Page 390 - High Power Laser Handbook

358 So l i d - S t at e La s e r s The National Ignition Facility Laser 359

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controlled laboratory environment. In 1962, a small laser-fusion
project, under the leadership of Kidder, was established in the Liver-
more lab’s physics department to explore this possibility.
Over the following decade, this group produced a number of
advances, including the development of the Lasnex laser-fusion simu-
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lation code and the seminal first open-literature publication of the
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physics behind inertial fusion. In this publication, the authors esti-
mated that thermonuclear burn in a compressed hot spot might be
observed with laser irradiation of about 10 kilojoules (kJ), while
significant fuel burnup and high gain would require ~1 MJ in a 10-ns
temporally shaped pulse.
In 1973, the first Livermore inertial confinement fusion (ICF)
laser—the single-arm Cyclops laser—was commissioned. Cyclops
generated several hundred joules in a few hundred-picosecond pulse
and was used for laser research and development (R&D), especially
for developing techniques for controlling optical self-focusing.
Cyclops pioneered the use of specially engineered low-nonlinear-
index glass, of Brewster-angle amplifier slabs, and of spatial filtering.
The first experiments to generate x rays by irradiating the interior of
a hohlraum were carried out on Cyclops by Lindl, Manes, and Brooks
in 1976. 6
The two-beam Janus laser, built in 1974, was a 40-J, 100-ps laser
that used many of Cyclops’s component designs. This laser, which
was used for target irradiation experiments, was the first Livermore
facility to demonstrate target compression and the production of
thermonuclear neutrons. In 1976, the Argus laser built on both of
these successes to push the performance envelope. Argus’s two beams
had 20-cm output apertures and a series of five groups of amplifiers
and spatial filters. Because spatial filtering was built into the design,
the telescopes were longer, thus improving the beam smoothing
achieved. Argus could deliver as much as ~2 kJ in a 1-ns pulse into a
100-μm spot, generating as many as 10 neutrons per shot on direct-
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drive exploding-pusher targets. It also pioneered the use of nonlinear
crystals to convert light to the second or third harmonic, and signifi-
cant improvement in coupling the light into the target was noted.
The next step along the path to ICF was taken in 1977, when the
20-beam Shiva laser was commissioned. Compared with previous
ICF lasers, Shiva was a giant—about 100 m × 50 m. Shiva was able to
deliver as much as 20 TW in short (100-ps) pulses and up to 10 kJ at
nanosecond pulse lengths, approximately fivefold increases in both
energy and power over Argus. It is arguable that Shiva’s greatest suc-
cess was its failure to accomplish all that had been hoped for. Experi-
ments with Shiva were able to achieve capsule compressions of about
100 times, which is in the right ballpark for ignition targets; however,
both hohlraum temperature and capsule compression fell below
expectations. These effects were traced to laser-plasma instabilities
(2ω and forward stimulated Raman scattering), which coupled laser
pe