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C National Ignition Facility Research on inertial confinement fusion (ICF) and high energy density (HED) physics has been pursued intensively in the United States for many years. The Na- tional Ignition Facility (NIF) is being built to move that research program forward to a demonstration that ICF can be achieved in the laboratory. An additional goal is to enhance substantially the range of HED states of matter that can be studied in the laboratory. The NIF, under construction at Lawrence Livermore National Laboratory (LLNL) in California, will deliver up to 1.8 MJ of ultraviolet light (354 nm wavelength) in 192 convergent laser beams (Figure C.1). The NIF is being constructed as part of the Stockpile Stewardship Program by the National Nuclear Security Administration (NNSA) to ensure the safety, security, and reliability of the nation’s nuclear stockpile without underground nuclear testing. The NIF’s role in the stewardship program is to provide relevant data for the weapons program and to test our scientific understanding of the physics of nuclear weapon explosions through successful fusion ignition experiments in the laboratory. The completion of the NIF and the beginning of experiments that will lead to full-scale ignition tests are scheduled for 2009. These ignition experiments, which will utilize the most highly developed approach of indirectly driven hot spot ignition, will be the culmination of more than two decades of experimental campaigns that were per- formed at the Nova laser at LLNL (the predecessor of the NIF), the OMEGA laser at the University of Rochester, the Z-machine at Sandia National Laboratories, the Nike laser at the Naval Research Laboratory, and other lasers elsewhere. Successful ignition experiments at the NIF will be a key stepping-stone to inertial fusion as an energy source. 226
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FIGURE C.1 Rendering of the ~2 MJ National Ignition Facility (NIF) that is currently under construction at LLNL showing the location of various components and support facilities. When completed, the NIF will be the nation’s highest-power MJ-class HED physics facility; it is being built primarily for weapons-relevant HED physics research, including ICF. Up to 15 percent of the laser time is planned to be available for basic science experiments. Courtesy of LLNL. 227
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228 Plasma Science The flagship mission of the NIF is to demonstrate fusion ignition—the com- bining or “fusing” of two light nuclei to form a new nucleus. The NIF’s powerful array of lasers is intended to ignite enough fusion reactions in a carefully designed capsule containing the heavy hydrogen isotopes that constitute the fusion fuel to produce more fusion energy than the laser energy delivered to the target. The physi- cal processes involved in ICF and the physics challenges that must be overcome to achieve ignition are detailed in Chapter 2. The NIF is crucial to the NNSA Stockpile Stewardship Program because it will be able to create the extreme conditions of temperature and pressure that exist on Earth only in exploding nuclear weapons and that are therefore relevant to understanding the operation of our modern nuclear weapons. Understanding the physics of the ignition process and the dy namics of matter under HED conditions, together with the HED materials data that will be provided by the NIF, will allow supercomputer modeling tools to be used by our nuclear stewards to assess and certify the aging stockpile without carrying out actual nuclear tests. For example, NIF experiments will investigate the physics regimes associated with radiation transport, secondary implosion, and ignition and will enable testing the consequences for weapons operation of the effects of aging of some weapon components. Please see Chapter 3 for additional details on the scientific needs of stockpile stewardship. Other benefits to stockpile stewardship of the NIF are to help maintain the skills of present nuclear weapons scientists, who must assess the aging-related conditions that could compromise the reliability of nuclear weapons, as well as to attract bright young scientists to the program by offering them the excitement of working with a world-class laser facility. Finally, the committee notes that the NIF is to be used for basic science experiments 10-15 percent of the time after 2010. Although not directly relevant to stockpile stewardship, such use will encourage cross-fertilization of ideas and transfer of best-practices between HED scientists at universities and national laboratory scientists and help enhance the database on HED materials properties, extending it beyond the properties of direct relevance to weapon scientists. NATIONAL IGNITION FACILITY TECHNOLOGY The laser design at the National Ignition Facility (NIF) represented a break from the master-oscillator power-amplifier architecture that had been used in previous high power lasers used for ICF research, such as the Shiva or Nova lasers. This new multipass architecture (see Figure C.2 for a representation of 1 beamline out of 192) was chosen to increase wall-plug efficiency (from 0.2 percent) and de- crease cost by building only one type of amplifier component in a more compact footprint. In this design, light is injected from the preamplifier, passes through the power amplifier, then makes four passes through the main amplifier and, finally,
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A pp e n d i x C 229 FIGURE C.2 The multipass architecture that is common to all of the 192 beamlines of NIF. There are four passes through the main amplifier and two passes through the power amplifier. Courtesy of LLNL. another pass through the power amplifier and out to the final optics assembly. This strategy required development of several technologies: full-aperture (40-cm) optical switches, a full-aperture deformable mirror for wave front correction, full- aperture potassium dihydrogen phosphate (KDP) frequency conversion crystals, and full-aperture mirrors and polarizers. The optical switch is a Pockels cell that is energized by electrodes in the optical path. For this reason plasma electrodes are used. Providing enough KDP crystals for switches and frequency conversion (from 1.056 µm wavelength light to one-half or one-third of that) required development of rapid growth techniques; a factor of 6 was achieved. The wall-plug efficiency to produce the 0.33 µm light to be used for ICF experiments starting in 2009 is about 0.5 percent, much less than is needed for fusion energy but suitable for a research laser. (For the fusion-energy application, diode-pumped lasers are being developed so that broadband 10 percent efficient flashlamps pumping neodymium-doped glass can be replaced by 60-70 percent efficient narrow-band light-emitting diodes pumping crystals or ceramics. Efficiencies for these laser systems are projected to be 15-20 percent.) The NIF, which can produce 4.5 MJ (6 MJ if all possible amplifier glass slabs are installed) of 1.056 µm (infrared) light (3 MJ at half that wavelength and 1.8 MJ at one-third that wavelength) has an area of three football fields. The laser energy can be focused to a 100-µm spot. It was not possible to make the entire NIF laser bay into a clean room by optical standards. Therefore, individual components are packaged as line-replaceable-units that are assembled in a clean area and can be quickly installed in hermetic beam lines. This will also reduce downtime. The number of high-yield shots will be limited by the time for induced radio- activity of the chamber to decay (about a week) and the maximum yearly yield of 1,200 MJ specified in the Environmental Impact Statement.