Page 391 - High Power Laser Handbook

360 So l i d - S t at e La s e r s The National Ignition Facility Laser 361

energy into high-energy electrons. These instabilities both decreased
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x-ray generation and preheated the ablator and the fuel. It had been
previously demonstrated that shorter-wavelength lasers would
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couple more effectively to targets. The Shiva results, along with the
8
improved simulation and analysis that accompanied them, firmly
established that achieving DT ignition requires both more energy and
shorter wavelength. For neodymium-doped glass lasers, this means
that harmonic conversion is essential.
By June 1979, when the Shiva compression experiments were
completed, design work for Shiva’s successor was already well
advanced. Nova was envisioned as a 20-beam, 200-kJ, 100 ps to 10 ns
infrared (IR) laser that would achieve the long-sought goal of fusion
burn at laboratory scale. The Shiva results showed that coupling at
1-μm wavelength could not be coaxed to be good enough to effi-
ciently drive the capsule. In addition, reported results from Campbell
et al. , École Polytechnique, the University of Rochester, and KMS
Fusion, Inc., all showed that conversion to 351-nm wavelength could
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be carried out with efficiency in excess of 50 percent. Based on this
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information, a review chaired by John Foster confirmed that Nova
should not be expected to reach ignition and recommended that it be
reconfigured as a 10-beam, 100-kJ IR device with frequency conver-
sion to the third harmonic.
Even before Nova construction began, it broke new ground in a
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number of ways. In 1976, Bliss et al. reported measurements of the
rate of nonlinear growth of beam-intensity fluctuations (filamentation)
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versus spatial frequency. In the same year, Trenholme and Goodwin
developed and made available computational tools that quantita-
tively explained these measurements, demonstrated the efficacy of
spatial filtering at controlling filamentation, and enabled examina-
tion of alternative Nova architectures to assess their relative fila-
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mentation risk. Also in 1976, Hunt et al. invented the use of relay
imaging to allow high spatial fill factor. Both of these techniques
were built into the Nova design. The Nova laser was the first whose
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design was guided by numerical modeling and optimization and
the first whose construction was preceded by the building of a pro-
totype (the two-beam Novette, commissioned in 1983). When Nova
was commissioned in 1985, it could deliver as much as 100 kJ of IR
light or 40 to 50 kJ at 351 nm, with flexible pulse-shaping capability
ranging from ~100-ps impulses to ~10-ns multistep ramps for
target implosions.
For more than a decade, Nova was the premier fusion laser in the
world. Among the accomplishments achieved on Nova were:

• First quantitative measurements of beam-breakup threshold
(due to small-angle forward-rotational Raman scattering) in
long air-path beam propagation
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