.H. Wood L.L., Thiessen A.R., and Zimmerman G.B. “Laser compression of
matter to super-high densities: thermonuclear (CTR) applications” Nature 239, 139
(1972)
Snitzer, E. “Glass Lasers” Appl.Opt. 5, 487 (1966)
Tabak, M., Hammer, J., Kruer, B., Campbell,E.M., Wilks,S., Mason, R., Perry, M.K.
Glinsky, M., Woodworth, J., Physics of plasmas 1(5), 1626 (1994)
Trenhome, J.B.; See LLNL laser program annual report UCRL-50021-76 (1973 to 2006)
David Eimerl, Erlan Bliss, Alex Glass, and John Trenholme performed early critical
experiments and analyses, followed by John Hunt, Bill Simmons and others to determine
the limits of small scale self-focusing due to multiple-spatial-filtered, “Brewster-angled”
glass-disk amplifiers, in long, relay-imaged laser chains. John Trenholme also played a
major role working with John Emmett, myself, and others to optimize the design of each
the major laser systems - obtaining as much performance for the available funding and
physics limitations as possible.
Van Wonterghern, B.M., Murray, J.R., Campbell, J.H., Speck, D.R., Barker, C.E., Smith,
I.C., Browning, D.F., Behrendt, W.C. “Performance of a prototype for a large-aperture
33
multipass Nd:glass laser for inertial confinement fusion” Applied Optics 30 (20/21) 4932
(1997)
Velarde, G. ( 2006) “Inertial Confinement Nuclear Fusion: A Historical Approach by its
Pioneers” in press (Madrid, Spain)
34
Biography: The book, for which this article was written, called for a biography of the author. I
include it here because it gives me the opportunity to thank Arthur Schawlow for accepting me
into his group in the Stanford Physics Department and for his guidance while a graduate student
from 1965-1971. At that time I became acquainted with nascent laser science and technology,
and I met exceptionally talented people, among them Bob Byer, John Emmett, Ted Haensch, and
others. The picture above shows Prof. Arthur Schawlow and the author in 1997, shortly before
Art died in 1999. Arthur Schawlow and Charles Townes first published “Infrared and Optical
Masers” in the Physical Review, 1958. In that article they described the extension of the
microwave maser to the optical regime, forming the basis for the Laser. For my thesis I built a
tunable dye laser, generating 5 mJ of green light in 100 ns., to study induced magnetism in the
anti-ferromagnetic, MnF2 . He was a wonderfully generous and supportive advisor. Later in
1971 I joined John Emmett at the Naval Research Laboratory and in 1972 I came to the
Livermore Laboratory, to help start the LLNL ICF program, working with John Emmett, Carl
Haussman, Bill Krupke, and John Nuckolls . In 1987, I began managing the Livermore
Laboratory’s internal research program, working with John Nuckolls, who had became the
Livermore director. In this role, I continued to provide on-going support for new laser ideas,
plasma physics, algorithm development, and I paid attention to laser-fusion ideas such as the
“fast-igniter” (see Tabak et al 1994), peta-watt lasers (see work by Perry et al), and large optical
gratings (Britten et al). I retired from LLNL in 2000 and now spend most of my time as president
of the Hertz Foundation – an educational foundation supporting applied scientists and engineers
during their graduate school careers.
35

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