■ December 1994
&& ReviewReview
The National Ignition Facility
University of California Lawrence Livermore National Laboratory ■ December 1994 About the Cover This month’s E&TR is dedicated to discussions of various aspects of the National Ignition Facility (NIF). The cover features images of the heart of the & Review latest and largest inertial fusion laser being designed by LLNL researchers for use in the international laser science community. In the background on the front cover is an engineering drawing of the 192-beam target chamber where ignition of NIF targets takes place. The inset is an artist’s rendering of the NIF in operation. It shows an indirect-drive target contained within a metal cylinder called a hohlraum. The blue laser beams are depositing their energy on the inside of the hohlraum. There the energy is converted to x rays that heat the target intensely, causing it to implode and ignite for a fraction of a second with the energy intensity of the The National interior of a star. On the back cover is a photograph Ignition Facility of an indirect target that contains a tiny amount of University of California hydrogen-isotope fuel. The hohlraum is about Lawrence Livermore National Laboratory 6 millimeters in diameter; the target inside is about 3 millimeters in diameter. The design, manufacture, and testing of these targets by Laboratory scientists is integral to the success of experiments performed on the NIF.
About the Journal The Lawrence Livermore National Laboratory, operated by the University of California for the United States Department of Energy, was established in 1952 to do research on nuclear weapons and magnetic fusion energy. Since then, in response to new national needs, we have added other major programs, T O T Y O F E N F E S I C including laser science (fusion, isotope separation, materials processing), biology and biotechnology, T M N R A R E E L R V I environmental research and remediation, arms control and nonproliferation, advanced defense technology, A I F P G N O E Y U R and applied energy technology, and industrial partnerships. These programs, in turn, require research in
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C I T L E T I L T T I G H T R H E E engineering, and physics. The Laboratory also carries out a variety of projects for other federal agencies. E R D E B E S M Energy and Technology Review is published monthly to report on unclassified work in all our programs. T A F A • • T E S O 1 8 6 8 Please address any correspondence concerning Energy and Technology Review (including name and address changes) to Mail Stop L-3, Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA 94551, Prepared for DOE under contract or telephone (510) 422-4859, or send electronic mail to [email protected]. No. W-7405-Eng-48 ■ December 1994
SCIENTIFIC EDITOR William A. Bookless PUBLICATION EDITOR & Dean Wheatcraft Review WRITERS June Canada, Kevin Gleason, Arnold B. Heller, Robert D. Kirvel, Harriet Kroopnick, Tom Spafford
ISSUE COORDINATORS The National Ignition Facility: An Overview 1 Paul Harding, Ray Marazzi
DESIGNERS A Tour of the Proposed National Ignition Facility 7 Paul Harding, George Kitrinos, Through a series of artists’ sketches of the proposed NIF, we follow the path Ray Marazzi of a photon from the generation of a laser pulse to the target. This article also describes the results of our recent work on a single prototype beam, which GRAPHIC ARTIST demonstrates the feasibility of the NIF concept. Treva Carey
CONTRIBUTING ARTISTS NIF and National Security 23 Scott Dougherty, Sandy Lynn In the absence of underground nuclear testing, the National Ignition Facility Dan Moore, Leston Peck will be a main source of the increased understanding of the physics of nuclear weapons required for the maintenance of the U.S. weapons stockpile. COMPOSITOR Louisa Cardoza The Role of NIF in Developing Inertial Fusion Energy 33 PROOFREADER By demonstrating fusion ignition, NIF becomes an important means of proving Catherine M. Williams the feasibility of inertial fusion energy as an economical, environmentally safe alternative to fossil fuels and other energy sources. It will also help us optimize This document was prepared as an account and fabricate fusion targets for power plants of the future and will provide the of work sponsored by an agency of the United basis for future decisions about energy programs and facilities. States Government. Neither the United States Government nor the University of California nor any of their employees makes any warranty, Science on the NIF 43 expressed or implied, or assumes any legal Last March a group of scientists convened at the University of California, liability or responsibility for the accuracy, Berkeley, to identify areas of research in which the National Ignition Facility completeness, or usefulness of any information, apparatus, product, or process disclosed, or could be used to advance knowledge in the physical sciences and to define a represents that its use would not infringe privately tentative program of high-energy laser experiments. These scientists determined owned rights. Reference herein to any specific that this facility would have the most effective applications in astrophysics, commercial product, process, or service by trade hydrodynamics, high-pressure physics, plasma physics, and basic and applied name, trademark, manufacturer, or otherwise physics. does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of NIF Environmental, Safety, and Health Considerations 55 California. The views and opinions of authors Our analysis to date of ES&H issues related to the National Ignition Facility expressed herein do not necessarily state or reflect shows that the proposed system and its operations will result in no significant those of the United States Government or the University of California and shall not be used for environmental impact and safety risk to the Laboratory’s work force and the advertising or product endorsement purposes. general public.
Printed in the United States of America Abstracts 58 Available from National Technical Information Service U.S. Department of Commerce 1994 Index 60 5285 Port Royal Road Springfield, Virginia 22161
UCRL-52000-94-12 Distribution Category UC-700 December 1994
OVERVIEW
The National Ignition Facility: An Overview
HEN Secretary of Energy Hazel O’Leary visited areas. Additionally, the project will benefit dozens of Bay W LLNL last October 21, she brought a message Area and California construction and manufacturing long awaited by researchers here and throughout the companies, creating many new jobs. international scientific community. The Secretary NIF is comparable in size to a municipal sports stadium. announced to an enthusiastic crowd of employees, (See the illustration on p. 9.) The heart of the facility is a community leaders, and industry representatives that she neodymium laser system of 192 beams, with each beam had approved “Key Decision 1” to build the National optically independent for outstanding experimental design Ignition Facility (NIF) and that LLNL’s expertise in laser flexibility. Together, the laser beams will produce fusion made it the preferred site for the approximately 1.8 million joules (approximately 500 trillion watts of $1-billion facility. power for four billionths of a second) of energy. In An international research center comprising the comparison, LLNL’s Nova laser, currently the world’s world’s most powerful laser, NIF will achieve ignition largest, produces 45,000 joules (approximately 15 trillion of fusion fuel and energy gain for the first time in a watts for three billionths of a second). laboratory. When it begins operation in 2002 (see the box The beams will compress and heat to 100 million on p. 2), NIF will serve researchers from many different degrees 1- to 3-millimeter-diameter capsules containing institutions and disciplines for both classified and deuterium-tritium fuel, thereby producing ignition (self- unclassified projects. heating of the fusion fuel) followed by a propagating As a DOE–Defense Programs facility, NIF will be a thermonuclear burn. The implosion process will produce key component in the Department’s science-based fusion burns with significant energy gain, up to ten times Stockpile Stewardship Program to ensure the safety and the energy required to initiate the reaction. (See the box reliability of the nation’s enduring stockpile of nuclear on p. 5.) weapons. By yielding considerably more fusion energy This sequence of events will produce the equivalent of than is put in by the laser (energy gain), it also will bring a miniature star lasting for less than a billionth of a us a large step closer to an inertial fusion energy (IFE) second, yet long enough for researchers to make accurate power plant. NIF will also advance the knowledge of measurements of its temperature, pressure, and other basic and applied research in high-energy-density science. properties. Indeed, we will “look” at fusion Finally, the project to construct NIF and equip it with the microexplosions with a spatial resolution of 10 micrometers most modern components will spawn technological (about one-tenth the size of a human hair) and freeze the innovation in several U.S. industries and enhance their action at a time resolution of 30 picoseconds (trillionths international competitiveness. of a second). If NIF is sited at LLNL, it will be the largest construction project and permanent facility in our history. Culminates 30 Years of Research Scientists worldwide will be performing research here, NIF will represent the scientific culmination of more invigorating LLNL in many existing and new technical than 30 years of inertial confinement fusion (ICF)
1 Overview E&TR December 1994
research at LLNL and throughout the world. Calculations could be used to simulate the detonation of nuclear by Livermore physicists in the 1960s showed that a laser weapons and that such a fusion technology might one day generating a megajoule of light in ten billionths of a generate electrical power. second could ignite a fusion microexplosion in the Over the years, Livermore scientists built and operated laboratory. They reasoned that such microexplosions a series of laser systems, each five to ten times more powerful than its predecessor. (See Figure 1.) Long Path, Livermore’s 1000 first neodymium glass laser, was completed in 1970 and was our workhorse laser for five years. Our NIF two-beam Janus laser, completed in 100 1974, demonstrated laser compression and thermonuclear burn of fusion fuel for the first time. In 1975, our one-beam Cyclops laser Nova 10 became operational and was used to perform target experiments and to Power, TW Long test optical designs for the future Path Shiva Shiva laser. A year later, the two- 1 beam Argus laser increased our understanding of laser–target Argus interactions. Cyclops During this time, laser Janus 0 development and ICF experiments 0 0.01 0.1 1 proceeded rapidly at other facilities, Energy, MJ including KMS Fusion, the Laboratory for Laser Energetics Figure 1. The energy and power of neodymium glass lasers built for inertial confinement (LLE) at the University of fusion (ICF) research at LLNL have increased dramatically over the past two decades. Rochester, and the Naval Research
Planning for NIF
The Department of Energy’s analysis, cost and schedule validation, construction and major procurements. procedure for approving large and a two-year Environmental Impact KD 4, in late FY 2002, will authorize projects such as NIF is based on Statement, which will include public facility operation and the first “Key Decisions” (KDs) made by the involvement. NIF has been identified experiments. Secretary of Energy. In January 1993, as a low-hazard, non-nuclear facility Detailed planning for NIF has the Secretary approved KD 0, which based on the Preliminary Hazards been led by five institutions that have affirmed the need for NIF and Analysis Report. long collaborated on laser fusion authorized a collaborative effort by In addition, the DOE has agreed experiments: LLNL, Los Alamos the three DOE defense laboratories to steps before KD 2 that will National Laboratory, Sandia National and the University of Rochester’s examine NIF’s likely impact on Laboratory, the University of Laboratory for Laser Energetics to nonproliferation and stockpile Rochester, and General Atomics. produce a conceptual design report. stewardship issues. KD 2, scheduled The cooperative spirit of the five This report was completed in for late fiscal year (FY) 1996, institutions and their interactions with April 1994. includes detailed engineering industry and the public were cited by KD 1 was signed by the Secretary design, further cost and schedule Vic Reis, Assistant Secretary for in October 1994. This decision validation, and final safety analysis. Defense Programs, during a visit to initiated preliminary design, safety KD 3, in late FY 1997, will authorize LLNL last November.
2 E&TR December 1994 Overview
Laboratory. Major programs in the Soviet Union, Japan, place among decades-long international efforts to reach China, Germany, France, and the United Kingdom were fusion conditions in the laboratory. Unlike magnetic established or expanded. fusion designs, ICF strives to compress fusion fuel In 1977, the 20-beam Shiva laser was completed. The isentropically before raising its ion temperature to largest American ICF project at that time, it delivered ignition levels. more than 10 kilojoules of energy in less than a billionth Such an ignition facility was strongly recommended by of a second. Meanwhile LLE’s 24-beam Omega system the National Academy of Sciences and the DOE Fusion became operational in 1980. Novette, which came on line Policy Advisory Committee in their 1990 reports and by in 1983, was the first laser designed to generate green and the DOE Inertial Confinement Fusion Advisory ultraviolet light. It confirmed work done at several ICF Committee and the JASON Review Committee in 1994. centers, showing that plasma instabilities were suppressed This facility, eventually called the National Ignition by shorter wavelength light. Facility, would use improved laser design and As a result of this work, Nova was redefined as a 10- engineering as well as advanced optics, laser amplifiers, beam system with frequency conversion rather than the and frequency converters. 20-beam infrared system originally approved. Nova Two conceptual designs for NIF were prepared, one became operational in late 1985, the same time that the using 240 beams and the other employing 192. As French Phebus laser, consisting of two Nova-style Figure 3 indicates, the smaller and less expensive of these beamlines, was completed. designs adequately meets target requirements, with a Using the Nova and Omega lasers, as well as safety margin of about two for achieving ignition. underground nuclear experiments in the Halite-Centurion Program, scientists have made important progress in NIF Benefits understanding ICF. At the same time, the study of scaling By demonstrating thermonuclear ignition and burn in glass lasers to systems much larger than Nova provided the laboratory for the first time, NIF will play a critical the technical guidelines for a future system to create target role in the DOE’s science-based Stockpile Stewardship ignition and energy gain. (See Figure 2.) ICF takes its Program. With the end of the Cold War, America’s nuclear
Figure 2. NIF will be the culmination of over two decades of research by the 100 1000 10 international ICF community into the use of –2 10–1 –3 10 glass laser systems to create controlled –4 10 –5 10 target ignition and energy gain in the 10 NIF DT gain laboratory. Unlike magnetic fusion energy 1015 Nova (MFE) designs (e.g., the Princeton Large Taurus, Doublet II, the Tokamak Fusion Test sec
–3 Reactor, and the Joint European Taurus, which trap fuel in an intense magnetic force , cm
τ Shiva field to induce fusion), ICF strives to compress fusion fuel isentropically before Typical MFE projects raising its ion temperature to ignition levels. 1013 Quality of confinement n
1011
ICF
0.1 1.0 10 100 (keV) (1) (10) (100) (1000) (million °C) Ion temperature
3 Overview E&TR December 1994
weapons stockpile is being 800 significantly reduced. However, nuclear weapons will continue to exist for the foreseeable future. In the absence of underground testing, the Limit of laser operation-192 600 reliability, safety, and effectiveness of the remaining stockpile can be assured only through advanced Baseline operation computational capabilities and aboveground experimental facilities. 400 NIF is the only facility proposed for the program that addresses fusion and several other physical processes that Peak UV power, TW involve high-energy density. 200 Data from NIF will complement data from hydrodynamic tests and will also be used to improve the physics in computer codes that are 0 needed to certify the safety and 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 reliability of our remaining Total UV energy, MJ stockpile. These more accurate codes will better simulate potential problems in the enduring stockpile Figure 3. Two conceptual designs were prepared for NIF, one using 240 beams and the other as well as improve our interpretation employing 192. The smaller of these was chosen because it is more affordable than the 240- beam option. In addition, as the figure indicates, the 192-beam design will not only achieve the of data from the archives of past baseline operation requirement of 1.8 megajoules and 500 terawatts of power (the optimum underground tests. point for target ignition indicated by the yellow star), but it can also operate at higher energy NIF will also help to maintain the and power with increasing risk of damage to the system—up to a maximum acceptable risk or skills of the nation’s small cadre of “redline” performance (2.2 megajoules and 600 terawatts). The shaded area indicates the nuclear weapons scientists and to increasing target margin above the minimum power and energy required to achieve ignition. attract new scientists to help manage
Inward transported thermal energy Laser energy Blowoff
Laser beams rapidly heat Fuel is compressed by the During the final part of the laser Thermonuclear burn the surface of the fusion rocket-like blowoff of the pulse, the fuel core reaches spreads rapidly through target forming a surrounding hot surface material. 20 times the density of lead and the compressed fuel, plasma envelope. ignites at 100,000,000˚C. yielding many times the input energy.
Figure 4. The steps of an inertial confinement fusion reaction, which produces up to ten times the energy used to initiate ignition. Under laboratory conditions, the sequence produces energy gain equivalent to the power of a miniature star lasting for less than a billionth of a second.
4 E&TR December 1994 Overview
the Stockpile Stewardship Program, support U.S. nuclear have been the subject of more than 1000 scientific papers nonproliferation goals, aid in the safe dismantlement of published by ICF researchers since 1985. nuclear weapons, and respond to nuclear weapon crises. As the world’s largest optical instrument, NIF will Another major goal of NIF is to help establish the spur key U.S. high-technology industries, such as scientific basis for environmentally friendly electrical optics, lasers, materials, high-speed instrumentation, power generated by IFE. The National Energy Policy Act semiconductors, and precision manufacturing. U.S. of 1992 calls for DOE to support both IFE and magnetic industry has long been a major participant in the rapid fusion energy approaches to achieving fusion energy as a progress of ICF research. Today DOE ICF scientists are practical power source. involved in 24 cooperative research and development As envisioned, IFE power plants will use high- agreements (CRADAs) totaling over $160 million in the repetition-rate laser or ion drivers (about 10 pulses per fields of microelectronics, microphotonics, advanced second). The heat from the continual fusion reactions will manufacturing, biotechnology, precision optics, be absorbed by coolants surrounding the fuel pellets and environmental sensors, and information storage. converted to electricity. NIF will provide crucial data on ICF scientists have also won 26 R&D 100 Awards for the design requirements of these drivers and on other outstanding technological developments with critical components. Such data will also be used to help commercial application. Most recently, LLNL and design an Engineering Test Facility that is planned for Moscow State University received a 1994 R&D 100 early next century as the next step toward a functioning award for growing potassium dihydrogen phosphate IFE power plant. (KDP) crystals much more rapidly, an achievement with NIF will also provide new capabilities for the high- significant promise for NIF. energy-density physics community. Because fusion Much further development in manufacturing targets will experience temperatures and pressures similar technologies over the next three years is needed to meet to those found in stars, data from NIF experiments will the cost goals for NIF. For example, the size of NIF attract scientists working in such areas as astrophysics, optics, such as KDP crystals, is up to two times larger space science, plasma physics, hydrodynamics, atomic than those used in Nova. In addition, the required damage and radiative physics, material science, nonlinear optics, threshold of these optics is two to three times higher than x-ray sources, and computational physics. These fields that of Nova’s optics. We are planning a program to
Inertial Confinement Fusion
Thermonuclear fusion is the laboratory. Both magnetic fusion “inertially confined” capsule can energy source for our sun and the and inertial confinement fusion expand. The resulting fusion stars and for nuclear weapons. In (ICF) research are supported by reactions yield much more energy a fusion reaction, nuclei of light DOE. than was absorbed from the driver elements, such as deuterium and In ICF, energetic driver beams beams. tritium (isotopes of hydrogen), (laser, x-ray, or charged particle) There are two basic approaches combine at extreme temperatures heat the outer surface of a fusion to ICF. In the first, called direct and pressures to form a heavier capsule containing deuterium and drive, laser beams impinge directly element, in this case helium. The tritium (D-T) fuel (see Figure 4). on the outer surface of the fusion energy released in a fusion reaction As the surface explosively target. In the second approach, is about one million times greater evaporates, the reaction pressure called indirect drive, beams heat than that released from a typical compresses the fuel to the density the surface of a metal case chemical reaction. and temperature required for D-T (hohlraum), causing emission of There are essentially three fusion reactions to occur. The x rays that strike the fusion target methods for confining fusion energy released further heats the capsule and drive the implosion. fuel reactions: gravitational compressed fuel, and fusion burn (See the box and figure on p. 38, confinement, as inside stars, and propagates outward through the which provide additional magnetic and inertial confinement, cooler, outer regions of the capsule information about direct- and which can be achieved in the much more rapidly than the indirect-drive targets.)
5 Overview E&TR December 1994
help our suppliers substantially reduce their costs the science-based Stockpile Stewardship in which NIF to manufacture high-quality, state-of-the-art NIF will play an indispensable role; NIF’s potential components, an achievement that will help them contributions to energy research; and NIF’s likely compete better in the international market. impact on advancing science and technology. We also When the first experiments are carried out on the NIF describe more fully the NIF facility by taking a tour of in 2002, they will begin a new era of advanced research it from a laser beam’s point of view, and finally, we with a laser system so powerful it was only dreamed review the environmental, safety, and health about several decades ago. By achieving ignition and considerations relevant to NIF. energy gain for the first time in the laboratory, NIF will maintain U.S. world leadership in ICF research and will directly benefit many different research communities. If sited at LLNL, it will considerably strengthen this For further laboratory and make it an even greater center of information scientific research. contact Jeffrey Paisner In this special issue of Energy and Technology (510) 422-6211 or Review dedicated to NIF, we describe in separate Kenneth Manes articles the importance of NIF to weapons physics and (510) 423-6207.
6 A Tour of the Proposed National Ignition Facility
On a conceptual walk-through of the proposed National Ignition Facility, we follow the path of a photon from the master oscillator and preamplifier at the beginning of the laser chain, through the main laser components, to the target. We conclude with the results of our recent Beamlet Demonstration Project, which demonstrated a prototype of one of the 192 laser beams that will be required to achieve ignition and energy gain in inertial confinement fusion targets.
HE National Ignition Facility laser beamlines require more than To give some perspective on its T (NIF) will house the world’s 9000 discrete, large optics (larger overall dimensions, the NIF building most powerful laser system. Figure 1 than 40 ¥ 40 cm) and several is roughly 200 meters long ¥ 85 meters is an artist’s sketch of the proposed thousand additional smaller optics. wide (about 600 ¥ 250 feet). These laser and target area building. The The laser output beams strike a dimensions are a little smaller than overall floor plan is U shaped, with series of mirrors, which redirect them those of a typical covered football laser bays forming the legs of the U, to the large target chamber shown on stadium. As an example, Figure 2 and switchyards and the target area the right side of Figure 1. From compares the NIF building with the forming the connection. switchyards to the target chamber Minneapolis Metrodome. The NIF will contain 192 room, the beams are in groups of four This article takes us on a tour of independent laser beams, each of (2 ¥ 2 arrays) and follow the beam the proposed facility. First, we follow which is called a “beamlet.” Each paths shown in red. At the target the complex path of a photon from the beamlet will have a square aperture chamber, the beams pass through master oscillator at the beginning of of a little less than 40 centimeters frequency-conversion crystals that the laser chain, through the laser on a side. Beamlets are grouped convert the infrared laser output components, and on to the target. mechanically into four large arrays— beams to ultraviolet laser light. They After this tour, we discuss some of or bundles of beamlines—with the then pass through lenses that focus the principal laser components and beamlets stacked four high and the ultraviolet beams on a tiny target target experiments in more detail. twelve wide, as shown in blue on the located in the center of the target Finally, we describe the results of the left-hand side of the sketch. The 192 chamber. Beamlet Demonstration Project that
7 NIF Facility Description E&TR December 1994
was recently completed at LLNL. As fiber generate weak laser pulses at fiber communications networks. The part of its core activities, the Inertial four separate frequencies (or colors of modulators allow us to tailor the Confinement Fusion (ICF) Program light). Each pulse is launched into an pulse shape independently in each developed a prototype of a single optical fiber system that amplifies beamlet under computer control. beamlet of NIF to validate the and splits the pulse into 192 separate In this way, the 192 pulses can be technology needed for the next fibers, 48 of each color. The four carefully shaped and balanced to generation of glass-laser drivers. colors are used to smooth the set up exactly the conditions an intensity (power per unit area) of the experimenter needs on a target Master Oscillator laser spot on the target. without rearranging any laser The optical fibers carry the laser hardware in the facility. An optical We begin our tour approximately pulses to 192 low-voltage optical fiber then carries the individually in the center of the facility, where a modulators. The modulators for NIF tailored pulse to each beamlet. The laser pulse is born in the master were derived from the integrated- power in the laser pulse at this point oscillator room. Here, four oscillators optics modulators that are now being is a little less than a watt. Typical made of neodymium-doped optical installed in very-high-speed optical pulses are a few nanoseconds long
Amplifier columns Main amplifier power conditioning system
Cavity mirror Transport spatial filters mount assemblies Beam control and laser diagnostics
Laser and beam transport structural support
Cavity spatial filters
Pockels cell assembly
Polarizer mount assembly Control room
Preamplifiers Transport turning mirror mounts Master oscillator room Target chamber Final optics system
Figure 1. View of the proposed laser and target area building for NIF. This facility will contain the world’s most powerful neodymium glass laser system, 50 times more powerful than Nova, currently the world’s most powerful laser. This sketch shows the laser bays, which form the two legs of the U-shaped floor plan, and the switchyards and target area forming the connection.
8 E&TR December 1994 NIF Facility Description
(a nanosecond is 10–9 second), so Following a Pulse Through the A pulse of laser light from the the energy is a few nanojoules (a Main Laser Components preamplifier reflects from a small nanojoule is 10–9 joule). mirror labeled LM0 in Figure 3. This Figure 3 shows the layout of the mirror is located near the focal plane Preamplifier main laser components of a NIF of the pair of lenses labeled lens 1 beamlet. These components take the and 2 and identified as the transport Optical fibers carrying the pulses laser pulse from the preamplifier all spatial filter. Light comes to a focus from the master oscillator room the way to a frequency-converted at the focal plane of the transport spread out to 192 preamplifier pulse headed to the target. We first spatial filter and re-expands to a size packages. As shown in Figure 1, the follow a pulse through the various of a little less than 40 cm at lens 1 of NIF preamplifiers are located beneath components and then discuss their the spatial filter, where it again the focal plane at the center of the functions in more detail. becomes a parallel beam. The beam large transport spatial filters, which are located between the laser components and the target chamber. Figure 2. The NIF Each preamplifier package has a building is similar in regenerative amplifier in a ring cavity. size to a modern municipal stadium. This device amplifies the pulse by a For perspective, the factor of about a million, from about a NIF building is nanojoule to a millijoule, with very compared here to the high stability. The amplifier uses Minneapolis small neodymium glass amplifiers Metrodome. pumped by semiconductor laser diodes so that it has very stable gain and requires little servicing. The laser pulse then enters spatial beam-shaping optics and a flashlamp-pumped, four- pass rod amplifier, which converts it to about a 1-J pulse with the spatial intensity profile needed for injection into the main laser cavity.
Transport LM3Five slabs Lens 1 Lens 2 Elbow spatial Transport mirror Booster filter mirror amplifier LM0 Nine Cavity Five Pockels 3 LM4Ð8 slabs spatial slabs cell filter Focus Cavity Cavity lens/vacuum amplifier amp barrier 1 2 Beam Cavity Lens 1 Lens 2 Injection dump mirror Deformable pulse from Target Polarizer (LM2) mirror (LM1) preamplifier Frequency converter Debris shield/phase plate
Figure 3. One beamlet of the NIF laser, from pulse injection to final focus on the target. We designed the laser chain in this beamline using the CHAINOP family of numerical codes. These codes model the performance and cost of high-power, solid-state inertial confinement fusion laser systems. The path taken by a photon is described in the text.
9 NIF Facility Description E&TR December 1994
passes through booster amplifier 3, amplified by amplifier 3 to an energy profile of each spot individually. reflects from the polarizer, is of about 17 kJ for a typical ignition Smooth intensity profiles lead to amplified further in cavity amplifier target pulse shape. Now the pulse better-understood experimental 2, and goes through a second spatial passes through the transport spatial conditions and better target filter identified as the cavity spatial filter on a path slightly displaced performance. filter. After passing through from the input path. Because it is An important feature of NIF is its amplifier 1, the beam reflects from a displaced, the output pulse just integrated computer control system. deformable mirror (mirror LM1 at the misses the injection mirror LM0, the This system uses a high-speed optical far left end of Figure 3). After once mirror where laser light was first fiber network to connect roughly again passing through amplifier 1, the reflected when it came from the 25,000 control points, sensors, and beam comes back through the cavity preamplifier. distributed processors. The items that spatial filter and amplifier 2. The pulse travels through a long are controlled by computer include Meanwhile, the component beam path reflecting from several motors and switches for alignment, identified as the Pockels cell in transport mirrors until it reaches the diagnostic systems for the laser Figure 3 is energized. This important target chamber. (For simplicity, beams, data from target diagnostics, component rotates the plane of Figure 3 does not show all the data processing stations, and all other polarization of the laser light from transport mirrors that will be installed control and information features of horizontal to vertical. In this in NIF.) Mounted on the target the facility. polarization, the pulse passes through chamber is a frequency converter that the polarizer and strikes mirror LM2, changes the infrared laser pulse to More About the Main Laser which redirects it back once again ultraviolet laser light. A focusing lens Components towards mirror LM1. The Pockels brings the ultraviolet pulse to a focus cell rotates the polarization back to at the center of the target chamber. A Amplifiers horizontal, and the beam passes back debris shield protects the focusing Figure 4 is a top-down view of a through amplifier 2 and the cavity lens from any target fragments and glass amplifier. The NIF amplifiers spatial filter and makes another may also have a pattern etched into are constructed from slabs of double pass through amplifier 1, its surface to reshape the distribution neodymium-doped phosphate glass reflecting from LM1. It then passes of laser intensity in the focal spot on set vertically on edge at Brewster’s through the cavity spatial filter and the target. angle to the beam. At this angle, amplifier 2 one more time. Four pulses from different horizontally polarized beams have By this time, the Pockels cell has beamlets, each at a slightly different very low reflective losses while been de-energized so that it no longer color or frequency, come to a focus at propagating through the plates. The rotates the polarization of the pulse. a single spot on the target. The glass slabs are 46 ¥ 81 cm to give a As a consequence, the pulse reflects intensity profile of the sum of these clear aperture of 40 ¥ 40 cm from the from the polarizer and is further four spots is much smoother than the beam’s point of view. Figure 5 shows how the amplifier slabs are grouped together into large arrays. Such grouping reduces the Xenon Neodymium-doped glass slabs set flashlamps at Brewster’s angle to the beam number of required parts and floor space, hence the cost of the facility. Long xenon flashlamps extend Beam vertically across a stack of four slabs, propagation an arrangement resulting in a length that is convenient for the pulsed- power system that drives the flashlamps. The width of 12 slabs is Reflector Glass blast shield convenient for design of the mechanical structures. The NIF amplifiers are suspended Figure 4. Top view of a large glass amplifier using slabs of neodymium-doped glass set at Brewster’s angle to the beam and pumped by xenon flashlamps. The glass blast shields beneath a support frame in order to protect the slabs from acoustic disturbances and dirt generated by the flashlamps. provide access from the bottom to replace slabs, flashlamps, and blast
10 E&TR December 1994 NIF Facility Description
shields. It is extremely important to Figure 5. The main minimize contamination of the Amplifier column cavity amplifier amplifiers by particles that otherwise assembly is typical of all the NIF amplifiers. might burn into the glass surfaces Notice that a column when illuminated by the intense consists of an flashlamp or laser light. Access from amplifier that is 4 the bottom of the amplifiers ensures beams high. Groups that service personnel and equipment of columns that are are always below and downwind 12 beams wide form from sensitive surfaces, so that dirt an amplifier falls to the floor and not into open assembly. amplifier structures. Figure 6 shows Amplifier a servicing cart in the process of glass slab installing a stack of four laser slabs To target into the amplifier structure. The pulsed-power system for the NIF uses advanced self-healing energy storage capacitors developed for use in Strategic Defense Initiative projects. These capacitors store energy at about four times the density and half the cost per joule of conventional capacitors, such as those used in the Nova laser facility at LLNL. The number of amplifier slabs and their distribution among the three amplifiers were chosen to maximize Slab the output power of the laser over the cassette desired range of pulse shapes. For short pulses, the limit is set by nonlinear effects in amplifiers 2 and 3. For long pulses, the limit switches over to nonlinear effects in amplifier 1 and the limit set by the total amount of energy stored in the slabs. We might have eliminated amplifier 3 and placed 9 or 11 slabs in the amplifier 2 position. However, the polarizer coating suffers optical damage at a rather low fluence (energy per unit area). Such damage limits the laser output fluence to significantly less than we can run with the configuration shown in Figure 3. Notice that amplifier blocks are restricted to have an odd number of slabs (see Figure 4) so that gain Amplifier servicing cart gradients in the end slabs cancel.
Spatial Filter Figure 6. To facilitate maintenance, the slab and flashlamp cassettes shown at the top will be The NIF has two spatial filters in changed from underneath an amplifier column using a special cart. This approach will allow each beamline. In essence, a spatial the critical amplifier components to be protected from the laser bay environment at all times.
11 NIF Facility Description E&TR December 1994
filter—or image relay pair—is a pair small-scale noise comes to a focus Pockels Cell and Polarizer of lenses separated by the sum of displaced to the side of the main A Pockels cell uses electrically their focal lengths. A parallel beam focus in the spatial filter. This induced changes in the refractive incident on one lens comes to a displaced focus means that we can index of an electro-optic crystal, such focus in the center and emerges as a place a small pinhole at the focal as KDP (potassium dihydrogen parallel beam at the other lens. plane that blocks this noise while phosphate or KH2PO4) to rotate the Spatial filters serve several allowing the main beam to go polarization of light. When combined functions in large lasers. For NIF, through. In addition, the focal plane with a polarizer, the Pockels cell can we positioned a pair of lenses so inside the spatial filter gives us a serve as an optical switch that directs that an image of the very clean convenient location for injecting the light into one or the other of two intensity profile injected from the input pulse from the preamplifier possible paths, and it is used for this preamplifier reforms near the without requiring any additional, function in the NIF laser. amplifiers and at the frequency expensive, large-aperture optical Conventional Pockels cells require converter. Diffraction causes components. a crystal that is roughly the same intensity noise to grow in laser Because the intensity near the focal thickness as the beam diameter. A systems, but this growth is reset to plane is very high, the spatial filters in crystal this thick is completely zero in the vicinity of an image of large lasers such as NIF must be impractical for the NIF’s 40-cm beam. the original input profile. In operated in a vacuum. Figure 7 shows Instead, the Pockels cell used in the addition, nonlinear effects in the the large transport spatial filter vacuum NIF laser is a new type developed at laser cause small-scale intensity vessel for NIF. The cavity spatial filter LLNL. As shown in Figure 8a, it noise in the laser to grow, but this is similar, but somewhat shorter. contains a thin plate of KDP placed
Figure 7. Spatial filters in large lasers must contain a vacuum. Shown here is the transport spatial filter 17.3 meters vacuum vessel for NIF. The basic functions of the transport spatial 65 meters filter and cavity spatial filter are similar, but their lengths and internal components are different. Each vessel contains 48 beams. The entire structure is supported from the floor by reinforced concrete pillars. The pulse- generation system is located below the center section. Laser diagnostic units are supported by a frame attached to the filter top. Access to internal mechanisms is via doors on the vessel sides.
12 E&TR December 1994 NIF Facility Description
between two gas-discharge plasmas. thin-film polarizers are difficult to cost of fabrication is higher than we The plasmas serve as conducting manufacture, but research supported would like. electrodes, which allow us to charge by LLNL at commercial We can use less expensive the surface of the thin crystal plate manufacturers has improved their components if we design the laser to electrically, but they are so tenuous process control and demonstrates that include an adaptive correction system that they have no effect on the high- large polarizers meeting the NIF that compensates for distortions in power laser beam passing through specifications are now available. the beam. An adaptive system also the cell. Figure 8b shows a plasma allows us to compensate for other electrode Pockels cell of this sort Deformable Mirror important distortions, such as thermal with a clear aperture of 35 cm. The The NIF laser must have beam gradients. Recent advances in device is now operating in our quality high enough that it can place adaptive optics in the Atomic Vapor Beamlet Demonstration Project at all of the energy from each beamlet Laser Isotope Separation program at the full laser fluence proposed for into a circle of about half a millimeter LLNL, and at several commercial the NIF. in diameter at the center of the target companies and government research The polarizer for the switch is a chamber. It is possible to purchase facilities, show that the cost of the multilayer dielectric coating set at optical components finished well deformable mirror, sensor, and Brewster’s angle to the beam. These enough to achieve this goal, but the processor technology required to
(a)
Cathode Cathode
Electron Electron emission emission KDP Side 1 Side 2
Switch Plasma Plasma Switch pulser pulser Discharge Discharge current current
KDP charging current
Anode Anode
z (cable)
Switch pulser
Figure 8. (a) The Pockels cell optical switch. This optical switch uses plasma as transparent, high-damage-threshold electrodes to charge the potassium dihydrogen phosphate (KDP) crystal. (b) A large-aperture, plasma electrode Pockels cell installed in the Beamlet Demonstration Project. Tests show that this device meets the requirements for timing, efficiency, and stability needed for NIF.
13 NIF Facility Description E&TR December 1994
implement such a system has fallen fusion targets perform much better efficiency can exceed 60% for the to the point that adaptive correction when they are driven with ultraviolet complex pulse shapes used to drive systems are very desirable for the NIF radiation. The NIF laser will convert ignition targets. facility. Figure 9 shows a typical the infrared (1.05-µm) light to deformable mirror that uses ultraviolet (approximately 0.35 µm) Target Area and Target electrostrictive actuators to bend using a system of two nonlinear Diagnostics the mirror surface to compensate crystal plates made of KDP, the same for wavefront error. This mirror type of crystal that is used in the Figure 11 shows an end view of is installed on the Beamlet Pockels cell. the NIF target area. From this Demonstration Project, where we are Figure 10 shows the arrangement perspective, we can see the beam studying its performance. The NIF of the two crystal plates. The first paths from the laser output through will use a similar, but larger, mirror plate converts two-thirds of the the turning mirror array to the target as mirror LM1 in Figure 3. incident 1.05-µm radiation to the chamber. The target chamber is a second harmonic at 0.53 µm. Then 10-m-diameter aluminum sphere. Frequency Converter the second crystal mixes that The beams enter the chamber in two A neodymium glass laser generates radiation with the remaining conical arrays from the top and two light at a wavelength of about 1.05-µm light to produce radiation from the bottom through final optics 1 micrometer in the infrared region. at 0.35 µm. This process has a peak packages mounted to the target However, we know that inertial efficiency greater than 80%, and the chamber. Figure 12 shows a final
Ordinary axis
Extraordinary axis
Third-harmonic generation KDP
7 cm crystal Extraordinary Second-harmonic axis generation
Polarization Ordinary (input radiation) axis
Figure 10. The NIF configuration for frequency conversion to the third harmonic using two KDP crystal plates. The NIF laser generates light in the infrared region (this 1.05-µm wavelength light Figure 9. The cavity mirror farthest from the target area (LM1 in is shown as red in the drawing). However, inertial fusion targets Figure 3) is a deformable mirror used for performing wavefront perform better with ultraviolet radiation. This 0.35-µm wavelength corrections of the beam. Electrostrictive actuators bend the mirror light is called the third harmonic (shown as blue). The first KDP surface to compensate for wavefront error. This photograph shows crystal (left) converts two-thirds of the input radiation (red) to a 70- ¥ 70-mm deformable mirror currently used on the Beamlet second-harmonic radiation (green). The second crystal mixes the Demonstration Project. The NIF will use a mirror that is similar to remaining input radiation with the second harmonic to produce this one, but larger. third-harmonic radiation (blue). Peak conversion efficiencies can exceed 80%.
14 E&TR December 1994 NIF Facility Description
optics assembly subsystem. and is lined with a layer of solid Figure 14 is an x-ray image of a Diagnostic instruments, such as deuterium–tritium (DT) fusion target shot from the Nova laser x-ray spectrometers, microscopes, fuel. The hollow interior contains showing the glowing spots where and cameras, are mounted around a small amount of DT gas. Nova’s ten beams strike the inside the equator and at the poles of the Figure 13b is an artist’s 3-D surface. In this image, artists drew in target chamber. rendering showing how laser the laser beams and the outline of Figure 13a is a scale drawing of a beams deposit their energy on the the cylinder, which are invisible typical fusion ignition target that will inside surface of the metal cylinder in an x-ray photograph. For this be used with this chamber. The or “hohlraum” where the energy is experiment, the metal wall was made target is a metal cylinder—typically converted to thermal x rays. The thin enough that some of the x rays made of gold or lead—about 6 mm in x rays heat and ablate the plastic leaked through to be photographed. diameter and 10 mm long. It contains surface of the ignition capsule, Thicker walls are used for most a plastic fusion capsule about 3 mm causing a rocket-like pressure on targets. In the language of fusion in diameter. The capsule is chilled to the capsule and forcing it to research, this target is called an a few degrees above absolute zero implode. “indirect-drive” target because the
Domed roof Target area building Upper mirror support frame
Target diagnostics Turning mirrors
Switchyard building Laser ports
Switchyard mirror
Personnel access area Base mat Target chamber
Final optics assembly Lower mirror support frame
Figure 11. Cut-away view of the NIF target area showing the major subsystems. The laser beams focus energy onto a target located at the center of the target chamber, which is housed in a reinforced-concrete building. Target diagnostics mounted on the chamber will collect the experimental data.
15 NIF Facility Description E&TR December 1994
Figure 12. The final laser beams do not strike the fusion optics assembly is KDP frequency- capsule directly. NIF can also study a single, integrated conversion crystals “direct-drive” targets in which there structure that is mechanically is no hohlraum and the beams do Focus lens supported by and strike the capsule directly. fastened to the target To illustrate the type of diagnostics chamber. The Debris shield we use to study fusion targets, frequency-conversion Figure 15 shows a sequence of crystals (see Bellows pictures of a fusion capsule implosion assembly Figure 10) are shown from a direct-drive target on the at the top. This Omega laser at the University of system will convert Calorimeter Rochester. Here, the fusion capsule is four infrared beams similar to, but smaller than, the one to the third harmonic, shown in Figure 14. (Indirect-drive focus the beams onto a target, and provide targets give similar pictures, but the beam smoothing and hohlraum and other complications color separation. The make the pictures less clear.) The calorimeter is used to images are from an x-ray framing Target chamber obtain energy interface spool camera microscope (that is, the measurements on the sequence of frames was taken by a incident beam. very-high-speed motion picture camera). Each frame lasts for 50 picoseconds (50 trillionths of a second).
Figure 13. (a) Scale drawing of a (a) (b) typical fusion target. The outer metal cylinder, usually made of either gold or lead, is about 6 mm in diameter. Inside is a plastic fusion capsule that is about 3 mm in diameter. The capsule is lined with a layer of solid deuterium–tritium (DT) fusion fuel, and the hollow interior contains a Shield small amount of DT gas. Laser beams enter the target in two conical arrays. Capsules The outer and inner cones are shown at the top and bottom of the target. (b) An artist’s 3-D rendering of the laser beams depositing their energy on the inside surface of the hohlraum where they are converted to x rays that heat the target internally, causing it to implode and ignite.
0.35-µm laser beams in two cone arrays
16 E&TR December 1994 NIF Facility Description
In Figure 15a, the 0.25-mm- Ten megajoules is not an and better target performance. diameter capsule lights up brightly as enormous amount of energy. It Nevertheless, these pictures are the laser beams first strike its cold roughly corresponds to the heat good illustrations of how target surface. In the next several frames, the released in burning an 11-ounce experimenters study the effects of surface of the capsule blows outward, water glass full of gasoline (about nonideal conditions, such as and the gas fill is accelerated towards 312 cm3). However, the energy in a nonuniform drive pressure, and the center, compressing to a very high fusion target is produced and radiated compare test results to their density. In Figure 15g, the gas begins away in less than a nanosecond from theoretical models. to collide with other imploding gas at a volume with a diameter of only the center of the capsule and comes to about a fifth of a millimeter. To do NIF Beamlet Demonstration a stop. As it stops (or stagnates), the that, the target material must reach Project temperature shoots up to a value at conditions that are found in nature which fusion reactions can begin. The only deep within stars and other hot Glass lasers are a popular and high-temperature region at stagnation celestial objects or deep within well-known technology, but the NIF can be seen as a bright x-ray spot in nuclear weapons. laser design is significantly different Figures 15g through 15j. If this were In Figure 15, the drive on the in many ways from existing large an ignition target shot using the NIF, target was deliberately distorted, so glass lasers (see the box on p. 18). To the hot gas core or “spark plug” the implosion formed a little to the be quite sure that the NIF system will would ignite the surrounding, right of center from the camera’s function as we project, it is prudent to relatively cold DT layer, producing point of view. A perfectly centered test some of the differences well in roughly 10 megajoules of fusion implosion gives higher temperatures advance of construction. In 1992, we yield. The hot target plasma then expands and cools in Figures 15k and 15l.
(a) (b) (c) (d)
(e) (f) (g) (h)
(i) (j) (k) (l)
Figure 14. X-ray image of a target shot from the Nova laser. The glowing spots are where Nova’s ten beams strike the inside surface of the target’s thin, cylindrical metal wall. This target is called “indirect Figure 15. Sequence of pictures of fusion-target implosion taken with an x-ray framing drive” because the laser beams do not camera microscope. Each frame lasts for 50 ps and is a positive image, so bright areas are directly strike the inner spherical capsule, regions of bright x-ray emission. (a) Laser beams strike the 0.25-mm-diameter cold target which contains DT fuel. The outer shell shell and start the implosion. (b) through (g) The outer surface of the shell blows off, and and beam paths, invisible in an x-ray the inner part of the shell and gas implode. In (g), the gas fill collides with itself near the photograph, are drawn by an artist to show center of the implosion, and the rapidly increasing temperature produces a bright spot of where they were located in the original x-ray emission. The central hot spot develops through (j) and then begins to expand and target. cool in (k) and (l).
17 NIF Facility Description E&TR December 1994
How the NIF Differs from Other Glass Lasers
The neodymium glass laser, invented in 1961, system allows us to put a laser with a typical output was one of the first types of laser to be developed. that is 40 times larger than Nova’s into a building only Researchers quickly realized that it could be scaled up about twice the size (although the Nova laser does not to large beam apertures and extreme peak power. Many completely fill the building). laboratories soon began developing glass laser hardware for research on nuclear fusion and other high-energy- density physics. Nova building To reach high energy and power, large glass without lobby amplifiers are required. However, it is difficult and Proposed inefficient to generate a high-quality output beam from NIF building large amplifiers. The design that evolved was called the single-pass master-oscillator/power-amplifier (MOPA) chain. As shown in the illustration below, a MOPA design uses a small oscillator to produce a pulse of a few millijoules with a beam diameter of a few millimeters. After passing through other components, such as a preamplifier, the pulse is divided and makes a single pass through a chain of amplifiers of gradually increasing size. The amplifiers are separate components rather than being grouped in large arrays, as in the NIF design. For truly large lasers, such as NIF, the MOPA design has another important disadvantage. The NIF fusion- ignition targets require pulses with a length of 3 to 5 ns. Laser chains – single-pass amplifiers of increasing size At this pulse duration, we can extract most of the energy stored in the laser glass. When we do, the tail end of a pulse has a much lower gain than the front end, a Beam- splitters difference we call saturation pulse distortion (SPD). (more laser chains) MOPA designs require larger and more expensive amplifiers and preamplifers under these conditions. Multipass lasers, such as NIF, solve the problem of SPD Master oscillator without the need for larger preamplifiers. In contrast, Nova uses much shorter pulses and does not extract as much of the stored energy. The MOPA is a low-risk, but expensive, approach to Even when the Nova laser was designed in 1978, we constructing large glass lasers. Limitations include knew that a multipass design would be potentially much considerable setup time and cost; very long propagation less expensive to build. However, the necessary paths; the need for critical alignments and physical component development that was still required meant changes to many different components; and the additional risks and possible delays. The NIF laser fabrication, assembly, and maintenance of a large design is a result of development efforts started many number of parts. years ago for components such as advanced oscillators, The Nova laser at LLNL is the largest operating amplifiers, and Pockels cells. Today, our Beamlet glass MOPA fusion laser. It contains five sizes of Demonstration Project integrates all of these new amplifiers with a total of 41 slabs of glass in each of developments. This effort is showing that the ten laser chains. Compared to the size of the Nova technology has now progressed to the point that a large, facility, the compact design of the NIF multipass multipass glass laser can be built with low risk.
18 E&TR December 1994 NIF Facility Description
established the Beamlet understand the engineering of large The NIF laser design has evolved Demonstration Project (or the amplifier arrays. To reduce costs, over the past two years as we try to Beamlet for short) to test some of the only one of the four apertures in the optimize its cost and match its new features. The Beamlet has now array contains high-quality laser performance to the wide range of been completed and is currently glass. The other three apertures possible experiments that might be operating reliably up through the contain an inexpensive glass that conducted. This evolution has led to a frequency converters at fluence and absorbs flashlamp light in a way that few other small differences between intensity levels projected for NIF. resembles the laser glass. In addition, the Beamlet and the NIF design We built a single prototype rather the amplifiers and other laser presented in the Conceptual Design than a large array of beams because it hardware rest on the floor rather than Report and discussed in the first part was clearly much too expensive to hanging from a support frame as they of this article. For example, the beam construct a full NIF beamline with will in the NIF design. Components apertures for the Beamlet are slightly 4 ¥ 12 amplifier blocks and multiple resting on the floor were more smaller than those for NIF, and the beamlets. We did, however, build the convenient for the room we had laser is shorter. We used 16 rather main laser amplifiers as an array available, although the system is than 19 slabs of glass, and these slabs stacked two high and two wide more difficult to keep clean than that are distributed in a ratio of 11-0-5 because it was important to begin to in the NIF design. (eliminating amplifier 2) rather than
Frequency Amplifier 1 converters Master Beam dump oscillator LM1
Cavity spatial filter
Laser diagnostics instruments Deformable mirror
Preamplifier
Transport spatial filter
Power supply Amplifier 3 (in basement) LM2 Pockels cell and polarizer
Figure 16. The Beamlet is a scientific prototype of the NIF multipass laser. This demonstration unit, which is now operating at LLNL, uses essentially the same technology as that for NIF but with one output channel rather than 192.
19 NIF Facility Description E&TR December 1994
9-5-5 in the three amplifiers shown in LM1, returns through the amplifier and many of the costs of a system Figure 4. The NIF distribution gives and cavity spatial filter to the Pockels scale with the number of beamlets better performance for pulses of 4 to cell and LM2, makes a second round rather than their size. However, 5 ns and longer, whereas the Beamlet trip through amplifier 1, and returns amplified spontaneous emission distribution is better for short pulses. through the Pockels cell to reflect causes the gain of large amplifiers to The input pulse from the preamplifier from the polarizer, just as described drop by a few percent at the edges of is injected into the cavity spatial filter for NIF. the aperture. We compensate for this rather than the transport spatial filter. The output pulse then reflects from gain drop on Beamlet (and NIF) by This design makes the front end three turning mirrors that are used to making the input pulse more intense slightly larger because the pulse does fold the Beamlet optical path in two around the edges. not see the small-signal gain of so that it fits in the available space. The large plasma-electrode amplifiers 2 and 3. However, the The pulse passes through amplifier 3 Pockels cell switch installed on design simplifies alignment if and the transport spatial filter, then Beamlet is shown in Figure 8b. Only the input pulse is used for laser enters the frequency-conversion about 30 J leak through the polarizer alignment, as it is in Beamlet (but not crystals. Beam splitters direct when we set this switch to reflect 6 kJ NIF). Otherwise, the Beamlet has all samples of the infrared and ultraviolet from the polarizer, so the Pockels cell the NIF components and features beams to diagnostic instruments and polarizer are remarkably shown in Figure 4. located near the frequency converters. efficient. Figure 16 is a drawing of the Most of the beam energy is absorbed Figure 18 shows the infrared output Beamlet facility. The master oscillator by an absorbing glass beam dump at energy from Beamlet as a function of and preamplifier are the same the end of the beamline. the input energy from the preamplifier. technology as that for NIF, but with Figure 17 is a photograph of the For this set of shots, we set the beam only one output channel rather than Beamlet amplifier. The amplifier’s aperture to 34 ¥ 34 cm, a value limited 192 . The laser pulse reflects from a clear aperture is 39 cm, or essentially by the 35-cm clear aperture of the deformable mirror (see Figure 9) and the same as the 40-cm aperture Pockels cell crystal. The output energy enters the cavity transport spatial proposed for NIF. The amplifiers matches the theoretical model (solid filter. We use a deformable mirror in perform exactly as predicted from line) very well. We have fired the this position rather than in the LM1 design codes (elaborate computer system at an output up to 13.9 kJ at position described for NIF because simulations) we developed for this smaller mirror could be adapted smaller amplifiers. from an existing design at very low Large amplifier apertures such as cost. The pulse then passes through this lead to a less expensive NIF 16 amplifier 1 and reflects from mirror because there are fewer beamlets, 12
Figure 17. The 8 2 ¥ 2 amplifier module 3-ns pulse for Beamlet shown 5-ns pulse during assembly. To 4 Output energy, kJ 8-ns pulse reduce costs, only one of the four 0 apertures in this 0 0.25 0.50 0.75 1.00 array contains high- Injected energy, J quality laser glass. The clear aperture of 39 cm is essentially Figure 18. Performance of the Beamlet the same as that amplifier for a 34- ¥ 34-cm beam at three proposed for NIF. different pulse lengths. The infrared output Such large amplifier energy is plotted as a function of the input apertures lead to a or injected energy from the preamplifier. less expensive NIF. This plot shows that the output energy matches our theoretical model (solid line) quite well.
20 E&TR December 1994 NIF Facility Description
5 ns and at somewhat higher output for likely that we can operate at It is important to have very longer pulses. At 5 ns, the fluence somewhat higher fluences, but we uniform intensity profiles in a fusion (energy per unit area) across the flat- will not push beyond the nominal NIF laser. Uniform profiles minimize the top part of the beam is 14.3 J/cm2. fluences and run the risk of damaging risk of damage caused by intensity This is about 7% above the nominal optical components until we have maxima in the beam and help to operating point for the NIF ignition conducted all of the most critical tests maintain high frequency-conversion target in the Conceptual Design on the system. We will certainly have efficiency. Figure 19 shows intensity Report. higher energies when we obtain larger profiles of the infrared and ultraviolet We recently conducted the first crystals and expand the beam size beams from Beamlet as recorded series of Beamlet frequency- later this year. We will not quite reach by television cameras in the laser conversion tests. For these tests, the the nominal NIF beam aperture of diagnostics area. The beam profiles aperture of the frequency-conversion 36 ¥ 38 cm because the Beamlet are very smooth and flat across the crystals limited the beam aperture to components are smaller and the beam center, as desired, and they roll off 29.6 ¥ 29.6 cm. We have operated the path is shorter than for NIF. smoothly to zero intensity in a small conversion crystals up to 8.7 J/cm2 Nevertheless, we should be able to margin around the edge. ultraviolet output in 3-ns pulses, which run experiments at apertures of about Low phase distortions on the beam is once again about 10% above the 34 to 35 cm. We have also run are also important. Phase distortions NIF nominal operating point. The complex shaped pulses of the sort not only prevent the beam from conversion efficiency from infrared required for ignition targets. In these focusing on a small spot, but they to ultraviolet was just over 80% experiments, we obtained conversion also degrade the process of frequency for square pulses, as anticipated. efficiency up to 65% and fluence and conversion. Figure 20 shows the The maximum ultraviolet energy energy similar to those for square distribution of Beamlet energy at the demonstrated to date is 6.7 kJ. It is pulses. focus of a lens (in the far field).
Laser Damage in Optical Materials
When a laser beam strikes a component, it can cause optical-induced damage. The damage usually appears only at high fluence (energy per unit area) and is caused by a small flaw or contamination by foreign material. Flaws or contaminants can absorb enough energy to melt or vaporize and then disrupt the surrounding optical material. The cost of a large laser is roughly proportional to the total beam area, so we push laser designs to the highest fluence that can be tolerated In the past 15 years, the number of defects that initiate without damage to minimize the beam area and cost. laser damage in optical materials has decreased On the left is a view through a microscope of a tiny dramatically. The decrease is a result of painstaking light-absorbing defect in a piece of optical glass. This research at commercial suppliers and various research particular defect is about a twentieth of a millimeter in laboratories. Much of the work was funded by the diameter, or about the diameter of a human hair, and it LLNL laser program and other programs interested in is 10 or 20 times larger than the typical defect that constructing large lasers. As a result, the NIF laser will concerns us. When an intense laser beam strikes the operate at fluence levels that would have been unthinkable defect, its surface evaporates and explodes causing tiny at the time Nova was constructed. As one example, the cracks in the glass. As shown on the right, the cracks average ultraviolet fluence at the output of our Beamlet tend to grow larger with each laser shot, and the Demonstration Project is more than three times the damaged spot eventually gets large enough to disrupt average fluence that would have been safe for the optical the laser beam quality seriously. components that were available when Nova was built.
21 NIF Facility Description E&TR December 1994
These data reveal that about 95% (smaller peaks on the curve) is caused on the spot size, and the spot size is of the energy is within an angle of by gas turbulence in the amplifiers and also somewhat smaller than that ±25 microradians from the center beam tubes. Beamlet’s deformable shown in Figure 20. We have only of the spot. Because the NIF mirror, which was in operation during begun to study the performance of the requirement for ignition targets is this test, can correct the long-term deformable mirror, but even these ±35 microradians, this spot meets that thermal distortions in the amplifier early results confirm that adaptive requirement. Nevertheless, we would slabs very effectively. Indeed, this spot optics will be a highly valuable like to improve this value even more is smaller—by a factor of four or addition to NIF. because some potential experiments five—than it would be without such could use a smaller spot. correction. However, the deformable Next Steps Toward NIF We obtained the far-field spot in mirror system cannot respond rapidly Figure 20 at the end of a sequence that enough to correct for gas turbulence. Our Beamlet tests to date included four full-power Beamlet When the glass amplifier slabs are demonstrate clearly that large, shots over about 10 hours. Much of cold and the temperature is uniform, multipass laser systems can operate the remaining structure in the spot the deformable mirror has less effect at the nominal operating conditions proposed for the NIF. We will now proceed to explore more extreme (a) Infrared (b) Ultraviolet operating conditions and to 250 demonstrate that the proposed target optics will give spots of the desired 200 size and uniformity at the plane of the target in the far field. We will also test many other features suggested for 150 NIF, such as new glass compositions, changes in amplifier pumping 100 conditions, and alternate switch technology. The results from our Beamlet Demonstration Project, 50 along with the models and design codes we are testing, will ensure that 0 we can have great confidence in the performance projected for NIF. Figure 19. A fusion laser must have uniform intensity profiles. These intensity profiles obtained from Beamlet are smooth and flat across the center during frequency conversion from (a) infrared to (b) ultraviolet light. The small margins indicate that the profiles roll off Key Words: adaptive optics; Beamlet smoothly to zero intensity around the edges of the 30- ¥ 30-cm beam. Demonstration Project; deuterium–tritium (DT) fuel; fusion energy; multipass lasers; National Ignition Facility (NIF); neodymium glass lasers; Pockels Figure 20. The distribution of Beamlet cell; potassium dihydrogen phosphate (KDP). energy at the focus of a lens (in the far field). This curve shows that about 95% of 1.1 For further the energy is within ±25 microradians of the information center of the spot, well within the design 0.7 contact requirements for NIF ignition targets. Center of spot John R. Murray 0.3 (510) 422-6152.
™0.1 Energy, normal to peak ™40 ™20 0 20 40 Divergence from center, µrad
22 NIF and National Security
Although producing total energies that are minuscule compared to those in a nuclear device, the National Ignition Facility will produce energy densities high enough to duplicate many of the physics phenomena that occur in nuclear weapons and will thus help the U.S. to maintain its enduring stockpile of nuclear weapons.
HE other articles in this issue Stockpile Stewardship models of performance–that is, T describe the intended features science-based Stockpile Stewardship. and capabilities of the National Since the Cold War ended with the The Stockpile Stewardship Program Ignition Facility and the diverse kinds dissolution of the Soviet Union, the is based on several assumptions and of research that it will support, such as U.S. nuclear weapons program has observations: attempts to achieve break-even energy changed dramatically. The U.S. • Nuclear weapons cannot be output through inertial confinement brought a unilateral halt to the uninvented and will not go away, fusion (ICF). These other applications development and production of new even if the U.S. were to dismantle its are important benefits that derive from nuclear weapon systems. Also, a entire nuclear stockpile. the availability of NIF, but here we moratorium on underground nuclear • U.S. defense policy will continue to describe the value of the facility testing was implemented to further rely on nuclear deterrence for the through its key contribution to negotiations on a Comprehensive foreseeable future. Therefore, experimental research in the physics of Test-Ban Treaty and to encourage the maintaining confidence in the nuclear weapons. With the moratorium broadest possible participation in the stockpile—in its safety, security, and on nuclear testing and the likelihood of Nuclear Non-Proliferation Treaty. reliability—is essential. a Comprehensive Test-Ban Treaty A major change in the nuclear • The moratorium on nuclear testing making such testing permanently weapons program has accordingly will likely be followed by a unavailable, NIF becomes one of a few been a move from nuclear test-based Comprehensive Test-Ban Treaty, means of maintaining and advancing weapon reliability and safety to which the U.S. must adhere to while our understanding of the weapons now reliance on a thorough scientific retaining confidence in its nuclear in the stockpile. understanding and better predictive arsenal.
23 NIF and National Security E&TR December 1994
• No new weapons are being have to be used to fill the gaps left by We do not completely understand developed, and currently there is the cessation of nuclear testing. (See the physical processes involved in no known need for future weapon Figure 1.) the operation of a nuclear weapon. development programs; moreover, Indeed, a complete, detailed, and some essential facilities of the U.S. Complexity of Nuclear Weapons mathematically exact description nuclear weapons production complex of the physics would exceed no longer exist. Maintaining confidence in the the capabilities of today’s • The U.S. nuclear stockpile will stockpile under the conditions supercomputers. We must therefore contain fewer weapons, of fewer described above is a challenge make approximations to the physics in types, as well as weapons that will because of the complexity of our evaluations of performance, become considerably older than their nuclear weapons design and of the although these approximations design lifetimes; this stockpile will phenomena that take place when introduce uncertainties in our require enhanced surveillance and nuclear weapons are operated— predictions. We must rely on our maintenance to recognize, evaluate, chemical explosion, hydrodynamic cumulative knowledge, including past and correct problems that may arise. implosion, mixing of materials, test data, to make valid inferences for • There may be a growing need to radiation transport, thermonuclear physics regimes that are inaccessible evaluate potential threats from ignition and burn, etc. with current experimental methods. unfriendly foreign powers and Nuclear testing provided a This expertise is also the only way terrorist groups. pragmatic solution—integrated tests we now have for the evaluation of We must continue to provide the of the devices—that is no longer many crucial issues, including: training and required information for available. With the moratorium on • The severity of age-related material a group of scientists and engineers nuclear testing, we must rely on changes discovered through routine that will be the stewards for this advanced computational modeling stockpile surveillance. stockpile under these conditions. The and non-nuclear experimental • The severity of unexpected effects remaining tools at their disposal will techniques for predictions and data. discovered with improved computer models. • Whether retrofits, such as to improve safety or reliability, will Past Future function properly. • Whether new technologies can or Warheads should be incorporated in a stockpiled weapon system. In a nuclear weapon, the System designs phenomena occur in two very different regimes of energy density. In Modernization the early phase of the implosion, before the development of significant nuclear yield, temperatures are Underground nuclear testing relatively low. This is the low-energy- density regime; here there is Numerical simulations considerable complexity because the strength of the materials and the Prenuclear (high-explosive) phase chemistry of their composition play a simulations role in how events proceed. Full-scale assemblies using mock nuclear Nuclear or high-energy-density material are used to test experimentally laboratory simulations the hydrodynamics of the implosion process at the beginning of a weapon’s operation. The study of this Figure 1. Graphic comparison of the situation of the nuclear-weapons stockpile before and subcritical regime is carried out at after the enormous changes of recent years. The challenge is to maintain confidence in a much smaller stockpile without nuclear testing and modernization. hydrodynamic test facilities using
24 E&TR December 1994 NIF and National Security
powerful x-ray machines, high-speed integrate and test all of our physics important in energy transport. optics, and other methods. Current knowledge but will also help the These phenomena occur in such hydrodynamic test facilities can Department of Energy maintain astrophysical realms as stellar access the precritical physics regime, expertise in weapons design. We interiors, accretion disks, and although, without complimentary envisage training designers both supernovae (and occurred in the Big nuclear tests, these facilities must be on specific stockpile stewardship Bang), but on Earth they occur only improved to provide much more issues and on broader NIF ignition in machines such as NIF (and spatial and temporal information. questions relevant to inertial formerly in testing environments After significant nuclear yield confinement fusion. created in the areas surrounding begins in a real device and fissile Weapons research on NIF is driven nuclear tests). material is heated, we enter the high- by a need to acquire a much more The weapon physics research energy-density regime. Although no detailed understanding of physics program at the NIF will stress laboratory experiment can duplicate processes at high energy densities, as investigations of these phenomena, the amount of energy released by a well as by a desire to achieve ignition. including: nuclear weapon, many of the physical We have been studying these • Material equation-of-state conditions relevant to such a weapon processes on Nova and other lasers, properties. can be created in the laboratory. but, again, NIF allows us to move our • Unstable hydrodynamics. Improving our predictive studies into those energy densities that • Radiation flow, including the capabilities for evaluating these occur in a nuclear weapon. opacity of ionized elements and x-ray processes will be difficult. Without The maximum total energies production. nuclear tests, we can never directly available on NIF will be an extremely • Nonequilibrium plasma physics, observe the full operation of this tiny fraction of the yield of the including short-wavelength lasing. high-energy-density regime. We must smallest nuclear weapon (see the box • Thermonuclear burn in the therefore improve our understanding on p. 26); but there are significant laboratory. of the relevant physics with better benefits to generating so much less The NIF will have the capacity for computations and new experiments, energy. The main event of a nuclear doing systematic, well-characterized techniques, and facilities. The test is both extremely brief and experiments because of the flexibility National Ignition Facility will allow extremely violent; it destroys most or of its multiple beam configuration. us—on a microscopic scale—to attain all of the diagnostic and measuring Considerable experience in doing the high-energy-density conditions instruments. It remains for the such experiments very successfully that exist in weapons. researchers afterwards to try to sort on Nova and other lasers will carry out all the physics and phenomena over to the NIF. Laser experiments on NIF and Weapons Physics submerged in the event in order the NIF, like those on Nova, can be to analyze it. By contrast, on the directly or indirectly driven. In direct As a versatile high-energy-density NIF, we can design and perform drive, multiple beams are directed at physics machine, the NIF will enable experiments that isolate whatever the target. In indirect drive, a set of us to gain an improved understanding physics phenomenon is of interest. laser beams is directed into a of the underlying physics and We can study the physics at relevant hohlraum, which is a tiny, hollow phenomena of nuclear weapons, to energy densities without having to cylinder (see Figure 2) made of a acquire and benchmark new data to deal with large total energies. We can high-Z material such as gold. The existing databases, and to test and thus build an incremental, exact beams enter through the open ends validate the physics computer codes description of the cumulative physics and strike the inner walls, where they for ensuring future reliability and that would make up a nuclear event. are absorbed and generate x rays that performance. The NIF will provide In the high energy densities at heat the interior of the hohlraum. The valuable data for predicting the which thermonuclear reactions occur, target is then bathed in this relatively performance of nuclear assemblies several distinctive phenomena uniform radiation field that heats it to and for testing the complex numerical predominate: very high material the desired temperature. codes used in weapons test compressions; unstable, turbulent In either direct- or indirect-drive calculations. hydrodynamic motion; highly ionized experiments, a second set of laser Creating thermonuclear burn in the atoms with high atomic numbers beams prepares a backlighter that laboratory will not only help us to (high-Z atoms); and radiation produces x rays that probe the target
25 NIF and National Security E&TR December 1994
NIF: High Energy Density at Low Total Energy The NIF will provide the high energy densities that to be achievable in the different regions of a burning are needed for thermonuclear reactions to occur. High DT capsule.1 Because the energy densities achieved in energy density, however, should not be confused with both modes of NIF operation—with and without high total energy. The two measures are independent: ignition—show significant overlap with the energy there can be high total energy with low energy density, density regime available from weapons tests, NIF can and high energy density with low total energy. Energy be used to investigate the high-energy-density physics density is the amount of energy per particle, or per unit subprocesses that occur in that regime. of volume. Nevertheless, physics investigations on NIF rely The NIF will operate at low total energies. High only on high energy density (i.e., how dense and hot energy densities can be produced on a small scale, and we can make a relevant target), not on total energy. in the case of the NIF, the scale is about a millimeter. (The total energy is only high enough to heat or drive This fact is vividly evident in Figure 2, which shows a a target that is big enough to yield measurable Nova hohlraum compared to a human hair; the large results.) The second, related, point is that the NIF or hohlraum used on NIF will be no larger than a dime. A any other AGEX (above-ground experiment) facility DT-filled capsule of the sort used for ignition studies cannot conduct integrative weapon tests because its on the NIF is very small—some 2 to 5 mm in diameter, total energy falls many orders of magnitude short of a tiny fraction the size of a thermonuclear secondary. that regime. NIF cannot be used as a testbed for Thus, although the energy density—the energy per weapon development. Our analysis of stockpile particle—in such a capsule is comparable to that in questions will therefore rely on computer calculations a thermonuclear secondary, the total energy is to put together different parts of the physics that we minuscule. Ignition on the NIF will have roughly study on NIF. the same explosive force—that is, total energy—as a gallon of gasoline. 4 The accompanying figure shows the importance of 10
the difference between the two measures of energy. 2 The figure plots energy density versus total energy 10 Weapons test for laser facilities, such as Nova and NIF, as well as 1 that achieved in a weapon test. Two regions are shown for NIF—without ignition and with ignition. 10™2 This distinction reflects the two alternative modes in Criticality
Energy, kt 10™4 which NIF will be used for experiments in physics Primary hydro NIF with ignition NIF related to weapons. NIF without ignition is ™6 characterized by the types of experiment described in 10 Nova this article. These experiments do not use fuel-filled 10™8 capsules; instead, the targets are foils and other materials that enable us to study the behavior of Specific energy density, kt/kg materials and media in the extreme conditions heated NIF energy densities will overlap those of nuclear weapons; the by x rays to high energy densities. shaded area represents the region of high energy density. Note, NIF with ignition characterizes experiments in however, that in measures of total energy, NIF energy regimes which the target is indeed a DT- or fuel-filled capsule, are well below the weapons test regime. and the calculated energy densities are those predicted
26 E&TR December 1994 NIF and National Security
and go to the detector. The measured we need know only an appropriate radiation by other parts of the absorption of this well-characterized average of the mean distances that a experiment. Figure 3 shows a typical and well-controlled x-ray source photon can travel before it is transmission experiment in local provides insight into the characteristics absorbed—that is, its Rosseland mean thermal equilibrium.2 Generally, as of the target material. The timing of free path between emission and the atomic number is increased, we the heating beams and backlighter absorption. (To analyze radiant need to either increase the temperature beams can be independently controlled energy transfer in a medium that is of the tamped target, or lower its to probe the targets under a wide not in thermal equilibrium, we would density, or both, in order to strip off variety of conditions. have to retain detailed transmission enough electrons from the atoms in Although the experimental information for every wavelength.) the target to ionize the plasmas to the program at NIF evolves from To achieve LTE for opacity desired level. Reaching such techniques developed on Nova, experiments, we use the indirect ionizations in the materials relevant to NIF experiments will probe the method described above, creating a weapons requires a much more qualitatively different regime of bath of x rays inside of a hohlraum. powerful laser than Nova, as Figure 4 plasmas characterized by radiation We thus make a diagnosable plasma shows. dominance at high-energy densities. in equilibrium and then determine Opacity research on the NIF will The three examples of laser-driven its x-ray transmission at the be conducted to evaluate new experiments described here figure appropriate wavelength. methods for predicting opacities. prominently in weapons research on To make a good plasma, the sample Such predictions are difficult, the NIF: opacity, equation-of-state, must be carefully tamped so that it because there are many transitions and hydrodynamic instability retains uniform density under heating and competing ionization stages that experiments. Each of these explores while hydrodynamically expanding can contribute to the opacity of a a different set of fundamental to the desired density during given element. The electrons in atoms phenomena characteristic of the measurement. The measurement is are arranged in shell structures of extreme conditions within a nuclear performed by passing backlighter increasing complexity (from weapon. In all three experiments, x rays through the hohlraum to probe innermost outward), and the shells are we can analyze how far the the tamped target. The target may conventionally labeled K, L, M, N, 1.8-megajoule drive of NIF can have sections of different thickness so etc. M-shell-dominated opacities push the energy density. that, when analyzing the film image, occur when an atom has been stripped we can separate the actual sample of enough electrons to open the M Opacity Experiments opacity from the absorption of shell. Complicated configurations of Loosely defined, opacity is the degree to which a medium absorbs radiation of a given wavelength. Knowledge of the opacity of a medium is crucial to understanding how the medium absorbs energy and transmits it from one place to another. This knowledge is important in nuclear weapons, where we care specifically about opacities at x-ray wavelengths, because this is the manner in which much of the energy in a weapon is transported. If we are analyzing radiant energy Figure 2. Two views of a typical Nova hohlraum shown next to a human hair. The end-on transfer in a medium that is locally in view shows a target within the hohlraum. Hohlraums for NIF will have linear dimensions thermodynamic equilibrium (LTE), about five times greater than those shown.
27 NIF and National Security E&TR December 1994
this sort play an important role in present ideas necessarily involve Equation-of-State Experiments determining opacity; for example, predicting key features with Understanding the physics of the M-band opacity of materials approximate statistical methods.3 nuclear weapons requires that we involves computing features of 108 Experiments are crucial in checking answer the practical question of how ionic configurations. Clearly this is that these models and predictions are much pressure is developed in a impossible in any direct way; correct. given material when a given amount of energy has been added. That is, we must determine the material’s equation of state: the thermodynamic relationship between the energy content of a given mass of the Film material and its pressure, temperature, and volume. Figure 5a shows a setup for a y X-ray shock breakout experiment—an source experiment for determining the x thermodynamic states created by the x passage of a single shock wave Bragg crystal through the subject material. By striking a material at standard temperature and pressure with single shocks of different strengths, we Hohlraum heated by 8 laser beams obtain a set of states that lie on the principal Hugoniot. Hugoniots not only describe how materials behave Figure 3. Schematic of a setup for absorption opacity experiments. The tamped target when shocked; they also serve as has two thicknesses. Laser beams entering the hohlraum generate an x-ray bath to heat the target. Backlighter x rays have significantly higher energies than the driver x rays. baselines for models of much of the thermodynamic space covered by the full equation of state. Hugoniot Figure 4. experiments present a rare case in Comparison of 90 which thermodynamic quantities opacity regimes such as pressure can be determined achievable on Nova Relativistic from the measurement of material and NIF and in M-shell velocities alone. weapons tests. In a shock breakout experiment,4 lasers create an x-ray bath inside a NIF 60 hohlraum. The x rays heat an absorbing material that ablates, or rockets off, and sends a shock wave Weapons test into a flyer plate. The flyer plate then hits a target that has two Nova precisely measured thicknesses or Atomic number 30 “steps.” The stepped target is observed end on by diagnostics that record the shock breakout. By measuring the difference in the timing of the shock breakout from the two sides of the step (Figure 5b), 0 we can determine the speed at which 050 200 350 500 650 the shock passed through the stepped Temperature, eV material. However, shock breakout
28 E&TR December 1994 NIF and National Security
experiments are difficult to interpret. This comparison is detailed in As before, the lasers heat the For example, we must determine Figure 6. hohlraum to create the x-ray heat whether the shock was planar: Did it bath that drives the experimental strike the surface of the stepped plate Hydrodynamic Instability package. Here, the x rays ablate a with uniform force, or did the flyer Experiments carefully designed sleeve that drives plate undergo “preheat” and The third experimental example a shock into the instability disassemble before it shocked the uses indirect drive to create experiment. Another laser beam step? Like opacity studies, equation- hydrodynamically unstable flows at makes an x-ray backlighter that of-state studies of these microscopic high compressions and Reynolds allows us to “photograph” the quantities are not only crucial to numbers. Unstable flows in highly growing instability. We must understanding the effects of high- compressed materials are ubiquitous carefully check the equation of state energy densities in weapons, but in weapon physics. We typically must of the subject materials, the planarity they are crucial to understanding determine the thickness of the mixing of the shocks, etc., before we can laser-based experiments themselves. layer between two materials caused compare the experiment with a Nova allowed us to study equations by the passage of a strong shock detailed simulation. Figure 8 shows a of state in the multimegabar pressure wave. Much research on turbulent comparison between a preliminary regions, but scaling equation-of-state flows relies on the assumption that instability experiment done on Nova experiments to pressures in the the flow is incompressible (like water and an arbitrary Lagrange-Eulerian important gigabar region require a in an ocean). However, here we are hydrodynamics calculation of the more powerful laser such as the NIF. interested in the situation where same setup done to test the ability of considerable compression and the code to probe large-scale sliding ionization can occur at the same time motions and deformations. (a) as turbulent, mixing motion. The program of work in instability Figure 7 shows the experimental research involves the study of setup for studying the instability shocked mixing layer growth, and the growth at an interface caused by the evolution of compressible turbulence passage of a controlled, planar shock. from the small-amplitude, linear X rays
Flyer Step plate Figure 6. target 104 Comparison of equation-of-state Weapons test regimes in flyer- plate experiments (b) NIF achievable on Nova 103 and NIF; also roughly indicated is the Time weapons-test regime.
Nova 102
Hugoniot
10 Pressure, Mbar, at constant volume Shock breakout
Figure 5. (a) Schematic of an x-ray-driven shock breakout experiment in colliding foils (the flyer plate and the step target are gold 1 foil); (b) shows the breakout timing 1 10 102 103 104 difference between the two sides of the Temperature, eV step, as captured on film.
29 NIF and National Security E&TR December 1994
growth regime (which is pertinent to control the width of such mixing To explore the full evolution to ICF implosions) to the full nonlinear layers as a function of time.5 It turbulence from the simpler linear evolution of turbulence. In the case would be of great importance to growth to the highly compressed of the mixing layers, there are weapons designers to pin down turbulent regime, the experiments suggestions for universal rules that these rules. naturally scale to a higher-energy laser, as Figure 9 shows.1
Other Weapons Experimental Realms The three types of experiments just described are among the most Four Four Nova Nova fundamental that will be performed on beams beams NIF—fundamental in the sense of X-ray drive probing phenomena that are virtually irreducible. Several areas of investigation are more integrative, probing phenomena arising from Doped polystyrene combinations or interactions of several different processes. These areas, Backlighter x rays described below, will also receive Instability surface intensive investigation on NIF. These investigations and those already Carbon foam enumerated will contribute to further X-ray imaging refinement of our weapons codes. Grid diagnostics Radiation Transport. We do not Laser Beryllium tube understand nuclear weapon processes beams well enough to calculate precisely the transfer of energy within a weapon.6 This transfer is crucial, since inadequate energy coupling can Figure 7. Schematic of a shock-driven hydrodynamic instability experiment. Changes in degrade yield or cause failure. In the the transmission profile of the backlighter x rays reaching the diagnostics allow the mixing at the shocked interface to be measured. era of nuclear testing, this incomplete
(a) 0 ns (b) 20 ns (c) 20 ns
Gold
2% brominated polystyrene
Beryllium 2 mm
0.1 g/cm3 carbon
Figure 8. Calculated development of an axial jet compared with experimental data. (a) The experimental setup in initial conditions (0 nanoseconds). Center and right are the event after 20 nanoseconds: (b) is the calculation of instability evolution by arbitrary Lagrange- Eulerian code; (c) shows the data from an instability experiment on Nova.
30 E&TR December 1994 NIF and National Security
understanding was not a problem Non-LTE Physics and X-Ray and perhaps the ICF implosions because the radiative energy transfer Lasers. The NIF will allow us to themselves. As the NIF program could be determined specifically. address important physics issues in for high-energy-density physics Weapon Output. The output of a situations in which plasmas are not in evolves, we expect these results to nuclear weapon includes neutrons, thermodynamic equilibrium (called influence the development of gamma rays, x rays, fission products, nonlocal thermodynamic further advanced diagnostics. activated elements, and exploding equilibrium, or non-LTE, physics). Code Development. To a large debris as kinetic energy. The ability Understanding non-LTE effects plays extent, our weapons computer to calculate the total spectral output an important role in determining x-ray codes embody the cumulative of a weapon is an ultimate measure outputs and in developing knowledge of weapons design. of our understanding of weapon temperature, ionization balance, and NIF data will help to improve and performance. kinetics diagnostics. It is also refine these codes to enable more Role of Ignition. Ignition essential in the development of laser- accurate modeling of results from encompasses two distinct weapons driven x-ray lasers. In the specific previous weapons tests and from physics issues: weapons physics area of x-ray lasers, NIF will allow us weapons physics experiments past measurements and maintenance of to explore x-ray laser pumping and future. Code development for critical skills. Several broad areas schemes with various materials and ICF research has focused on the appear uniquely accessible to an conditions. need to calculate many coupled ignition capsule: An important benefit of the x-ray physical processes in • The possible use to study onset of laser research is its use as an nonequilibrium conditions and to DT ignition in the presence of imaging system for ICF, weapons simulate all resulting experimental impurities, which can occur from a mix physics, and biology experiments. diagnostics within a single of intentional contaminants placed in High brightness, narrow bandwidth, computational model. The future the gas. The NIF will provide the only small source size, and short pulse need for higher accuracy and place where DT burn will be studied duration give the x-ray laser many increased engineering detail will in detail. advantages over conventional x-ray require better numerical methods, • The generation of x rays from the illumination sources.7 three-dimensional simulations, and capsules. This will challenge our Future imaging applications will massively parallel computers. ability to model and understand burn, include the study of laser-driven These growing ICF code energy balance, and transport mass ablation on the interior walls capabilities will be very important processes in a highly transient system of hohlraums, equation of state, to weapons researchers for having large gradients. • The NIF capsules will also provide an intense source of 14-MeV Figure 9. Whereas 5 1000 neutrons. These neutrons could be the moderate Weapons compressions on used to heat material instantaneously test Nova allow us to to temperatures more than 50 eV follow the transition without changing the material’s from linear instability to weak turbulence, volume. This unique capability may the high 4 NIF prove useful for other weapons 1.2 MJ compressions and 400 physics studies such as equation-of- 60–200 TW larger scale volumes state experiments. on NIF allow us to follow this all the Designing NIF fuel capsules to way to turbulent mix. tailor output and explore efficient Compression Compressive operation is a challenge to designers, 3 Nova hydrodynamics 100 computational physicists, and 30 kJ engineers. Such modeling will 3–10 TW challenge our understanding of many 2 25 fundamental processes associated Shock pressure, Mbar (at constant volume) with weapon design and will help 10 102 103 104 keep us intelligent in this critical Sample size/perturbation wavelength technology.
31 NIF and National Security E&TR December 1994
understanding the results of NIF combination of low total energy with 2. T. S. Perry et al., “Opacity Measurements in a weapons physics experiments. weapons-regime energy density will Hot Dense Medium,” Physical Review Letters allow us to pursue, besides ignition 67, 3784, (1991); L. Da Silva et al., “Absorption Measurement Demonstrating the Summary experiments, many nonignition Importance of ∆n = 0 Transitions in the Opacity experiments. These will allow us to of Iron,” Physical Review Letters 69, 438 Underground nuclear tests played improve our understanding of (1992); P. T. Springer et al., “Spectroscopic an important role in advancing materials and processes in extreme Absorption Measurements of an Iron Plasma,” knowledge of the physics of nuclear conditions by isolating various Physical Review Letters 69, 3735 (1992). 3. W. H. Goldstein, “A New Model for Heavy weapons. This knowledge led to fundamental physics processes and Element Opacity,” Procedures of the 8th progressively safer and more phenomena for separate investigation. Biennial Nuclear Explosives Design Physics effective performance and to retrofits Such studies will include opacity to Conference (NEDPC), Los Alamos National for older designs that improved their radiation, equations of state, and Laboratory, Los Alamos, NM, LA-12305-C, safety as well. Results from tests also hydrodynamic instability. In addition 364 (1992). 4. R. Cauble et al., “Demonstration of 0.75 Gbar enabled us to build and refine our to these, we will study processes in Planar Shocks in X-Ray Driven Colliding weapons computer codes. The which two or more such phenomena Foils,” Physical Review Letters 70, 2102 unilateral moratorium that the United come into play, such as in radiation (1993). States imposed on underground transport and in ignition itself. 5. D. L. Youngs, “Numerical Simulations of nuclear testing is likely to be Weapons physics research on NIF Turbulent Mixing by Rayleigh-Taylor Instability,” Physica (Utrecht) 12D, 32 (1984). followed by a Comprehensive Test- offers a considerable benefit to 6. T. S. Perry et al., “Experimental Techniques to Ban Treaty. The United States must stockpile stewardship not only in Measure Thermal Radiation Heat Transfer,” therefore have an alternative means enabling us to keep abreast of issues Journal of Quantitative Spectroscopy & of safely and securely maintaining its associated with an aging stockpile, Radiative Transfer 51, 273 (1994). stockpile of nuclear weapons and but also in offering a major resource 7. L. Da Silva et al., “X-Ray Lasers for Imaging and Plasma Diagnostics,” Proceedings of the ensuring their reliability. Stockpile for attracting and training the next 4th International Colloquium on X-Ray Lasers, stewardship is one of the functions to generation of scientists with nuclear Williamsburg, VA (1994). which the National Ignition Facility stockpile expertise. According to the 8. C. Callan et al., Science Based Stockpile will contribute by virtue of recent JASON report on stockpile Stewardship, The MITRE Corporation, JASON experimental work in the physics of stewardship, the NIF “will promote Program Office, 7525 Colshire Dr., McLean, VA, JSR-94-345 (1994). nuclear weapons. the goal of sustaining a high-quality The total energy output from group of scientists with expertise thermonuclear ignition on NIF will be related to the nuclear weapons For further an extremely tiny fraction of the program.”8 information energy from even the smallest nuclear contact Stephen B. Libby weapon—indeed it will be roughly Key Words: inertial confinement fusion; National (510) 422-9785. equivalent to the output of a gallon of Ignition Facility; Stockpile Stewardship Program. gasoline. Nevertheless, experiments will generate the same energy Notes and References densities—energies per particle—that 1. S. W. Haan et al., Design and Modeling of Ignition Targets for the National Ignition occur in nuclear weapons. This Facility, Lawrence Livermore National Laboratory, Livermore, CA, UCRL-JC-117034 (1994).
32 The Role of NIF in Developing Inertial Fusion Energy
The National Ignition Facility will demonstrate fusion ignition, which is central to proving the feasibility of inertial fusion energy. It will also help us determine the full potential of this alternate energy source.
MERICA’S dependence on burning hydrogen fuel by water only dependable in the limited desert A imported oil currently accounts electrolysis. Clearly, an alternative regions of the world, some fusion for a trade deficit of $60 billion per energy source is needed. fuels can be extracted from seawater, year. As time passes, the world At present, there are only three making them available to all demand for energy will continue to known inexhaustible primary energy countries of the world. Fusion power grow, in part for demographic sources for the future: the fission plants, if they can be developed reasons, such as the rapidly breeder reactor, solar energy, and economically, will also have many increasing energy use per capita in fusion. All are superior to coal or advantages over fission. The developing Asian and Latin oil-based power plants because they radiation hazard presented by fusion American countries together with are environmentally cleaner and power plants can potentially be the expected doubling of the world’s ecologically safer. They will release thousands of times smaller than that population over the next 50 years. little or no radioactivity per unit of fission power plants, with proper Our deficit and world energy of power, as do coal mining and choice of materials. demand will also grow for burning in the form of radon, environmental reasons, particularly uranium, and thorium,1 and they will Two Approaches to Fusion in the United States, which will need emit none of the gases (carbon a substantial new source of energy dioxide and nitrogen dioxide) that Fusion combines nuclei of light to power zero-emission contribute to greenhouse effects. elements into helium to release energy transportation and reduce urban air Fusion, however, offers certain and is the same process that powers the pollution, to charge batteries in advantages over fission and solar sun. As noted, the fuel for fusion electric cars, or to produce clean- energy. Unlike solar energy, which is (deuterium and lithium, which can
33 Inertial Fusion Energy E&TR December 1994
capture a neutron to regenerate tritium) plasma inside a large, high-vacuum, the most complex plant equipment. can be extracted from seawater. The toroidally-shaped vacuum chamber.3 The separation between the driver and most likely fuel for any approach to The IFE approach to fusion, in fusion chamber also allows a single fusion energy is DT (either liquid, gas, contrast, is one of the goals of the driver to drive multiple fusion or a combination as in inertial fusion National Ignition Facility (NIF), the chambers, thus permitting flexibility in energy targets), which is a mixture of subject of this article. This approach the required chamber pulse rate and deuterium and tritium isotopes of uses powerful lasers or ion beams lifetime and allowing for the staged hydrogen. This DT must be heated (drivers) to demonstrate fusion deployment of several fusion chambers until it is hotter than the interior of ignition and energy gain in the to achieve low-cost electricity.4 the sun, but it fuses at the lowest laboratory by imploding and igniting • Progress in inertial fusion temperature of any fusion fuel. small, spherical DT fuel capsules experiments on the Nova laser facility To explore the feasibility of (targets) to release fusion energy in a at LLNL allows the most important economical fusion power plants, the series of pulses (see box on p. 38). In target physics affecting target gain to Department of Energy is currently its quest to accomplish this goal, the be modeled successfully by computer developing two primary approaches NIF supports a primary national codes such as LASNEX. When these to fusion energy—magnetic fusion security mission for science-based computer models are better confirmed energy and inertial fusion energy stockpile stewardship (see preceding by target-ignition tests in the NIF, (IFE). Both approaches use DT fuel article) and secondary missions they can be used to design targets for and offer the potential advantages supporting energy and basic science. future IFE power plants. described above, but they must • IFE fusion chambers do not require be developed more fully before The IFE Power Plant a hard vacuum; therefore, a wider economical fusion energy can be range of materials can be used to assured. Because the two approaches Figure 1 is a conceptual view of a achieve very low activation and use different physics and present generic IFE power plant, showing its radioactive waste. IFE chamber different technological challenges, four major parts—the driver, target designs that protect the structural the National Energy Policy Act of factory, fusion chamber, and steam- walls with thick, renewable fluid 19922 calls for both to be developed turbine generator (balance of plant). flows are also possible, which will to the demonstration (DEMO) stage. This figure demonstrates some of the eliminate the need to replace the Magnetic fusion ignition is the principal advantages of IFE as an chamber’s internal structural goal of the proposed International energy source: components periodically.5,6 Thermonuclear Experimental Reactor, • The driver and target factory are • The cost of developing IFE can be which uses strong superconducting separated from the fusion chamber to diluted by sharing NIF for defense magnets to confine a low-density DT avoid radiation and shock damage to and energy missions.
How the NIF Can Help Develop IFE Beams Fusion Targets Target Driver chamber factory In 1990, the Fusion Policy Advisory Committee7 recommended Heat that inertial fusion ignition be Heat demonstrated in the NIF as a key Turbine prerequisite to IFE. In addition to generator Recycled target material ignition, IFE needs development in three major areas of technology: • High-gain, injectable, mass- produced, low-cost targets. • An efficient high-pulse-rate driver. • A suitable, long-lasting fusion chamber. Figure 1. Conceptual view of a four-part IFE power plant, showing the driver (either laser A major facility following the NIF, to or ion particle beams), the target factory, the fusion chamber, and the turbine generator that produces electricity. be called an Engineering Test Facility
34 E&TR December 1994 Inertial Fusion Energy
(ETF), is planned to test the feasibility (a) of these three areas of technology integrated together. The ETF will explore and develop the high pulse rate (several pulses per second) and overall system efficiency needed for economical IFE power production.
Filling Technological Needs Indirect-drive laser target Targets. The targets for IFE must be capable of high energy gain. Energy gain is achieved when the fusion energy released from a reaction exceeds the energy that was put into the target by a laser or ion-beam driver. For high gain, the energy released from the target should be more than 50 to 100 times greater than the driver energy. Tests of Laser-driven model inertial fusion target physics and of heavy-ion target ignition in the NIF will allow us to predict confidently the performance of several candidate IFE target designs. (b) For IFE targets to produce electricity at competitive rates (less than 5 cents per kilowatt-hour), they must be mass-producible at a cost of less than 30 cents each. This means new target-fabrication techniques must be researched and developed. In addition, we will have to develop and test methods of target injection and tracking for driver–target engagements at pulse rates of 5 to10 Hz. The NIF can test the performance of candidate mass-produced IFE targets and, at least for a limited number of pulses in Direct-drive laser target a short burst, the associated target- injection methods. The option of using direct-drive in addition to indirect-drive targets (see the box on pp. 38-39) is under consideration for the NIF. If direct- drive implosion experiments on the Omega Upgrade (an upgrade of the Figure 2. The NIF target area and beam-transport system (a) for indirect-drive experiments Omega glass laser to 60 beams) at the relevant to either laser or heavy-ion targets and (b) for direct-drive laser targets only. Note that the target area and beam-transport system in the baseline system (a) could be University of Rochester’s Laboratory reconfigured to design (b) by the repositioning of 24 four-beam clusters, making direct-drive of Laser Energetics are successful, experiments possible. this option will be exercised, and both direct- and indirect-drive targets will be examined on the NIF. Figure 2
35 Inertial Fusion Energy E&TR December 1994
shows the laser-beam configurations needed for ignition. Regardless of and develops more efficient, around the NIF fusion chamber that driver type, all IFE drivers for power high-repetition-rate IFE drivers will be used to conduct indirect- plants need a similar combination of (principally heavy-ion beam drive (Figure 2a) and direct-drive characteristics: high pulse-repetition accelerators) for power plants, the experiments (Figure 2b). Figure 2 rates (5 to 10 Hz) and high efficiency NIF can be built and achieve its also shows examples of indirect- and (i.e., driver output beam mission with current solid-state laser direct-drive targets that can be tested energy/electrical energy input to the technology. Diode-pumped solid- in each configuration. driver greater than 10 to 20%, state lasers (DPSSLs), which also Figure 3 shows a heavy-ion-driven depending on target gain). In build on NIF laser technology, may target for IFE (Figure 3a) compared addition, they should be highly prove to be a backup to the heavy-ion with a modified laser-driven target reliable and affordable when accelerator. Using laser-diode arrays (Figure 3b) that is designed to model compared with nuclear generator under development for industrial more closely the IFE heavy-ion plants. applications, DPSSL drivers may target. The latter (Figure 3b) The Energy Research branch of ultimately improve the efficiency, illustrates how the NIF could use a DOE is developing heavy-ion pulse rate, and cost of solid-state laser to test the soft-x-ray transport accelerators to meet the above lasers enough for use as IFE drivers. and plasma dynamics inside a higher- requirements.8 Heavy-ion drivers can Figure 4 shows a schematic DPSSL fidelity hohlraum geometry similar to be either straight linear accelerators driver layout that, except for the that in Figure 3a. Note that the (linacs) or circular (recirculating) diode pump arrays, has an capsule performance and implosion beam accelerators like that shown architecture similar to that being symmetry requirements for indirect- in the box on p. 39. The Defense developed for the NIF. drive targets are independent of Programs branch of DOE, in contrast, Fusion chambers. IFE needs whether the x rays are generated with is pursuing advanced solid-state fusion chambers where target fusion a laser or an ion-beam driver. We can lasers, krypton–fluoride lasers, and energy can be captured in suitable also use the NIF for special laser- light-ion pulsed-power accelerators coolants for conversion into target experiments that simulate many for defense applications that may, electricity. To allow high pulse rates, aspects of heavy-ion targets. with improvements, lead to alternate these chambers will have to be built Drivers. Although the key target- IFE drivers. Diode-pumped solid- so they can be cleared of target debris physics issues that NIF will resolve state lasers will be able to build on in fractions of a second. Further, they are largely independent of the type the laser technology being developed must be reliable enough to withstand of driver used, it is essential in for the NIF.9 the pulsed stresses of one billion evaluating the potential of IFE to While other DOE research shots (30 years of operation) without determine the minimum driver energy examines the direct-drive option structural failure. They should also
Figure 3. The NIF can (a) (b) test important heavy- ion physics issues, Hohlraum 0.35-µm laser beams such as soft-x-ray Shield in two cone arrays transport and drive symmetry, hohlraum Heavy-ion plasma dynamics, beams capsule-implosion hydrodynamics, and Hohlraum Shield mix. Here the modified Capsule laser-driven target in (b) shows how the NIF 14–16 mm could use a laser to Doped Doped gas test x-ray transport and plastic plasma dynamics in a hohlraum geometry like that shown in (a). DT fuel capsule
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use low-activation materials (such as Gas-cooled Gas-cooled Pockels cell molten salt coolants or carbon- gain medium Diodes composite materials) to minimize the Pulse Polarizer generation of radioactive waste. injection Many IFE power plant studies have already found conceptual designs that meet these goals, but actual tests will Gas-cooled Spatial filter be required. What we learn from the harmonic NIF fusion chamber can provide data Dichroic conversion Output pulse to benchmark design codes for future mirror IFE chamber designs. First wall Other Needs. In addition to fusion ignition, the NIF will provide important data on other key IFE Neutron power plant needs. These needs absorber include wall protection from target Heated fused debris and radiation damage, chamber silica final optic Reactor clearing, rapid target injection, and chamber precision tracking. The NIF will also be used to provide data that Figure 4. The diode-pumped, solid-state laser driver for IFE is similar in design to that can benchmark and improve the being developed for the NIF. Although the NIF architecture will not include the diode pump predictive capability of various arrays shown here, it will serve as an experience and technology base for the IFE driver. This computer codes that will be needed to figure shows a DPSSL IFE laser designed like the NIF in that it uses a multipass laser design future IFE power plants, to amplifier in which the laser beam is amplified by passing back and forth between the cavity select among possible IFE technology mirrors four times before a Pockels cell optical switch sends the amplified beam out to the options, and to improve our final optics and the target. However, the DPSSL uses light from arrays of efficient diode understanding of IFE target and lasers to pump the amplifier from the ends rather than using light generated from flash chamber physics. lamps on the sides of the amplifier as in the present NIF design. One predictive capability that can calculate and interpret material responses to neutron damage is a 100 technique called molecular dynamic Cu 25 keV PKA (a) simulation (MDS).10 MDS calculates Capsule responses at the atomic level by 1015 n/shot 50 quantifying how a three-dimensional array of atoms responds to knock-on 2–5 cm atoms that impinge on the matrix 0 from a range of angles and with a Shield Length, Å range of energies as a result of an –50 incident neutron flux. Potentially, Sample array MDS capabilities may include t = 4.61 ps predicting, for a material, the number –100 of vacancies and interstitials that will –100 –50 0 50 100 result from a neutron irradiation Length, Å pulse, as well as the cluster fraction of defects, atomic mixing and Figure 5. A molecular-dynamic simulation experiment on the NIF. Samples of 2- to 5-cm-width solute precipitation, and phase material placed within 20 cm of a NIF yield capsule (at left) will receive a significant exposure to transformations. Figure 5 shows how 14-MeV neutrons (1015 neutrons per shot per square centimeter of sample area). The tantalum samples of materials exposed to the shield will stop most x rays. Electron microscope images of the damage sites will be compared target neutron emission in a NIF shot to MDS code predictions as shown at right. The figure shows a typical damage site in a copper can provide data that confirm the sample due to primary knock-on copper atoms (25 keV primary knock-on atoms [PKA]) arising MDS model calculations. from collisions of fast neutrons with copper atoms in the sample.
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Producing Inertial Fusion Energy An inertial confinement fusion (ICF) capsule or An inertial fusion power plant would typically fire a target is a small, millimeter-sized, spherical capsule continuous series of targets at a pulse rate of 6 Hz. The whose hollow interior contains a thin annular layer of series of fusion energy releases thus created in the form liquid or solid DT fuel (a mixture of deuterium and of fast reaction products (helium alpha particles and tritium isotopes of hydrogen). The outer surface of the neutrons) would be absorbed as heat in the low- capsule is rapidly heated and ablated either directly by activation coolants (fusion chamber) that surround the intense laser or ion particle beams (drivers), called targets. Once heated, the coolants would be transferred direct drive (a below), or indirectly by absorption of to heat exchangers for turbine generators that produce soft x-rays in the outer capsule surface. These soft electricity. The inertial fusion power plant example x-rays are generated by driver beams hitting a nearby shown below uses jets of molten salt, called Flibe, metal surface, a process called indirect drive (b below). surrounding the targets inserted into the fusion The rocket effect caused by the ablated outer capsule chamber. The molten salt jets absorb the fusion energy material creates an inward pressure causing the capsule pulses from each target while flowing from the top to to implode in about 4 nanoseconds (a nanosecond is the bottom. The molten salt is collected from the one billionth of a second). The implosion heats the DT bottom of the chamber and circulated to steam fuel in the core of the capsule to a temperature of about generators (not shown) to produce steam for standard 50 million degrees Celsius, sufficient to cause the turbine generators. This particular power plant example innermost core of the DT fuel to undergo fusion. The uses a ring-shaped ion beam accelerator as a driver, but fusion reaction products deposit energy in the capsule, there are also laser driver possibilities. further increasing the fuel temperature and the fusion The minimum driver energy required to implode the reaction rate. Core fuel ignition occurs when the self- capsule fast enough for ignition to occur is typically heating of the core DT fuel due to the fusion reaction about a megajoule, the caloric equivalent of a large product deposition becomes faster than the heating due doughnut. Since this driver energy must be delivered to compression. The ignition of the core will then in a few nanoseconds, however, a power of several propagate the fusion burn into the compressed fuel hundred terawatts (1 terawatt = 1 million megawatts) layer around the core. This will result in the release of will be needed. For reference, the entire electrical much more fusion energy than the energy required to generating capacity of the United States is about one- compress and implode the core. half terawatt.
(a) Direct-drive targetstargets areare directlydirectly heatedheated andand implodedimploded (b) Indirect-drive target fuelfuel capsulescapsules areare implodedimploded byby softsoft xx raysrays by intenseby intense driver driver beams. beams.generated generated by intenseby intense lasers lasers or ionor ion beams beams at theat the ends ends of aof high-Z a high-Z radiationradiation case case (“hohlraum”). (“hohlraum”). Soft x ray DT fuel layer Vacuum Fuel capsule Ion particle or Laser beam laser beams
beams Heavy-ion
Ignition fusion reaction High-Z radiation case High-Z radiator Fuel capsule products (alpha particles) Indirect-drive Indirect-drive heat the compressed fuel laser target heavy-ion target core to fusion temperature causing implosion
38 E&TR December 1994 Inertial Fusion Energy
The rapid thermal motion of the deuterium and by many driver beams. In indirect-drive targets, the fuel tritium nuclei will cause a significant fraction of them to capsule is placed inside a thin-walled cylindrical container collide and fuse into helium ash before the compressed (hohlraum) made from a high-atomic-number material, fuel mass from the implosion has had time to rebound such as lead. Here a smaller number of driver beams (with and expand. The reaction products will fly away with a total energy similar to that required for direct drive) are several hundred times more kinetic energy than the directed at the two ends of the hohlraum cylinder, where thermal energy of the deuterium–tritium ion pair before the driver beam energy is converted to soft x rays, which, fusion occurred. If some inefficiency in coupling the in turn, lead to the compression of the fuel capsule. The driver laser or ion-beam energy into compressing and hohlraum spreads the soft x rays uniformly around the heating the capsule is taken into account, the ratio of capsule to achieve a symmetric implosion. fusion energy produced by the target to the driver beam For its driver, the NIF will use a solid-state glass energy input to the target—called the target gain—can laser to deposit the externally directed energy. This range from 50 to 100 in a typical power plant. Once this laser will deliver 1.8 MJ of laser light energy (in pulses fusion heat is converted into electricity, the average spaced several hours apart) to test the minimum energy amount of electricity needed to energize the driver required for target ignition and the scaling of target gain would be 5 to 10% of the total plant output. so that any type of target optimized for future power Inertial fusion targets are of two basic types: direct plants can be designed with confidence. DOE–Energy drive and indirect drive, both of which will be tested by Research is developing heavy-ion beam accelerators as the NIF to determine the best target for inertial fusion its leading candidate drivers for future IFE power energy. A direct-drive target consists of a spherical plants, while DOE–Defense Programs is developing capsule driven directly by laser or ion beams. So that the other driver technologies for ICF research, including capsule will implode symmetrically and achieve high advanced solid-state lasers, that could lead to alternative gain, it must be illuminated uniformly, from all directions, IFE drivers as well.
Oscillating Flibe jets Rotating shutter
Recirculating heavy-ion induction accelerator Bypass pumps
39 Inertial Fusion Energy E&TR December 1994
Developing Fusion Power to test FPT issues. NIF’s relevance • Neutronics data on radioactivity, Technology to FPT has to do with both its nuclear heating, and radiation The NIF can also help develop prototypical size and configuration shielding. fusion power technology (FPT), and its prototypical radiation-field The NIF will also be able to which includes the technologies (neutrons, x rays, and debris) spectra demonstrate the safe and needed to remove the heat of fusion and intensity per shot. The most environmentally benign operation and deliver it to the power plant. The important limitation of NIF for FPT that is important for IFE, including primary functions of such components experiments is its low repetition rate handling tritium safely and in IFE power plants are to convert (low neutron fluence), and its most maintaining minimum inventories energy, to produce and process important contributions to FPT of low-activation materials. It is tritium, and to provide radiation development for IFE are related to: designed to keep radioactive shielding. The dominant issues for • Fusion ignition. inventories low enough to qualify as FPT in IFE power plants concern • Design, construction, and operation a low-hazard, non-nuclear facility component performance (both nuclear of the NIF (integration of many according to current DOE and federal and material) so as to achieve prototypical IFE subsystems). guidelines, thus setting the pattern for economic competitiveness and to • Viability of first-wall protection future IFE plants. Similar non-nuclear realize safety and environmental schemes. design goals will also be met for IFE advantages. In this regard, NIF will • Dose-rate effects on radiation power plants if the design selected provide valuable FPT information damage in materials. for the fusion chamber is carefully gained from the demonstrated • Data on tritium burnup fractions in followed and the low activation performance and operation of the the target, tritium inventory and flow- materials for it are used. The NIF NIF facility itself, as well as from rate parameters, and the achievable will also demonstrate proper quality experiments designed specifically tritium breeding rate in samples. assurance in minimizing both
Defense Programs (ICF) Target physics National ignition Design Construct Facility (NIF) experiments
Ignition and gain Energy Research (IFE) demonstration Heavy-ion driver Tests ILSE experiments technology (ILSE Facility at LBL) Driver feasibility demonstration Target fabrication and fusion chamber Technology development and tests technologies Energy Target and Power applications fusion plant decision chamber ETF Power plant demonstration Engineering Test Facility (ETF) Stage 1 Stage 2 TestsTechnology DEMO integration and DEMO
1995 2000 2005 2010 2015 2020 2025
Figure 6. The timeline for IFE development includes ICF ignition and gain, IFE technologies, and the IFE power plant.
40 E&TR December 1994 Inertial Fusion Energy
occupational and public exposures to 10 Hz). As indicated, it will also opportunity for fusion development radiation. drive multiple test fusion chambers afforded the DOE by a modest for defense, IFE (ETF), basic heavy-ion driver program that An Integrated IFE Development science, and materials research, using leverages off the extensive target Plan a single driver to save costs. Its total program being conducted by Defense construction cost is expected to be Programs. Consequently, we urge To capitalize on the success of $2 billion in today’s dollars, and its the DOE to reexamine its many fusion ignition in the NIF, which is life-cycle costs to the year 2020 are programs, both inside and outside of expected to occur around the year expected to be $3 billion. Then a Energy Research, with the view to 2005, an Engineering Test Facility successful IFE chamber from embark more realistically on a (ETF) will be needed. This facility previous tests will be upgraded to a heavy-ion program. Such a program will test the fusion power plant higher average fusion power level. would have the ILSE as a technologies called for in the 1990 This upgrade, which is expected to centerpiece, and be done in Fusion Policy Advisory Committee7 provide a DEMO (net electric-power coordination with the program to the 1992 National Energy Policy Act demonstration) by the year 2025, demonstrate ignition and gain by of 19922 plans. A decision to move is shown as the last phase of the Defense Programs.”8 forward with the ETF will also upgradable ETF/LMF facility. depend on the timely demonstration Note in Figure 6 that the decision Summary of a feasible, efficient, high-repetition to initiate the ETF/LMF facility, IFE driver. including selecting an ETF/LMF When the NIF demonstrates fusion Figure 6 shows existing and driver, will be made after ignition is ignition, which is central to proving proposed facilities in an integrated demonstrated in the NIF. An ETF the feasibility of IFE, it will tell us plan for IFE development. In addition with a single driver can be designed much about IFE target optimization to the NIF, they include: to test several types of fusion and fabrication, provide important • The Induction Linac Systems chambers at reduced power, greatly data on fusion-chamber phenomena Experiment (ILSE). Plans call for reducing the cost of IFE development and technologies, and demonstrate this proposed heavy-ion accelerator through a demonstration power the safe and environmentally benign test facility to be built at the plant. This parallel approach to operation of an IFE power plant. In Lawrence Berkeley Laboratory. Its IFE development has already accomplishing these tasks, the NIF mission will be to demonstrate the been endorsed by many review will also provide the basis for future feasibility of a heavy-ion driver for committees, including the National decisions about IFE development IFE by testing critical, high-current, Academy of Sciences,11 the Fusion programs and facilities such as the ion-beam-induction accelerator and Policy Advisory Committee,7 the ETF. Furthermore, it will allow the focusing physics with properly Fusion Energy Advisory Committee, U.S. to expand its expertise in inertial scaled-down ion energy and mass. and the Inertial Confinement fusion and supporting industrial ILSE may be built in two stages for Fusion Advisory Committee.12 technology, as well as promote U.S. a total construction cost of about DOE–Defense Programs (using the leadership in energy technologies, $46 million. The ILSE experiments NIF for fusion ignition and gain provide clean, viable alternatives to should also be completed by the year demonstration) and DOE–Energy oil and other polluting fossil fuels, 2005. Research will play complementary and reduce energy-related emissions • The ETF/Laboratory roles in driver development and other of greenhouse gases. Microfusion Facility (ETF/LMF). IFE technologies. This multiuser facility for both Chairman Robert Conn, in Key Words: drivers—laser drivers, heavy-ion defense experiments and IFE reporting the recommendations of drivers; energy sources—fission breeder reactors, technology development will be able the 1993 Fusion Energy Advisory fossil fuels, inertial fusion energy, magnetic fusion energy, solar energy; fusion chambers; fusion to produce target-fusion energy Committee to then DOE Energy power technology; International Thermonuclear yields at full-power plant scale (200 Research Director Will Happer, Experiment; National Ignition Facility; targets— to 400 MJ) and high pulse rates (5 to wrote: “We recognize the great direct-drive targets, indirect-drive targets.
41 Inertial Fusion Energy E&TR December 1994
References Energy Research Director William Happer 1. Sources, Effects, and Risks of Ionizing (April 1993). Radiation, United Nations Scientific Committee 9. C. D. Orth, S. A. Payne, and W. F. Krupke, A on the Effects of Atomic Radiation, 1988 Diode-Pumped Solid-State Laser Driver for Report to the General Assembly, United Inertial Fusion Energy, Lawrence Livermore Nations, ISBN-92-1-142143-8, 090000P National Laboratory, Livermore, CA, UCRL- (1988), Table 33, p. 114 and Table 72, p. 232. JC-116173 (1994). 2. The National Energy Policy Act of 1992, Public 10. J. Wong, T. Diaz de la Rubia, M. W. Guinan, Law 102-486 (1992). M. Tobin, J. M. Perlado, A. S. Perez, and J. 3. E. M. Campbell and D. L. Correll, “Lasers,” Sanz, The Threshold Energy for Defect Energy and Technology Review, Lawrence Production and FIC: A Molecular Dynamics Livermore National Laboratory, Livermore, Study, Lawrence Livermore National CA, UCRL-52000-94-1/2, (1994), pp. 53–55. Laboratory, Livermore, CA, UCRL-JC-114789 4. B. G. Logan, R. W. Moir, and M. A. Hoffman, (1993). Also see Procedures 6th International Requirements for Low Cost Electricity and Conference on Fusion Reactor Materials, Hydrogen Fuel Production from Multi-Unit Stresa, Italy (1993). Inertial Fusion Energy Plants with a Shared 11. National Research Council, Second Review Driver and Target Factory, Lawrence of the Department of Energy’s Inertial Livermore National Laboratory, Livermore, Confinement Fusion Program, Final Report, CA, UCRL-JC-115787 (1994), to be published (National Academy Press, Washington, DC, in Fusion Technology. 1990). 5. R. W. Moir, R. L. Bieri, X. M. Chen, T. J. 12. Inertial Confinement Fusion Advisory Dolan, M. A. Hoffman, P. A. House, R. L. Committee for Defense Programs, letter to Leber, J. D. Lee, Y. T. Lee, J. C. Liu, G. R. Assistant Secretary for Defense Programs Longhurst, W. R. Meier, P. F. Peterson, R. W. (May 20, 1994). Petzoldt, V. E. Schrock, M. T. Tobin, and W. H. Williams, “HYLIFE-II: A Molten-Salt Inertial Fusion Energy Power Plant Design— Final Report,” Fusion Technology 25, 5–25 (1994). 6. J. H. Pitts, R. F. Bourque, W. J. Hogan, W. M. Meier, and M. T. Tobin, The Cascade Inertial Confinement Fusion Reactor Concept, Lawrence Livermore National Laboratory, Livermore, CA, UCRL-LR-104546 (1990). 7. Fusion Policy Advisory Committee, Review of the U.S. Fusion Program, Final Report, Washington, DC (1990). 8. “Findings and Recommendations for the Heavy- Ion Fusion Program,” Report of the Fusion For further information contact Energy Advisory Committee (FEAC) to DOE B. Grant Logan (510) 422-9816 or Michael T. Tobin (510) 423-1168.
42 Science on the NIF
The National Ignition Facility will allow scientists to explore a previously inaccessible region of physical phenomena that could validate their current theories and experimental observations and provide a foundation for new knowledge of the physical world.
AST March, at the behest of the of research in which the NIF and other anything from a terrestrial lightning L Department of Energy, a group high-energy lasers could be used to bolt (approximately 104 K) to the core of scientists from around the world advance knowledge in the physical of a carbon-burning star (109 K). convened at the University of sciences and to define a tentative In short, this versatile and powerful California, Berkeley, to discuss program of high-energy laser research tool would enable scientists in potential scientific applications of the experiments. The scientists determined these fields to explore previously National Ignition Facility (NIF). The that the NIF as well as other high- inaccessible regions of the physical NIF is a 192-beam, neodymium glass energy lasers have effective parameter space that could validate laser that the Department of Energy application in areas relating to current theories and experimental will use to obtain information on high- astrophysics, hydrodynamics, material observations and provide a foundation energy-density matter, which is properties, plasma physics, and for new knowledge. Following are the important for the generation of energy radiation physics. Their determination areas where the NIF is expected to through inertial confinement fusion. was based on the wide range of make notable contributions to science This information will also be used to experiments already being performed and applied science. maintain the skills and information on high-energy lasers, the diverse base necessary to manage the nation’s interests of the scientific community, Astrophysics nuclear stockpile, and most and the extraordinary range of physical importantly from a scientific conditions that would be achievable To obtain information about stars perspective, to pursue basic and with the NIF—densities from one and other astronomical bodies, the applied research. millionth the density of air to ten times astrophysicist produces a sample The objective of the gathering in the density of the solar core and plasma in the laboratory and studies Berkeley was to identify those areas temperatures that would be relevant to its physical properties. For example,
43 Science on the NIF E&TR December 1994
to determine a star’s structure astrophysical community is interested forces some of the electrons to be throughout the various stages of its in developing this potential with very energetic, thereby affecting the lifetime—that is, its mass, heat, high-energy lasers, especially with pressure and internal energy. luminosity, and pulsational the NIF. The theory of stellar evolution is instabilities—the astrophysicist may affected by uncertainties in the require information on the radiative Equation of State equation of state in a few areas. For opacity of a plasma that mimics Under many circumstances, the example, in white dwarfs—the the outer stellar envelope and/or equation of state of a stellar interior “nuclear ashes,” or compressed information on the equations of state is simple: most of the gas is cores—of stars that have shed their (how density and temperature relate to hydrogen and other light elements hydrogen-containing outer layers and the pressure or internal energy) of a that have lost a good portion of their gone through most of their evolution, plasma that resemble the dwarf star electrons. Unfortunately, the the pressure from degenerate electrons interior. Furthermore, to get a better equation of state of the star’s interior supports the material against gravity. idea of a star’s structure during is not as simple when the star is in Near the surface of the material, various stages of evolution, the its later stages of evolution. Density however, degenerate electrons lose astrophysicist may be interested in is quite high, and the material their dominance. The ions then take producing a stellar-like plasma to becomes strongly coupled; that is, over, setting the specific heat and investigate its nuclear reaction rates. the ions interact strongly and no establishing the rate at which the The key to success in all of these longer behave as free particles. This white dwarf will cool, a process that experiments is the ability to behavior is often accompanied by takes many millions of years. synthesize the very hot plasmas that electron degeneracy. This leads to characterize the stellar environment the tendency of the electrons to fill Radiative Opacity during stages of stellar evolution. The up certain energy states in a way that The radiative opacity of the material in stellar interiors plays a key role in determining how stars evolve: what the maximum mass of a stable star is, how hot and how luminous the star is while it burns its 0.74 hydrogen fuel, what pulsational instabilities may occur. Previous to 6 Mfl recent experiments, astrophysicists 5 Mfl were using a set of opacity 0 calculations that predicted a very /P
1 0.72 P narrow range of surface temperatures 4 Mfl for the hydrogen-burning phase of 4 Mfl 5 Mfl stellar evolution for all stars—in other 6 Mfl words, all stars were very hot at this 0.70 time despite their differences in mass. 7 Mfl These calculations also tied pulsation instability to stellar luminosity and 1.0 3.0 5.0 7.0 mass, which resulted in the wrong P0 pulsation periods. The solution then was to correct the opacities used in the calculations. Figure 1. The effects of opacity on pulsations of Cepheid stars. The stellar mass (Mfl) is In the last few years, a group of predicted from the ratio of the first harmonic, P1, to the fundamental, P0. The black lines are the results based on the new opacity calculations; the red lines are the results based on the physicists at LLNL has been able to old opacity calculations; the dots are the observed ratios. The new calculations, which reduce the discrepancy by using a predict a much wider range of surface temperature for the hydrogen-burning phase of new set of opacity calculations. stellar evolution, put observation and theory in agreement. Experiments on the NIF will Although these are definitely closer allow us to verify these effects and confirm our theoretical predictions. to observation (Figure 1) than the
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previous calculations, they still could be counted after the event, or oxygen cycle; the latter two form embody many approximations. Thus, scintillators could be positioned part of the proton–proton chain of to verify opacity at the relevant around the target to detect their pulse hydrogen-burning reactions in solar- conditions, astrophysicists will need of 2-MeV gamma rays during the like stars. to conduct direct experiments. A event. Because the events would be high-energy facility like the NIF will produced all at once in this type of Hydrodynamics allow them to do this. experiment, the usual low signal-to- noise ratio would be avoided, making Hydrodynamics is the study of Thermonuclear Reaction Rates it easy to distinguish the reactions fluid motion and the fluid’s Although astrophysicists have from ambient room background noise. interaction with its boundaries. The been studying nuclear reactions for NIF experiments may further our NIF will allow us to further our decades, their experiments have understanding of nuclear reactions understanding of the hydrodynamics rarely achieved the energies at which that explore the proton–proton chain of inertial confinement fusion and such reactions occur in stellar of hydrogen-burning reactions in shock wave phenomena in the galaxy. environments. With the NIF, they solar-type stars and also the carbon- Because the NIF will be capable of will be able to conduct experiments nitrogen-oxygen cycle. As examples, depositing a large quantity of energy that achieve such energies. three reactions of interest in in a large amount of material over a Figure 2 shows the temperature and astrophysics include the 12C(p,Ì)13N long time and at high densities, it will density regimes attainable with the reaction, the 3He(3He,2p)4He be able to generate hydrodynamic NIF and compares them to the reaction, and the 3He(4He,Ì)7Be flow conditions that are much more conditions of a star as it progresses reaction. The first plays an important extreme than those generated by through each phase of evolutionary role in every star’s carbon-nitrogen- wind tunnels, shock tubes, or even nuclear burning. The first regime, which extends up to about 14 keV, shows the temperatures and densities Neon- that may be reached in a laser-heated Carbon- oxygen- hohlraum or an imploding capsule burning Helium- silicon- without nuclear ignition and includes 1030 Hydrogen- burning burning a star’s hydrogen- and helium- burning burning phases. The second regime, 3 which is between 9 and 60 keV, – Burning shows the conditions that might exist NIF capsule in a deuterium–tritium capsule after NIF laser only ignition and includes the temperatures 1020 and densities achieved up to a star’s Density, cm carbon-burning phase. The nuclear cross sections depend on energy (temperature), and we are currently limited by conventional accelerator methods’ inability to 1010 probe the relevant energy regimes of 10–1 10 101 102 103 interest (see Figure 2). The NIF will Temperature, keV allow us to measure the nuclear reaction rates at precisely the energies Figure 2. Temperature and density regimes attainable on the NIF overlap with the relevant to stellar interiors. conditions of a star as it progresses through evolutionary nuclear burning. The first regime, In a thermalized NIF capsule, which overlaps with a star’s hydrogen- and helium-burning phases, could be reached in a where a temperature of 8 keV could laser-heated hohlraum or an imploding capsule without nuclear ignition. The second be attained, the number of radioactive regime, which overlaps with the conditions achieved up to a star’s carbon-burning phase, 13N nuclei, which would have a half- might exist in a deuterium–tritium capsule after ignition. Experiments conducted in these life of approximately 10 minutes, regimes could greatly enhance our knowledge of stellar evolution.
45 Science on the NIF E&TR December 1994
(a) (b) high-energy lasers such as Nova. Thus, scientists will be able to investigate a number of flow problems under previously unattainable conditions. NIF experiments designed to study these problems—for example, the growth of perturbations at a fluid interface (unstable flow) and shock–shock boundary interactions (stable flow)—will lead to new understandings in fluid dynamics.
Imposed Perturbations Time Position To study the growth of an imposed Position Position perturbation under continuous acceleration, we shock planar foils Figure 3. One- and two-dimensional views of the backlight absorbed by a foil with initial of fluorosilicone by x-ray ablation. perturbation. The one-dimensional view (a) shows that the end of the bright spot occurs when the backlight ceases to radiate effectively. The face-on, two-dimensional view (b) The foil trajectory is recorded by a shows little if any distortion of the foil. radiographic streak camera so that we can check the bulk movement of the sample. The image’s contrast in optical depth is then measured Figure 4. Traces for as a function of time. From this an accelerated foil measurement, we deduce the evolution with an imposed Burn-through 2.7 of the imposed perturbation. perturbation. The curves are offset Figure 3 shows two images of a foil Spike Bubble from a perturbation experiment vertically to allow 2.3 simple comparison. conducted on the Nova laser. The The dashed lines image on the left shows that foil has indicate the backlight 1.9 become increasingly bright; this is intensity, which the result of thinning caused by the varies across the foil 1.5 “bubble and spike” shape as a function of time. Non-linear characteristic of the nonlinear regime. The image on the right, taken 1.1 approximately 2.6 µs after the start of the drive for a duration of 100 ps, In, exposure shows no transverse distortion of 0.7 the foil. We obtain quantitative data by 0.3 taking intensity traces at different times transverse to the grooved structure, as shown in Figure 4. Note that the –0.1 Linear curves, which represent different times in the growth of the perturbation, are t, ns offset for ease of comparison. At early times, the growth is small and still 50-µm increments sinusoidal, indicating that the instability is in the linear regime. Late
46 E&TR December 1994 Science on the NIF
in time, the growth is larger and impact as a point source of high Material Properties distinctly nonsinusoidal, exhibiting energy and momentum density. the characteristic bubble-and-spike This modeling is usually done by For the last several decades, shape. The rapid flattening of the depositing focused laser energy in scientists have been studying the modulations in the top two curves small spheres of high-Z (high-atomic- physical properties of materials— results from the burn-through that number) material or by generating e.g., their equations of state, occurs when the bubbles break out of prodigious shocks and post-shock opacity, and radiative transport—by the back side of the foil. At this point, pressures with flyer foils. conducting experiments with gas the spikes are still being ablated away; A study of concentrated impacts guns, high explosives, and high- however, they can no longer be on surfaces shows that scaling laws pressure mechanical devices. replenished by matter flowing down apply to craters formed by impact and Although these experiments have from the bubbles. surface energy deposition. A proof- provided a wealth of precise data on These experiments extend from the of-principle experiment designed to a wide range of materials, they are single-mode example described here explore the effects of impact cratering limited because they do not provide to multiple-mode experiments and to on simulated soil (Figure 5) indicates information on material behavior at buried interfaces with imposed mode that further research in this area the extreme pressures and structures. The limiting case of a would be of great interest. The ability temperatures of scientific interest, random set of perturbations at a of the laser to deposit large amounts i.e., pressures from 1 to buried interface requires a somewhat of energy in a spot volume without 100 terapascals and temperatures different technique. residual gases—the by-product of the up to a few hundred electron volts. same experiment performed with high Although a few laser-driven and Impact Cratering explosive—indicates its utility as a shock-wave experiments have been Many phenomena in impact simulation source. On the NIF, the carried out in this range of interest, cratering occur on temporal and amount of energy deposited and the the resulting data are quite spatial scales that are very large when in situ diagnostic potential would imprecise and do not validate any compared to those of the impacting make investigation of hydrodynamic of the theoretical models of material object; as a result, we can model the response possible in real time. behavior.
(a) (b)
Soil sample
Laser 14° Gold hollow sphere
Figure 5. To explore the hydrodynamic response of a simulated soil to impact cratering, we deposited 4 kJ of laser energy into the 1.5-mm radius cavity of a 16 x 16 x 16-cm aluminum-plate cube filled with grout (a). The energy density provided by 4 kJ of laser energy in 1 ns was enough to vaporize a hollow gold target at the bottom of the 6-cm deep cavity and the surrounding grout. Less than 200 J of laser radiation escaped the target, as indicated by the top surface of the cube (b). Although this surface is crazed and slightly bowed, the cavity and the entry cone are clearly visible. There is also a profusion of radial cracks and a faint but definite indication of tangential (spherical) cracks. This diagnostic is an example of what we can learn from scaled experiments.
47 Science on the NIF E&TR December 1994
With the NIF, scientists will be equation-of-state data) and high- In this experiment, any spatial able to investigate material behavior resolution x-ray measurements (for imbalance in the drive or any in this range and obtain the primary radiative opacity data). unpredicted edge effects (for data needed to test their theoretical example, those from interactions models. The experimental methods Indirectly Driven Colliding Foil between the flyer foil and sleeve used to obtain these data include Experiments containing the target assembly) could colliding foil experiments (for To reach a regime of very high cause the flyer foil to tilt or curve pressure without sacrificing laser spot and drive a nonplanar shock into the size and one-dimensionality, we target. However, any nonplanarity (a) employ a variation of the well-known would be readily observed because flyer-plate technique. In this of the relatively large diameter of the Drive beams technique, a flyer—in this case a foil— foils; furthermore, any edge-induced stores kinetic energy from the driver nonuniformities would be minimized during an acceleration and rapidly because the step in the target is at the delivers it as thermal energy when it center of the foil. (See pp. 28–29 for X rays collides with another foil. The flyer a discussion of this experiment foil also acts as a preheat shield so relative to weapons physics 3-µm gold 50-µm flyer foil polystyrene that the target remains on a lower equation-of-state experiments.) 50-µm void ablator adiabat, that is, for a given pressure If the target foil is preheated by the temperature is kept lower than it high-energy x rays from the 2-µm/6-µm 1-mm-long would be if exposed to the driver. hohlraum before the flyer foil hits it, gold ¥ 700-µm target foil gold sleeve In this type of experiment the measurement is compromised. (Figure 6), the laser beams are focused To test this possibility, we altered Streak into a millimeter-scale cylindrical gold the x-ray drive in one experiment so camera hohlraum; the radiation that escapes that the overall intensity would be from a hole in the hohlraum becomes identical to that in other experiments the x-ray drive. The hohlraum x rays but the intensity of high-energy (b) ablate a 50-µm layer of polystyrene x rays (those ≥2.5 keV) would be with a 3-mm-thick gold flyer foil. reduced by more than a factor of Time This foil then accelerates through a five. The result indicated that the void and, near the end of the laser measurement was not affected by pulse, collides with a stationary, two- preheat. step (two-thickness) gold target foil. The shock on the rear side of the target X-Ray Opacity Measurements foil is then imaged with an optical To understand the plasma state streak camera. and radiative transport, we need to Figure 6b is a typical streak obtain high-quality measurements camera image of shock breakout on a of the radiative opacity of materials. Shock breakout two-step target foil. The time interval To do this, we must simultaneously between the two breakout times (one measure the x-ray transmission, Figure 6. (a) Schematic of a radiation- for each thickness) measures the temperature, and density of a driven shock experiment. The x rays shock speed in the target. An interval material sample in a single escape from a hole in the cylindrical gold of 57 ps between breakout on the two experiment. These measurements hohlraum and ablate a 50-µm layer of steps corresponds to an average shock have been done successfully on polystyrene with a 3-µm gold flyer foil. The flyer foil accelerates through a 50-µm velocity of 70 km/s. According to our Nova using point-projection void, collides with a two-step gold target equation-of-state tables, this shock spectroscopy (see Figure 7); we foil, and launches a compression wave in speed corresponds to a density of believe that this technique will be the target foil. (b) A typical streak camera 90 g/cm3 and a pressure of 0.74 Gbar even more successful in similar image of a shock breakout showing the in the gold target, which is by far the experiments on NIF because we will time interval between the two sides of highest inferred pressure obtained in be able to access larger ranges of the step. a laboratory. material densities and temperatures.
48 E&TR December 1994 Science on the NIF
When we use point-projection sample that is tamped, also maintains an aluminum–niobium sample. The spectroscopy on Nova, we use eight the relatively high density of the sample contained 14% aluminum laser beams to heat the sample. Then, a sample and ensures that it is in local by weight for the temperature point source of x rays is produced by thermodynamic equilibrium. measurement. Figure 8 shows the tightly focusing one of the remaining The sample is tamped by plastic so transmission of the aluminum and laser beams onto a small backlight that as it expands, its density remains the transmission of the niobium. target of high-Z material. X rays from constant. The thickness of the tamper The dotted lines overlaying the the backlight pass through the sample is determined by calculations, and the experimental data are the calculations. onto an x-ray diffraction crystal and are density of the sample is determined In general, there is excellent then recorded on x-ray film. Other by imaging. Usually, a second point- agreement. This experiment is a x rays from the same point bypass the projection spectrometer images the milestone. It shows that we can obtain sample but are still diffracted from the expansion of the sample. The first opacity measurements accurate crystal onto the film record. The ratio of point-projection spectrometer is used enough to serve as an in situ attenuated to unattenuated to measure the sample’s absorption. temperature diagnostic for the sample. x rays provides the x-ray transmission The relative intensities of the The accuracy of the sample’s spectrum of the sample. Proper transitions from the different ion temperature, measured to be 48 eV collimation allows a highly quantitative species give the ion balance in the (±2 eV), represents a very important analysis of the spectrum. Background sample, which, when coupled to the advance in measuring temperatures from film chemicals, sample emission, density measurement, gives the of high-energy-density matter. (See and crystal x-ray fluorescence can all be temperature of the sample. pp. 27–28 for a discussion of this separately determined from the x-ray The two point-projection experiment in relation to weapons film record. measurements allow density to be physics opacity experiments.) The sample itself must be uniform measured to an accuracy of ±10% and in temperature and density. the temperature to an accuracy of Plasma Physics Uniformity of temperature is achieved about 5%. With these accuracies by heating the sample in a hohlraum it is possible to make a quantitative In the broadest sense, plasma that does not allow laser light to comparison between the experimental physics is the scientific investigation reflect or impinge on it directly; thus, results and the theoretical calculations of the predominant state of matter in the sample is heated only by x rays. of the opacity. our universe, plasma. The study of The hohlraum, by providing x-ray In one experiment on Nova, we plasma physics has been stimulated drive that volumetrically heats the measured the opacity of niobium in over the past four decades by its close
Figure 7. Spectrum Absorption Schematic of point- spectrum projection spectroscopy Tamped opacity sample with two different thicknesses for opacity Backlight measurements. The laser laser-produced y, ¬ Film backlight x rays pass through the target y and are imaged. A Bragg crystal disperses the Point source x spectrum so that x Occluded of x rays a spatially and area: emission spectrally resolved Bragg crystal and fog image is obtained. Temporal resolution is provided by Hohlraum heated by 8 laser beams backlight duration.
49 Science on the NIF E&TR December 1994
connection with the goal of creating turn, creates an unstable feedback (e.g., temperature, density, and fusion as an energy source and with loop; the more tightly focused the average ion charge). the exploration of various laser light is, the higher its intensity The primary diagnostic for astrophysical plasmas. and the lower its electron density. filamentation would be the stimulated The advanced experimental Eventually, this instability is Raman scattering signal, which is capabilities of the NIF will allow us saturated by diffraction effects, indicative of the low density in the to produce and characterize large, thermal absorption, or parametric filaments. That could be coupled with hot, uniform plasmas. With large instabilities. high spatial-resolution imaging, high- uniform plasmas, we will be able to The growth of filamentary resolution optical probing, and a measure electron and ion structures can be determined by the study of the angular distribution of temperature, charge state, electron width and length of speckles in the scattered light. density, and flow velocity. In short, incident laser beam. Because long Thomson scattering could be used we will be able to perform a wide speckles are more likely to self-focus for these investigations to make range of quantitative experiments on than short speckles, filamentation can highly localized measurements of a medium that is a very good be described by a growth rate along plasma temperature and density. It approximation of a real test bed for the length of a speckle, or 8f2l, where could also be used to measure the plasma physics. Many experiments f is the f-number of the beam (i.e., coherent motion of electrons involved will be extensions of the fusion beam focal length divided by effective in ion acoustic and electron plasma energy experiments that have been maximum beam diameter) and l is waves. This measurement would performed on smaller, less powerful the wavelength of the laser light. If provide a temporally and spatially high-energy lasers. Many others, the speckle lengths are smaller than resolved measure of the coherent however, will go beyond the the scale length of the plasma, the fluctuation amplitude in a specific requirements of fusion energy to f-number and the wavelength of the direction, as determined by the explore a range of basic topics in incident beam can be very powerful detector angle, the scattering volume, plasma physics, a few of which are levers for modifying filamentation. and the scattering light source. discussed here. By using large uniform plasmas on Measurement of the background the NIF, we will be able to produce fluctuation levels would provide Filamentation sufficiently large filaments to study information about the initial level of When a small hot spot, or speckle, this process over a wide range of the coherent fluctuations, their in the laser-intensity profile wavelengths and f-numbers. We will amplification, and their saturation. It undergoes self-focusing, also be able to explore this process also could provide useful information filamentation occurs: that is, electrons over a broad range of experimental about the coupling between (and eventually ions) are expelled parameters by varying the color and stimulated Raman and Brillouin from the filament, causing laser light f-number of the filamentation beam scattering if it were done at the same to focus more tightly. This process, in and by varying the plasma conditions location and at the same time as the
1.0
0.8
0.6
0.4 Experimental data Absorbtion 0.2 Prediction
0 1520 1540 1560 1580 1600 2100 22002300 2400 2500 2600 2700 2800 Energy, eV
Figure 8. Absorption of an aluminum–niobium sample. The experimental data are in the solid black line and the opacity prediction is the dashed line. The spectrum of the aluminum-potassium alpha lines, which were verified to yield an accurate temperature, were measured on the same experiment as the niobium spectrum.
50 E&TR December 1994 Science on the NIF
coherent motion measurements. Gated imaging Streaked high- Developing an x-ray Thomson (GXI-3) resolution scattering measurement to study spectrometer coherent plasma motion in high (HIX) density plasmas is another exciting Gated spectrometer Streaked possibility. (SXRFC) spectrometer (SSC) Formation of Large, Uniform Plasmas Presently, we can produce relatively high-temperature (3000-eV), millimeter-scale plasmas Laser Laser using the diagnostic complement and experiments shown in Figure 9. Gated These large, uniform plasmas are imaging used to study phenomena as diverse (WAX) as plasma–laser interactions and nuclear reaction rates. The experimental geometry should be directly scalable to the NIF, with Gated nine-tenths of the laser being used to imaging (GXI-2) Interaction form the plasma and one-tenth being beam used to create interactions. The Streaked imaging ability to produce these large uniform (SSC) plasmas on the NIF will allow us to study fundamental aspects of our Figure 9. Schematic of the experiments and diagnostic complement (red arrows) used to form experiments, such as hohlraum a large, hot, uniform plasma. A gas bag (light green) is filled through two tubes in the hoop (dark environments and sidescatter, that green). The pressure is stabilized by a pressure transducer that causes the fully ionized species have been virtually impossible to to yield electron densities of approximately 10–1 cm3. The roughly spherical plasma, which has a interpret quantitatively. volume of 0.066 cm3 and a radius of 2.5 mm, is heated to a temperature of about 3000 eV by heating lasers (blue arrows). A separate interaction beam (light blue arrow) drives the Short-Pulse, High-Power instabilities in a controlled way. This geometry should be directly scalable to the NIF. Experiments There is widespread agreement that the NIF should include a beam (a) High compression (b) Channeling laser beam (c) Ignitor laser beam line for short-pulse, high-power implosion experiments. This capability is especially important for studying such basic topics as relativistic, ultra- high-intensity regimes of laser–matter interaction; high-gradient accelerator schemes; and fast ignition (Figure 10). It is also more amenable Compressed material to detailed simulation and to Pusher systematic exploration of linear and nonlinear behavior of plasmas. Figure 10. Fast ignition requires high compression, two laser systems, and system diagnoses. The high-gradient accelerator (a) In the first step of this process, a gas-filled sphere is imploded. The core of the compressed schemes employ plasmas that support gas is at densities of 600 g/cm3. (b) In the next step, a laser with a pulse duration of 100 ps and an much higher energy fields than those intensity of 1018 W/cm2 creates a channel by pushing the critical density surface toward the core. associated with conventional (c) Finally, the heater, or ignitor, beam is turned on. This beam interacts with the density gradient accelerator schemes. As a result, the and generates hot electrons at MeV energies. These electrons penetrate into the core of the device will be much more compact compressed gas and cause an instantaneous rise in the local temperature of the core.
51 Science on the NIF E&TR December 1994
and potentially cheaper. A number of able to convert laser energy to a wide types of radiation and particle novel schemes have been proposed variety of x-ray and particle sources fluxes will play an important role and studied at laser powers not quite needed to address several important in benchmarking our basic high enough to produce the desired questions in basic and applied understanding of laser–plasma electron velocity. If these schemes physics. For example, we will be interactions and atomic physics. prove successful, applications to able to produce intense broadband Broadband x rays generated by tunable sources of x rays are also thermal x rays from high-Z targets, NIF laser plasmas will be used to envisioned. coherent amplified x rays (x-ray produce and characterize large, lasers) from high-gain plasmas, uniform plasmas relevant to inertial Radiation Sources intense neutron pulses from confinement fusion and astrophysics. implosion plasmas, and intense The high temperatures and densities The conversion of laser energy into pulses of hard x rays from fast produced during implosion and short wavelength radiation is a major electrons. Accurate energy spectra subsequent ignition will be an goal of many high-energy laser and absolute measurements of the excellent source of continuum experiments. On the NIF, we will be conversion of laser energy into all x rays—those extending from the soft x-ray region to MeV with pulse durations of less than l00 ps. Besides producing important 0.40 coherent radiation sources (i.e., x-ray 1-ns pulse Without M band lasers), NIF will offer a critical test 500 µm ¥ 500 µm With M band of our atomic modeling, allowing us to extrapolate existing neon-like and 0.30 2-ns pulse nickel-like collisional x-ray lasers to 1000 µm ¥ 1000 µm wavelengths of about 20 angstroms (Å = 10–10 m). At these wavelengths, we can use x-ray laser interferometry (the interference created by splitting 0.20 and then recombining the x-ray laser beam) to measure electron densities 1-ns pulse
Temperature, keV in plasmas exceeding solid densities. 1600 µm ¥ 2800 µm Also, the short-pulse capability of 0.10 the NIF may enable us to develop new x-ray lasers that emit radiation at wavelengths shorter than 10 Å; such bright, coherent sources would be very useful in characterizing solid 0 0 0.5 1.0 1.5 2.0 2.5 3.0 matter for materials science and Time, ns biophysics research. The NIF will be able to generate more than 1018 neutrons in a single Figure 11. Equivalent radiation temperature vs time for three gold hohlraum experiments 100-ps pulse, making it very useful performed on Nova. The experiments used a total energy of 18 kJ. Laser entrance holes were in the sides of the 500- and 1000-µm-length hohlraums and through the ends of the for producing uniform, high-density, 2800-µm-length hohlraum. The curves for each hohlraum show the reproducibility of the low-temperature plasmas. It will be data. The black line indicates the equivalent radiation temperature with the M band; the red able to generate fast electrons with line indicates the equivalent radiation temperature without the M band. The contribution of hundreds of kiloelectron volts in the M band to the radiation temperature was greatest in the small hohlraums because of the energy—which is a potential closure of the holes; it was very small in the large hohlraums because they were large and source of high-energy x rays for because their viewing angle did not accommodate a view of the laser-irradiated spots. backlighting and probing plasmas.
52 E&TR December 1994 Science on the NIF
These radiation sources are • Second, radiative properties serve absorption of the probe, the effects supplemented by the possibility of as primary data for numerous other of refraction, and the impossibility using radiation enclosures, or studies. For example, spectral line of going beyond critical densities. hohlraums (such as those shown in lists are inadequate for many of the Recently, we did an experiment to Figure 11), to generate radiation charge states of heavier elements. see whether an optical measuring environments and x-ray drive fluxes. Thus, scientists measure and device, known as a Mach-Zehnder These sources will be able to produce categorize the energies of highly interferometer, and a standard 3-cm- far in excess of the approximately ionized species for a variety of uses. long yttrium x-ray laser could be 200 eV produced by hohlraum • Third, radiative properties serve as used to probe these plasmas more sources available on current high- noninterfering probes. For example, successfully. energy lasers. These sources will be by looking at the emission or In this experiment (shown of higher effective temperature and absorption spectrum of a plasma, schematically in Figure 12), the also will be able to provide uniform scientists can obtain fundamental output from a standard 3-cm-long x-ray drive over far larger areas than information about the plasma’s yttrium x-ray laser was collimated is possible with today’s sources. ionization balance, rate processes, by a multilayer mirror and injected Thus, the advantages of using x-ray densities, temperatures, and into the interferometer. An imaging heating for the study of hot, dense fluctuation levels. The radiative optic from the interferometer then matter will be greatly enhanced on properties are therefore a powerful imaged a plane within the the NIF. diagnostic of the plasma state. interferometer where a plasma was Experiments on high-energy lasers produced. Figure 13 shows the Radiative Properties have done much to enhance our recorded interferogram of the knowledge of the radiative properties plasma. The fringes, or contrast The importance of radiative of hot, dense matter; thus, we expect modulations, due to the plasma properties in high-energy-density that experiments on the NIF, such as are clearly visible, indicating the plasma derives from three factors: those employing interferometry and feasibility of this technique. • First, the radiative property can be plasma spectroscopy, will advance However, plasma blow-off, evident the best indicator of the level of that knowledge even further. in the central region of the image, scientific knowledge in a particular completely obscures the laser, area. For example, when scientists Interferometry Experiments indicating that the technique still want to develop new descriptions For years, optical probing of has its limits. On the NIF we will of atomic structure, they look at high-density or large plasmas has be able to push the x-ray laser transition energies. been difficult because of the high interferometer to shorter x-ray laser
Figure 12. Schematic of the experimental setup for x-ray laser Imaging interferometry. multilayer Laser irradiated target mirrors X-ray CCD detector
Mach-Zehnder X-ray laser interferometer Collimating multilayer mirror
53 Science on the NIF E&TR December 1994
wavelengths, making it an even more Summary physics. Finally, the NIF will enable important diagnostic tool in the study us to push the x-ray laser and characterization of large-scale The extraordinary range of interferometer to shorter x-ray laser plasmas. physical conditions that will be wavelengths, making it an even more achievable on the NIF will advance important diagnostic tool in the study knowledge in the physical sciences. It and characterization of large-scale will give us the ability to synthesize plasmas. The NIF will allow us to 1200 and analyze the plasmas that explore a previously inaccessible characterize the stellar environment region of physical phenomena that during its evolution. It will enable us could validate our current theories µm 600 to investigate a number of stable and and experimental observations and unstable flow problems under provide a foundation for new conditions that cannot be obtained by knowledge. 0 conventional means, such as wind 0 1000 2000 µm tunnels, shock tubes, or other high- Key Words: astrophysics; high-pressure physics; energy lasers. It will give us the hydrodynamics; National Ignition Facility—high- Figure 13. Interferogram of a high- ability to investigate material energy laser experiments; plasma physics; radiation density plasma produced by x-ray laser behavior at pressures from 1 to sources; radiative properties. interferometry. The plasma is made by 100 terapascals and temperatures up irradiating the surface of a mylar plastic to a few hundred electron volts so sample (solid horizontal band) with an x-ray For further that we can validate our theoretical information beam. The bright spot above the plastic contact sample is the self-emission of the plastic. understanding of material behavior at extreme conditions. We will be able Richard W. Lee (510) 422-7209. to convert NIF laser energy to a wide variety of x-ray and particle sources needed to address important questions in basic and applied
54 NIF Environmental, Safety, and Health Considerations
The proposed NIF has been rated by the DOE as a radiological low-hazard, non-nuclear facility. Our analysis to date of environmental, safety, and health issues related to the NIF, presented in documents that are available to the public, shows that the system will have no significant environmental impact and present no significant safety or health risk to the work force and the general public.
NVIRONMENTAL, safety, and safety and health assessments. The dose at the site boundary, the E health (ES&H) considerations topics fall broadly into three maximum annual dose and average are of paramount importance in all categories presented in the following annual dose to any NIF worker, and phases of the design of the National order: radiation exposure, waste the effects of a worst-case accident Ignition Facility (NIF). From the generation, and fusion targets. assuming release of the maximum outset, maximizing public and The NIF will maintain a small planned tritium inventory. We have employee safety and minimizing inventory of less than 300 curies also estimated the quantities of health risk and environmental impact (0.03 grams) of tritium fuel and hazardous, mixed, and low-level have been integral parts of the design unburned tritium from inertial radioactive wastes. Before turning to process. In a broader sense, inertial confinement fusion experiments. The these matters, a brief overview of the confinement fusion has long-term fusion reactions in the 1-mm capsule review process will help to put our potential as a source of future energy will release a neutron for every efforts to date into perspective. for the world. Earning the trust of the tritium atom consumed, about 1017 public and operating in a manner that atoms for an ignition experiment. The The Assessment and Review protects the environment over time NIF target area employs heavy Process are requirements for any future shielding, including 2-meter-thick energy source. concrete walls and a shielded vessel Safety and environmental analysis This article describes the results of against the radiation. We have specific to the NIF began more than environmental analyses as well as estimated the routine annual radiation two years ago. This work was
55 Environmental, Safety, and Health Issues E&TR December 1994
an outgrowth of continuing damage to humans as 1 roentgen exit sign. The dose to a member of environmental impact and safety of high-energy x rays. Natural the public expected from all NIF analyses of general concepts for background radiation from the effluents is 600 times less than that future inertial confinement fusion environment—that is, from naturally from a single cross-country airline (ICF) facilities. More than a year ago, occurring elements, cosmic radiation, flight. a standing NIF working group was and so forth—averages 0.3 to The worst-case accident formed at LLNL. The group is made 0.5 rem/yr depending on where an considered in our safety assessment up of environmental and safety individual lives. This natural assumes the release of all the tritium experts in radiation protection, safety background radiation does not (300 Ci) in its worst biohazardous analysis, environmental evaluation, include exposure to other potential form (tritiated water) immediately laser operations, occupational safety, sources of radiation, such as that from after a maximum-yield experiment tritium handling, waste handling, airplane flights and some types of (20 megajoules). This postulated, but quality assurance, and fire protection. medical diagnoses or treatments. highly unlikely, accident would result The group meets biweekly to ensure The routine annual dose from NIF in a calculated dose of 0.056 rem at a consistent and well-documented at a site boundary 300 meters from site boundary 300 meters from NIF. evaluation of environmental and NIF is expected to be 0.00013 rem. This dose is 0.2% of the DOE siting safety aspects of the NIF design. Put into perspective, this value guidelines for annual exposure. The NIF working group has represents 0.13% of the DOE and prepared the Preliminary Hazards Environmental Protection Agency Waste Generation Analysis for the NIF1 as well as guideline and is 350 times less than radiation protection,2 safety, the annual radiation dose arising from NIF will generate three types of environmental, quality assurance, emissions from a 1-GWe coal-fired waste: hazardous, low-level and decommissioning evaluations of power plant. radioactive, and mixed (a the NIF design.3 These analyses are The average annual dose received combination of hazardous and low- available to the public as published by flight attendants is 0.5 rem, a dose level radioactive). To be on the reports.4 not monitored by the airline industry. conservative side, we estimated ES&H issues are extensively In comparison, the maximum annual higher waste quanlities than are likely analyzed by experts, documented, and dose to any NIF worker will be less from the NIF’s waste streams. reviewed by the public as part of the than 0.5 rem, which is less than 10% Moreover, our assessment has not process established by the DOE for of the DOE guidance. The average fully considered waste-minimization major system acquisitions such as the dose to any NIF worker is estimated techniques, such as frozen carbon NIF. Whereas major steps toward to be about 0.01 rem. dioxide pellet cleaning. Waste safety and environmental analyses The maximum tritium inventory minimization will be an important have already been done for NIF, for the NIF will be 300 curies (Ci). and continuing design activity. additional analyses will include an This amount is the equivalent of The annual hazardous waste Environmental Impact Statement, 0.03 grams of tritium. The maximum stream associated with NIF will be Preliminary and Final Safety NIF inventory is less than 3% of the about 3180 kg (7000 lb) of solid Analyses, and Operational Readiness routine inventory of the National waste and 2270 L (600 gal) of liquid Reviews. The DOE requires all of Tritium Labeling Facility in waste.3 Most of this waste stream, these studies to ensure that essential Berkeley, California, which uses about 2270 kg (5000 lb), will be in aspects of the project are thoroughly tritium for tagging biomedical the form of 20 boxes of paper soaked analyzed and are completely samples. One NIF target will contain with capacitor oil. Such waste is satisfactory before operations begin. less that 2 Ci of tritium, one-fifth the similar to but smaller in quantity than amount of tritium in some typical that generated in the same time by an Radiation Doses theater exit signs of which more than automobile oil-changing facility. The one million are sold annually. waste will be routinely disposed of by In our radiological assessments, There are no significant certified contractors. the unit of measure is the rem, which radioactive or hazardous effluent Mixed waste is both radioactive stands for roentgen-equivalent in man levels for NIF. For example, the and chemically hazardous. The and is a unit of biological radiation projected maximum emission of annual mixed waste stream associated dose. It is the amount of ionizing tritium is less than 10 Ci/yr, the with NIF will be about 135 kg radiation that produces the same equivalent of the tritium in a single (300 lb) of solid and about 2000 L
56 E&TR December 1994 Environmental, Safety, and Health Issues
(530 gal) of liquid,3 which represent a National Laboratory in Los Alamos, Environmental Impact Statement small fraction of the quantities NM; and LLNL). process will also allow participation currently generated at LLNL. These The filling activity for NIF targets by the public in reviewing the mixed waste quantities take into requires a total inventory of less potential environmental impacts of consideration some use of frozen CO2 than 5 grams of tritium. The target the NIF. pellet cleaning, a dry, high-pressure manufacturing and filling facilities scouring technique that have their own National Key Words: environmental safety and health decontaminates objects and avoids the Environmental Policy Act (NEPA) (ES&H); National Ignition Facility (NIF)—radiation need for alternate methods that use documentation and engineered dose; tritium inventory; waste stream. hazardous liquid solvents, thereby systems to protect workers, the generating large quantities of liquid public, and the environment by safely Notes and References 1. S. J. Brereton, Preliminary Hazards Analysis for mixed waste. confining any tritium. the National Ignition Facility, Lawrence The annual solid low-level Because of the existing national Livermore National Laboratory, Livermore, CA, radioactive waste stream, 400 kg capability, the NIF project will UCRL-ID-116983 (1993). (850 lb), will be a small fraction of not include a dedicated target 2. M. S. Singh, Radiological Analysis of the the quantity currently generated at manufacturing and tritium filling National Ignition Facility, Lawrence Livermore National Laboratory, Livermore, CA, UCRL- LLNL and is less than one-sixth of facility. Instead, it will receive LR-115188 (1993). that produced by a major university’s several types of targets from several 3. National Ignition Facility Conceptual Design medical center. The annual liquid different sites. Targets for NIF will be Report, Lawrence Livermore National low-level radioactive waste stream transported in certified containers Laboratory, Livermore, CA, UCRL-PROP- (aqueous waste) will be disposed of in prescribed by the Department of 117093 (1994) and National Ignition Facility 3 Seting Proposal–LLNL, Lawrence Livermore several 55-gallon drums, along with Transportation and in accordance National Laboratory, Livermore, CA, UCRL- two drums of vacuum pump oil, with the Code of Federal Regulations PROP-119353 (1994). according to applicable guidelines. (Title 49, section 173). 4. To obtain copies of references 1 and 2, contact the National Technical Information Services Fusion Targets Summary (NTIS), U. S. Department of Commerce, 5285 Port Royal Road, Springfield, VA 22161. Reference 3 cites ongoing documents that will ICF targets are used in many After reviewing the Preliminary not be available to the public through NTIS until facilities throughout the country. Hazards Analysis report, the DOE complete; copies, however, are available to read Examples of current or future concurred with the preliminary at the Livermore Public Library, Livermore, facilities using such targets include categorization of the NIF as a CA, and at the Visitors Center at LLNL. 5. S. Brereton, G. Greiner, M. Singh, and M. Trent the Nova laser; the Particle Beam radiological low-hazard, non-nuclear also contributed to this article. Fusion Accelerator II (PBFA-II) at facility. This means that operation of Sandia National Laboratory in the NIF will have minor onsite and Albuquerque, NM; the planned negligible offsite consequences. The upgrade of the Omega laser at the hazards categorization will be University of Rochester; and the reviewed in each subsequent safety proposed NIF. The manufacture and analysis report. filling of fusion fuel capsules is a The conservative safety and separate and ongoing activity of the environmental analyses outlined in national ICF Program that supports this article are the first of a series of present and future facilities. These studies required to ensure the safety functions are carried out at several of workers, the public, and the DOE and commercial sites (for environment. The NEPA process of example, at General Atomics in La the DOE ensures joint participation For further information contact Jolla, CA; the University of Rochester by the public and those states that Jon M. Yatabe (510) 422-6115 or in Rochester, NY; Los Alamos may be affected by the project. The Michael T. Tobin (510) 423-1168.
57 ABSTRACTS
A Tour of the Proposed National Ignition Facility The National Ignition Facility (NIF) will enable us to When built, the National Ignition Facility (NIF) will produce energy densities (energies per particle) that house the world’s most powerful neodymium glass laser overlap with the energy densities produced in nuclear system. NIF will be 50 times more powerful than the weapons, yet the total energy available on NIF will be a Laboratory’s Nova laser, currently the world’s most minuscule fraction of the total energy from a weapon. powerful. The NIF will contain 192 independent laser This combination of low total energy with weapons- beams, or “beamlets,” each with a square aperture of a regime energy density will allow us to pursue, besides little less than 40 cm on a side. For economy and ignition experiments, many nonignition experiments. efficiency, beamlets will be stacked four high and twelve These will allow us to improve our understanding of wide into four large arrays. The beamlines will require materials and processes in extreme conditions by more than 9000 large-format optics (greater than 40 ¥ isolating various fundamental physics processes and 40 cm) and several thousand smaller optics. Compared to phenomena for separate investigation. Such studies will the size of the current Nova facility at LLNL, which uses include opacity to radiation, equations of state, and a single-pass amplifier laser architecture, the compact hydrodynamic instability. In addition to these, we will multipass design of the proposed NIF system allows us to study processes in which two or more such phenomena put a laser with a typical output that is 40 times greater come into play, such as in radiation transport and in than Nova’s into a building only about twice the size. ignition. This article follows the path of a photon from the master Weapons physics research on NIF offers a oscillator and preamplifier, through the NIF main laser considerable benefit to stockpile stewardship, not only in components, to the target. It also highlights some of enabling us to keep abreast of issues associated with an the development efforts, begun many years ago, for aging stockpile, but also in offering a major resource for components, such as the multipass glass amplifiers and training the next generation of scientists who will monitor plasma electrode Pockels cell, that allow us to design a the stockpile. large, multipass glass laser economically and at very low Contact: Stephen B. Libby (510) 422-9785. risk. Results from our recently completed Beamlet Demonstration Project, involving a prototype NIF The Role of NIF in Developing Inertial Fusion beamline, along with the models and design codes we are Energy testing ensure that we can have great confidence in the The proposed National Ignition Facility (NIF) will performance projected for NIF. provide LLNL researchers as well as others in the Contact: John R. Murray (510) 422-6152. scientific community committed to developing Inertial Fusion Energy (IFE) with the means of developing and NIF and National Security testing data and materials that are key to the long-term Since the end of the Cold War with the demise of the goal of building and operating IFE power plants as clean, Soviet Union, the U.S. nuclear weapons program has viable, environmentally safe sources of inexhaustible changed dramatically. A major change has been the energy. When the NIF demonstrates fusion ignition, moratorium on underground nuclear testing, which is which is central to proving the feasibility of IFE, it likely to be extended indefinitely by a Comprehensive will tell us much about IFE target optimization and Test-Ban Treaty. Although there are now far fewer fabrication, provide important data on fusion-chamber weapons and weapon types than only a few years ago, the phenomena and technologies, and demonstrate the safe nuclear stockpile nevertheless remains, and U.S. policy and environmentally benign operation of an IFE power will continue to rely on nuclear deterrence for the plant. In accomplishing these tasks, the NIF will also foreseeable future. Because the U.S. must be confident provide the basis for future decisions about IFE that the nuclear arsenal would perform reliably if needed, development programs and facilities, such as the planned reliance on testing to assess weapon performance must be Engineering Test Facility (ETF). Furthermore, it will replaced by reliance on thorough scientific understanding allow the U.S. to expand its expertise in inertial fusion and better predictive models of performance—that is, and supporting industrial technology as well as promote science-based stockpile stewardship. U.S. leadership in energy technologies, provide clean, viable alternatives to oil and other polluting fossil fuels, and reduce energy-related emissions of greenhouse gases. Contact: B. Grant Logan (510) 422-9816 or Michael T. Tobin (510) 423-1168.
58 E&TR December 1994 Abstracts
Science on the NIF NIF Environmental, Safety, and Health Last March, a group of scientists convened at the Considerations University of California, Berkeley, to discuss the To ensure the safety of workers and the public and potential scientific applications of the National Ignition to assess potential environmental impacts, we have Facility (NIF)—a 192-beam, neodymium glass laser that completed the first of a series of safety and environmental will be used to obtain the high-energy physics data analyses related to the proposed National Ignition Facility needed to maintain the nation’s nuclear stockpile. The (NIF). On the basis of its review of the Preliminary objective of the gathering was to identify areas of Hazards Analysis report, the DOE has concurred with the research in which the NIF could be used to advance categorization of the NIF as a radiological low-hazard, knowledge in the physical sciences and to define a non-nuclear facility. Our studies to date show that the NIF tentative program of high-energy laser experiments. The will present no significant environmental or health and scientists determined that the most effective scientific safety risk. For example, the average annual biological applications of the NIF would be in astrophysics, radiation dose to a NIF worker is estimated to be about hydrodynamics, high-pressure physics, and plasma 0.01 rem. This value is less than 10% of the DOE physics. In astrophysics, the NIF would give scientists the guideline. As part of the National Environmental ability to synthesize and analyze the plasmas that occur at Protection Act (NEPA) determination process established all stages of stellar evolution. In hydrodynamics, it would by the DOE, the public will be invited to participate in enable them to investigate flow problems under reviewing environmental, safety, and health issues related conditions that cannot be obtained by the conventional to the NIF. wind tunnel or shock tube. In high-pressure physics, it Contact: Jon M. Yatabe (510) 422-6115 and Michael T. Tobin (510) 423-1168. would allow scientists to investigate material behavior at pressures from 1 to 100 terapascals and temperatures up to a few hundred electron volts so that they could validate their theoretical models of material behavior. Scientists would also be able to convert NIF laser energy to a wide variety of x-ray and particle sources needed to address important questions in basic and applied physics. With the NIF, scientists could push the x-ray laser interferometer to shorter x-ray laser wavelengths so that it would be a more valuable diagnostic tool in the study and characterization of large-scale plasmas. In short, the NIF would enable scientists to explore a previously inaccessible region of physical phenomena that could validate their current theories and experimental observations and provide a foundation for new knowledge of the physical world. Contact: Richard W. Lee (510) 422-7209.
59 INDEX
1994 Index January–February 1994 Page The State of the Laboratory 1 Economic Competitiveness 4 National Security 10 Nonproliferation, Arms Control, and International Security 12 Defense Systems 18 Nuclear Testing and Experimental Science 24 Lasers 30 Environment 36 Environmental Research and Technology Development 38 Environmental Restoration, Protection, and Waste Management 44 Energy 50 Biology and Biotechnology 56 Engineering 62 Physics 68 Chemistry and Materials Science 74 Computers and Computing 80 Education 86 Administration and Institutional Services 92 Awards 98
March 1994 Forensic Science Center 1 Melanoma at LLNL: An Update 9 Center for Healthcare Technologies 21
April 1994 World’s Fastest Solid-State Digitizer 1 The MACHO Camera System: Searching for Dark Matter 7
May 1994 Modified Retorting for Waste Treatment 1 Cleaning Up Underground Contaminants 11
June 1994 The Clementine Satellite 1 Uncertainty and the Federal Role in Science and Technology 13
July 1994 Pathfinder and the Development of Solar Rechargeable Aircraft 1 ASTRID Rocket Flight Test 11
60 E&TR December 1994 Index
August–September 1994 Overview Materials by Computer Design: An Introduction 1 Research Highlights Using Computers to Build Better Computers 4 Modeling Large Molecular Systems 6 Massively Parallel Computing CRADA 8 Designing Better Microelectronics Devices 9 Polymeric Nitrogen: A Potential Compound to Store Energy 10 Feature Articles Nanotribology: Modeling Atoms When Surfaces Collide 13 Toward Improved Understanding of Material Surfaces and Interfaces 25 Predicting the Structural and Electronic Properties of Scintillators 33
October 1994 Feature Articles The Industrial Computing Initiative 1 Artificial Hip Joints: Applying Weapons Expertise to Medical Technology 16 Research Highlights KEN Project: Real-World Face Recognition 22 Modeling Groundwater Flow and Chemical Migration 24 Gas and Oil National Information Infrastructure 26
November 1994 Feature Articles LLNL’s 1994 R&D 100 Awards 7 Legacy of the X-Ray Laser Program 13 Research Highlights Elevated CO2 Exposure and Tree Growth 22 Meniscus Coating 24
December 1994 The National Ignition Facility: An Overview 1 A Tour of the Proposed National Ignition Facility 7 NIF and National Security 23 The Role of NIF in Developing Inertial Fusion Energy 33 Science on the NIF 43 NIF Environmental, Safety, and Health Considerations 55
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