Review& Review&

■ December 1994

University of California

Lawrence Livermore National Laboratory

The National Ignition Facility

Review& Review&

■ December 1994

University of California

Lawrence Livermore National Laboratory

The National Ignition Facility

About the CoverThis 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 thelatest and largest inertial fusion laser being designedby LLNL researchers for use in the internationallaser science community. In the background on thefront cover is an engineering drawing of the 192-beam target chamber where ignition of NIFtargets takes place. The inset is an artist’s renderingof the NIF in operation. It shows an indirect-drivetarget contained within a metal cylinder called ahohlraum. The blue laser beams are depositing theirenergy on the inside of the hohlraum. There theenergy is converted to x rays that heat the targetintensely, causing it to implode and ignite for afraction of a second with the energy intensity of theinterior of a star. On the back cover is a photographof an indirect target that contains a tiny amount ofhydrogen-isotope fuel. The hohlraum is about 6 millimeters in diameter; the target inside is about3 millimeters in diameter. The design, manufacture,and testing of these targets by Laboratory scientistsis integral to the success of experiments performedon the NIF.

About the JournalThe 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 magneticfusion energy. Since then, in response to new national needs, we have added other major programs,including laser science (fusion, isotope separation, materials processing), biology and biotechnology,environmental research and remediation, arms control and nonproliferation, advanced defense technology,and applied energy technology, and industrial partnerships. These programs, in turn, require research inbasic scientific disciplines, including chemistry and materials science, computing science and technology,engineering, and physics. The Laboratory also carries out a variety of projects for other federal agencies.Energy and Technology Review is published monthly to report on unclassified work in all our programs.Please address any correspondence concerning Energy and Technology Review (including name and addresschanges) to Mail Stop L-3, Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA 94551,or telephone (510) 422-4859, or send electronic mail to etr-mail@llnl.gov.Prepared for DOE under contract

No. W-7405-Eng-48

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The National Ignition Facility: An Overview 1

A Tour of the Proposed National Ignition Facility 7Through a series of artists’ sketches of the proposed NIF, we follow the path 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 demonstrates the feasibility of the NIF concept.

NIF and National Security 23In the absence of underground nuclear testing, the National Ignition Facility 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.

The Role of NIF in Developing Inertial Fusion Energy 33By demonstrating fusion ignition, NIF becomes an important means of provingthe feasibility of inertial fusion energy as an economical, environmentally safealternative to fossil fuels and other energy sources. It will also help us optimize and fabricate fusion targets for power plants of the future and will provide the basis for future decisions about energy programs and facilities.

Science on the NIF 43Last March a group of scientists convened at the University of California, Berkeley, to identify areas of research in which the National Ignition Facility could be used to advance knowledge in the physical sciences and to define a tentative program of high-energy laser experiments. These scientists determined that this facility would have the most effective applications in astrophysics,hydrodynamics, high-pressure physics, plasma physics, and basic and appliedphysics.

NIF Environmental, Safety, and Health Considerations 55Our analysis to date of ES&H issues related to the National Ignition Facility shows that the proposed system and its operations will result in no significantenvironmental impact and safety risk to the Laboratory’s work force and the general public.

Abstracts 58

1994 Index 60

Review&■ December 1994

SCIENTIFIC EDITOR

William A. Bookless

PUBLICATION EDITOR

Dean Wheatcraft

WRITERS

June Canada, Kevin Gleason, Arnold B. Heller, Robert D. Kirvel,Harriet Kroopnick, Tom Spafford

ISSUE COORDINATORS

Paul Harding, Ray Marazzi

DESIGNERS

Paul Harding, George Kitrinos, Ray Marazzi

GRAPHIC ARTIST

Treva Carey

CONTRIBUTING ARTISTS

Scott Dougherty, Sandy LynnDan Moore, Leston Peck

COMPOSITOR

Louisa Cardoza

PROOFREADER

Catherine M. Williams

This document was prepared as an accountof work sponsored by an agency of the UnitedStates Government. Neither the United StatesGovernment nor the University of California norany of their employees makes any warranty,expressed or implied, or assumes any legalliability or responsibility for the accuracy,completeness, or usefulness of any information,apparatus, product, or process disclosed, orrepresents that its use would not infringe privatelyowned rights. Reference herein to any specificcommercial product, process, or service by tradename, trademark, manufacturer, or otherwise does not necessarily constitute or imply itsendorsement, recommendation, or favoring by theUnited States Government or the University ofCalifornia. The views and opinions of authorsexpressed herein do not necessarily state or reflectthose of the United States Government or theUniversity of California and shall not be used foradvertising or product endorsement purposes.

Printed in the United States of AmericaAvailable from

National Technical Information ServiceU.S. Department of Commerce

5285 Port Royal RoadSpringfield, Virginia 22161

UCRL-52000-94-12Distribution Category UC-700

December 1994

1

HEN Secretary of Energy Hazel O’Leary visitedLLNL last October 21, she brought a message

long awaited by researchers here and throughout theinternational scientific community. The Secretaryannounced to an enthusiastic crowd of employees,community leaders, and industry representatives that shehad approved “Key Decision 1” to build the NationalIgnition Facility (NIF) and that LLNL’s expertise in laserfusion made it the preferred site for the approximately $1-billion facility.

An international research center comprising theworld’s most powerful laser, NIF will achieve ignition of fusion fuel and energy gain for the first time in alaboratory. When it begins operation in 2002 (see the boxon p. 2), NIF will serve researchers from many differentinstitutions and disciplines for both classified andunclassified projects.

As a DOE–Defense Programs facility, NIF will be akey component in the Department’s science-basedStockpile Stewardship Program to ensure the safety andreliability of the nation’s enduring stockpile of nuclearweapons. By yielding considerably more fusion energythan is put in by the laser (energy gain), it also will bringus a large step closer to an inertial fusion energy (IFE)power plant. NIF will also advance the knowledge ofbasic and applied research in high-energy-density science.Finally, the project to construct NIF and equip it with themost modern components will spawn technologicalinnovation in several U.S. industries and enhance theirinternational competitiveness.

If NIF is sited at LLNL, it will be the largestconstruction project and permanent facility in our history.Scientists worldwide will be performing research here,invigorating LLNL in many existing and new technical

areas. Additionally, the project will benefit dozens of BayArea and California construction and manufacturingcompanies, creating many new jobs.

NIF is comparable in size to a municipal sports stadium.(See the illustration on p. 9.) The heart of the facility is aneodymium laser system of 192 beams, with each beamoptically independent for outstanding experimental designflexibility. Together, the laser beams will produce 1.8 million joules (approximately 500 trillion watts ofpower for four billionths of a second) of energy. Incomparison, LLNL’s Nova laser, currently the world’slargest, produces 45,000 joules (approximately 15 trillionwatts for three billionths of a second).

The beams will compress and heat to 100 milliondegrees 1- to 3-millimeter-diameter capsules containingdeuterium-tritium fuel, thereby producing ignition (self-heating of the fusion fuel) followed by a propagatingthermonuclear burn. The implosion process will producefusion burns with significant energy gain, up to ten timesthe energy required to initiate the reaction. (See the box on p. 5.)

This sequence of events will produce the equivalent of a miniature star lasting for less than a billionth of asecond, yet long enough for researchers to make accuratemeasurements of its temperature, pressure, and otherproperties. Indeed, we will “look” at fusionmicroexplosions with a spatial resolution of 10 micrometers(about one-tenth the size of a human hair) and freeze theaction at a time resolution of 30 picoseconds (trillionths of a second).

Culminates 30 Years of ResearchNIF will represent the scientific culmination of more

than 30 years of inertial confinement fusion (ICF)

W

The NationalIgnition Facility: An Overview

OVERVIEW

Planning for NIF

Overview E&TR December 1994

2

research at LLNL and throughout the world. Calculationsby Livermore physicists in the 1960s showed that a lasergenerating a megajoule of light in ten billionths of asecond could ignite a fusion microexplosion in thelaboratory. They reasoned that such microexplosions

could be used to simulate the detonation of nuclearweapons and that such a fusion technology might one daygenerate electrical power.

Over the years, Livermore scientists built and operateda series of laser systems, each five to ten times more

powerful than its predecessor. (SeeFigure 1.) Long Path, Livermore’sfirst neodymium glass laser, wascompleted in 1970 and was ourworkhorse laser for five years. Ourtwo-beam Janus laser, completed in1974, demonstrated lasercompression and thermonuclear burnof fusion fuel for the first time. In1975, our one-beam Cyclops laserbecame operational and was used toperform target experiments and totest optical designs for the futureShiva laser. A year later, the two-beam Argus laser increased ourunderstanding of laser–targetinteractions.

During this time, laserdevelopment and ICF experimentsproceeded rapidly at other facilities,including KMS Fusion, theLaboratory for Laser Energetics(LLE) at the University ofRochester, and the Naval Research

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Figure 1. The energy and power of neodymium glass lasers built for inertial confinementfusion (ICF) research at LLNL have increased dramatically over the past two decades.

The Department of Energy’sprocedure for approving large projects such as NIF is based on “Key Decisions” (KDs) made by theSecretary of Energy. In January 1993,the Secretary approved KD 0, whichaffirmed the need for NIF andauthorized a collaborative effort bythe three DOE defense laboratoriesand the University of Rochester’sLaboratory for Laser Energetics toproduce a conceptual design report.This report was completed in April 1994.

KD 1 was signed by the Secretaryin October 1994. This decisioninitiated preliminary design, safety

analysis, cost and schedule validation,and a two-year Environmental ImpactStatement, which will include publicinvolvement. NIF has been identifiedas a low-hazard, non-nuclear facilitybased on the Preliminary HazardsAnalysis Report.

In addition, the DOE has agreed to steps before KD 2 that willexamine NIF’s likely impact onnonproliferation and stockpilestewardship issues. KD 2, scheduledfor late fiscal year (FY) 1996,includes detailed engineering design, further cost and schedulevalidation, and final safety analysis.KD 3, in late FY 1997, will authorize

construction and major procurements.KD 4, in late FY 2002, will authorizefacility operation and the firstexperiments.

Detailed planning for NIF has been led by five institutions that havelong collaborated on laser fusionexperiments: LLNL, Los AlamosNational Laboratory, Sandia NationalLaboratory, the University ofRochester, and General Atomics. The cooperative spirit of the fiveinstitutions and their interactions withindustry and the public were cited byVic Reis, Assistant Secretary forDefense Programs, during a visit toLLNL last November.

E&TR December 1994 Overview

3

Laboratory. Major programs in the Soviet Union, Japan,China, Germany, France, and the United Kingdom wereestablished or expanded.

In 1977, the 20-beam Shiva laser was completed. Thelargest American ICF project at that time, it deliveredmore than 10 kilojoules of energy in less than a billionthof a second. Meanwhile LLE’s 24-beam Omega systembecame operational in 1980. Novette, which came on linein 1983, was the first laser designed to generate green andultraviolet light. It confirmed work done at several ICFcenters, showing that plasma instabilities were suppressedby shorter wavelength light.

As a result of this work, Nova was redefined as a 10-beam system with frequency conversion rather than the20-beam infrared system originally approved. Novabecame operational in late 1985, the same time that theFrench Phebus laser, consisting of two Nova-stylebeamlines, was completed.

Using the Nova and Omega lasers, as well asunderground nuclear experiments in the Halite-CenturionProgram, scientists have made important progress inunderstanding ICF. At the same time, the study of scalingglass lasers to systems much larger than Nova providedthe technical guidelines for a future system to create targetignition and energy gain. (See Figure 2.) ICF takes its

place among decades-long international efforts to reachfusion conditions in the laboratory. Unlike magneticfusion designs, ICF strives to compress fusion fuelisentropically before raising its ion temperature toignition levels.

Such an ignition facility was strongly recommended bythe National Academy of Sciences and the DOE FusionPolicy Advisory Committee in their 1990 reports and bythe DOE Inertial Confinement Fusion AdvisoryCommittee and the JASON Review Committee in 1994.This facility, eventually called the National IgnitionFacility, would use improved laser design andengineering as well as advanced optics, laser amplifiers,and frequency converters.

Two conceptual designs for NIF were prepared, oneusing 240 beams and the other employing 192. As Figure 3 indicates, the smaller and less expensive of thesedesigns adequately meets target requirements, with asafety margin of about two for achieving ignition.

NIF BenefitsBy demonstrating thermonuclear ignition and burn in

the laboratory for the first time, NIF will play a critical role in the DOE’s science-based Stockpile StewardshipProgram. With the end of the Cold War, America’s nuclear

Figure 2. NIF will be the culmination ofover two decades of research by theinternational ICF community into the use ofglass laser systems to create controlledtarget ignition and energy gain in thelaboratory. Unlike magnetic fusion energy(MFE) designs (e.g., the Princeton LargeTaurus, Doublet II, the Tokamak Fusion TestReactor, and the Joint European Taurus,which trap fuel in an intense magnetic forcefield to induce fusion), ICF strives tocompress fusion fuel isentropically beforeraising its ion temperature to ignition levels.

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weapons stockpile is beingsignificantly reduced. However,nuclear weapons will continue toexist for the foreseeable future. In theabsence of underground testing, thereliability, safety, and effectiveness of the remaining stockpile can beassured only through advancedcomputational capabilities andaboveground experimental facilities.NIF is the only facility proposed forthe program that addresses fusion andseveral other physical processes thatinvolve high-energy density.

Data from NIF will complementdata from hydrodynamic tests andwill also be used to improve thephysics in computer codes that areneeded to certify the safety andreliability of our remainingstockpile. These more accuratecodes will better simulate potentialproblems in the enduring stockpileas well as improve our interpretationof data from the archives of pastunderground tests.

NIF will also help to maintain theskills of the nation’s small cadre ofnuclear weapons scientists and toattract new scientists to help manage

Overview E&TR December 1994

4

Figure 4. The steps of an inertial confinement fusion reaction, which produces up to ten times the energy used to initiate ignition. Underlaboratory conditions, the sequence produces energy gain equivalent to the power of a miniature star lasting for less than a billionth of a second.

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Figure 3. Two conceptual designs were prepared for NIF, one using 240 beams and the otheremploying 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 thebaseline operation requirement of 1.8 megajoules and 500 terawatts of power (the optimumpoint for target ignition indicated by the yellow star), but it can also operate at higher energyand power with increasing risk of damage to the system—up to a maximum acceptable risk or“redline” performance (2.2 megajoules and 600 terawatts). The shaded area indicates theincreasing target margin above the minimum power and energy required to achieve ignition.

Laser beams rapidly heat the surface of the fusion target forming a surroundingplasma envelope.

Fuel is compressed by the rocket-like blowoff of the hot surface material.

During the final part of the laser pulse, the fuel core reaches 20 times the density of lead and ignites at 100,000,000˚C.

Thermonuclear burnspreads rapidly through the compressed fuel, yielding many times the input energy.

Inward transported thermal energyLaser energyBlowoff

have been the subject of more than 1000 scientific paperspublished by ICF researchers since 1985.

As the world’s largest optical instrument, NIF willspur key U.S. high-technology industries, such as optics, lasers, materials, high-speed instrumentation,semiconductors, and precision manufacturing. U.S.industry has long been a major participant in the rapidprogress of ICF research. Today DOE ICF scientists areinvolved in 24 cooperative research and developmentagreements (CRADAs) totaling over $160 million in thefields of microelectronics, microphotonics, advancedmanufacturing, biotechnology, precision optics,environmental sensors, and information storage.

ICF scientists have also won 26 R&D 100 Awards foroutstanding technological developments withcommercial application. Most recently, LLNL andMoscow State University received a 1994 R&D 100award for growing potassium dihydrogen phosphate(KDP) crystals much more rapidly, an achievement withsignificant promise for NIF.

Much further development in manufacturingtechnologies over the next three years is needed to meetthe cost goals for NIF. For example, the size of NIFoptics, such as KDP crystals, is up to two times largerthan those used in Nova. In addition, the required damagethreshold of these optics is two to three times higher thanthat of Nova’s optics. We are planning a program to

E&TR December 1994 Overview

5

the Stockpile Stewardship Program, support U.S. nuclearnonproliferation goals, aid in the safe dismantlement ofnuclear weapons, and respond to nuclear weapon crises.

Another major goal of NIF is to help establish thescientific basis for environmentally friendly electricalpower generated by IFE. The National Energy Policy Actof 1992 calls for DOE to support both IFE and magneticfusion energy approaches to achieving fusion energy as apractical power source.

As envisioned, IFE power plants will use high-repetition-rate laser or ion drivers (about 10 pulses persecond). The heat from the continual fusion reactions willbe absorbed by coolants surrounding the fuel pellets andconverted to electricity. NIF will provide crucial data onthe design requirements of these drivers and on othercritical components. Such data will also be used to helpdesign an Engineering Test Facility that is planned forearly next century as the next step toward a functioningIFE power plant.

NIF will also provide new capabilities for the high-energy-density physics community. Because fusiontargets will experience temperatures and pressures similarto those found in stars, data from NIF experiments willattract scientists working in such areas as astrophysics,space science, plasma physics, hydrodynamics, atomicand radiative physics, material science, nonlinear optics,x-ray sources, and computational physics. These fields

Inertial Confinement FusionThermonuclear fusion is the

energy source for our sun and thestars and for nuclear weapons. In a fusion reaction, nuclei of lightelements, such as deuterium andtritium (isotopes of hydrogen),combine at extreme temperaturesand pressures to form a heavierelement, in this case helium. Theenergy released in a fusion reactionis about one million times greaterthan that released from a typicalchemical reaction.

There are essentially threemethods for confining fusion fuel reactions: gravitationalconfinement, as inside stars, andmagnetic and inertial confinement,which can be achieved in the

laboratory. Both magnetic fusionand inertial confinement fusion(ICF) research are supported byDOE.

In ICF, energetic driver beams(laser, x-ray, or charged particle)heat the outer surface of a fusioncapsule containing deuterium andtritium (D-T) fuel (see Figure 4).As the surface explosivelyevaporates, the reaction pressurecompresses the fuel to the densityand temperature required for D-Tfusion reactions to occur. Theenergy released further heats thecompressed fuel, and fusion burnpropagates outward through thecooler, outer regions of the capsulemuch more rapidly than the

“inertially confined” capsule canexpand. The resulting fusionreactions yield much more energythan was absorbed from the driverbeams.

There are two basic approachesto ICF. In the first, called directdrive, laser beams impinge directlyon the outer surface of the fusiontarget. In the second approach,called indirect drive, beams heatthe surface of a metal case(hohlraum), causing emission of x rays that strike the fusion targetcapsule and drive the implosion.(See the box and figure on p. 38,which provide additionalinformation about direct- andindirect-drive targets.)

Overview E&TR December 1994

6

the science-based Stockpile Stewardship in which NIFwill play an indispensable role; NIF’s potentialcontributions to energy research; and NIF’s likelyimpact on advancing science and technology. We alsodescribe more fully the NIF facility by taking a tour ofit from a laser beam’s point of view, and finally, wereview the environmental, safety, and healthconsiderations relevant to NIF.

For furtherinformationcontactJeffrey Paisner(510) 422-6211 orKenneth Manes(510) 423-6207.

help our suppliers substantially reduce their costs to manufacture high-quality, state-of-the-art NIFcomponents, an achievement that will help themcompete better in the international market.

When the first experiments are carried out on the NIFin 2002, they will begin a new era of advanced researchwith a laser system so powerful it was only dreamedabout several decades ago. By achieving ignition andenergy gain for the first time in the laboratory, NIF willmaintain U.S. world leadership in ICF research and willdirectly benefit many different research communities. Ifsited at LLNL, it will considerably strengthen thislaboratory and make it an even greater center ofscientific research.

In this special issue of Energy and TechnologyReview dedicated to NIF, we describe in separatearticles the importance of NIF to weapons physics and

HE National Ignition Facility(NIF) will house the world’s

most powerful laser system. Figure 1is an artist’s sketch of the proposedlaser and target area building. Theoverall floor plan is U shaped, withlaser bays forming the legs of the U,and switchyards and the target areaforming the connection.

The NIF will contain 192independent laser beams, each ofwhich is called a “beamlet.” Eachbeamlet will have a square apertureof a little less than 40 centimeters on a side. Beamlets are groupedmechanically into four large arrays—or bundles of beamlines—with thebeamlets stacked four high andtwelve wide, as shown in blue on theleft-hand side of the sketch. The 192

To give some perspective on itsoverall dimensions, the NIF buildingis roughly 200 meters long ¥ 85 meterswide (about 600 ¥ 250 feet). Thesedimensions are a little smaller thanthose of a typical covered footballstadium. As an example, Figure 2compares the NIF building with theMinneapolis Metrodome.

This article takes us on a tour ofthe proposed facility. First, we followthe complex path of a photon from themaster oscillator at the beginning ofthe laser chain, through the lasercomponents, and on to the target.After this tour, we discuss some ofthe principal laser components andtarget experiments in more detail.Finally, we describe the results of theBeamlet Demonstration Project that

laser beamlines require more than9000 discrete, large optics (largerthan 40 ¥ 40 cm) and severalthousand additional smaller optics.

The laser output beams strike aseries of mirrors, which redirect themto the large target chamber shown onthe right side of Figure 1. Fromswitchyards to the target chamberroom, the beams are in groups of four(2 ¥ 2 arrays) and follow the beampaths shown in red. At the targetchamber, the beams pass throughfrequency-conversion crystals thatconvert the infrared laser outputbeams to ultraviolet laser light. Theythen pass through lenses that focusthe ultraviolet beams on a tiny targetlocated in the center of the targetchamber.

A Tour of the ProposedNational Ignition Facility

On a conceptual walk-through of the proposed National IgnitionFacility, we follow the path of a photon from the master oscillator and

preamplifier at the beginning of the laser chain, through the mainlaser components, to the target. We conclude with the results of our

recent Beamlet Demonstration Project, which demonstrated aprototype of one of the 192 laser beams that will be required to achieve

ignition and energy gain in inertial confinement fusion targets.

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was recently completed at LLNL. Aspart of its core activities, the InertialConfinement Fusion (ICF) Programdeveloped a prototype of a singlebeamlet of NIF to validate thetechnology needed for the nextgeneration of glass-laser drivers.

Master Oscillator

We begin our tour approximatelyin the center of the facility, where alaser pulse is born in the masteroscillator room. Here, four oscillatorsmade of neodymium-doped optical

fiber generate weak laser pulses atfour separate frequencies (or colors oflight). Each pulse is launched into anoptical fiber system that amplifiesand splits the pulse into 192 separatefibers, 48 of each color. The fourcolors are used to smooth theintensity (power per unit area) of thelaser spot on the target.

The optical fibers carry the laserpulses to 192 low-voltage opticalmodulators. The modulators for NIFwere derived from the integrated-optics modulators that are now beinginstalled in very-high-speed optical

fiber communications networks. Themodulators allow us to tailor thepulse shape independently in eachbeamlet under computer control. In this way, the 192 pulses can becarefully shaped and balanced to set up exactly the conditions anexperimenter needs on a targetwithout rearranging any laserhardware in the facility. An opticalfiber then carries the individuallytailored pulse to each beamlet. Thepower in the laser pulse at this pointis a little less than a watt. Typicalpulses are a few nanoseconds long

NIF Facility Description E&TR December 1994

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Amplifier columns

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support

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Figure 1. View of the proposed laser and target area building for NIF. This facility will contain the world’s most powerful neodymium glasslaser system, 50 times more powerful than Nova, currently the world’s most powerful laser. This sketch shows the laser bays, which form thetwo legs of the U-shaped floor plan, and the switchyards and target area forming the connection.

(a nanosecond is 10–9 second), so the energy is a few nanojoules (ananojoule is 10–9 joule).

Preamplifier

Optical fibers carrying the pulsesfrom the master oscillator roomspread out to 192 preamplifierpackages. As shown in Figure 1, theNIF preamplifiers are located beneaththe focal plane at the center of thelarge transport spatial filters, whichare located between the lasercomponents and the target chamber.

Each preamplifier package has aregenerative amplifier in a ring cavity.This device amplifies the pulse by afactor of about a million, from about ananojoule to a millijoule, with veryhigh stability. The amplifier usessmall neodymium glass amplifierspumped by semiconductor laserdiodes so that it has very stable gainand requires little servicing. The laserpulse then enters spatial beam-shapingoptics and a flashlamp-pumped, four-pass rod amplifier, which converts itto about a 1-J pulse with the spatialintensity profile needed for injectioninto the main laser cavity.

Following a Pulse Through theMain Laser Components

Figure 3 shows the layout of themain laser components of a NIFbeamlet. These components take thelaser pulse from the preamplifier allthe way to a frequency-convertedpulse headed to the target. We firstfollow a pulse through the variouscomponents and then discuss theirfunctions in more detail.

A pulse of laser light from thepreamplifier reflects from a smallmirror labeled LM0 in Figure 3. Thismirror is located near the focal planeof the pair of lenses labeled lens 1and 2 and identified as the transportspatial filter. Light comes to a focusat the focal plane of the transportspatial filter and re-expands to a sizeof a little less than 40 cm at lens 1 ofthe spatial filter, where it againbecomes a parallel beam. The beam

E&TR December 1994 NIF Facility Description

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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 usingthe CHAINOP family of numerical codes. These codes model the performance and cost of high-power, solid-state inertial confinement fusionlaser systems. The path taken by a photon is described in the text.

Figure 2. The NIFbuilding is similar insize to a modernmunicipal stadium.For perspective, theNIF building iscompared here to theMinneapolisMetrodome.

passes through booster amplifier 3,reflects from the polarizer, isamplified further in cavity amplifier2, and goes through a second spatialfilter identified as the cavity spatialfilter. After passing through amplifier 1, the beam reflects from adeformable mirror (mirror LM1 at thefar left end of Figure 3). After onceagain passing through amplifier 1, thebeam comes back through the cavityspatial filter and amplifier 2.

Meanwhile, the componentidentified as the Pockels cell inFigure 3 is energized. This importantcomponent rotates the plane ofpolarization of the laser light fromhorizontal to vertical. In thispolarization, the pulse passes throughthe polarizer and strikes mirror LM2,which redirects it back once againtowards mirror LM1. The Pockelscell rotates the polarization back tohorizontal, and the beam passes backthrough amplifier 2 and the cavityspatial filter and makes anotherdouble pass through amplifier 1,reflecting from LM1. It then passesthrough the cavity spatial filter andamplifier 2 one more time.

By this time, the Pockels cell hasbeen de-energized so that it no longerrotates the polarization of the pulse.As a consequence, the pulse reflectsfrom the polarizer and is further

amplified by amplifier 3 to an energyof about 17 kJ for a typical ignitiontarget pulse shape. Now the pulsepasses through the transport spatialfilter on a path slightly displacedfrom the input path. Because it isdisplaced, the output pulse justmisses the injection mirror LM0, themirror where laser light was firstreflected when it came from thepreamplifier.

The pulse travels through a longbeam path reflecting from severaltransport mirrors until it reaches thetarget chamber. (For simplicity,Figure 3 does not show all thetransport mirrors that will be installedin NIF.) Mounted on the targetchamber is a frequency converter thatchanges the infrared laser pulse toultraviolet laser light. A focusing lensbrings the ultraviolet pulse to a focusat the center of the target chamber. Adebris shield protects the focusinglens from any target fragments andmay also have a pattern etched intoits surface to reshape the distributionof laser intensity in the focal spot onthe target.

Four pulses from differentbeamlets, each at a slightly differentcolor or frequency, come to a focus ata single spot on the target. Theintensity profile of the sum of thesefour spots is much smoother than the

profile of each spot individually.Smooth intensity profiles lead tobetter-understood experimentalconditions and better targetperformance.

An important feature of NIF is itsintegrated computer control system.This system uses a high-speed opticalfiber network to connect roughly25,000 control points, sensors, anddistributed processors. The items thatare controlled by computer includemotors and switches for alignment,diagnostic systems for the laserbeams, data from target diagnostics,data processing stations, and all othercontrol and information features ofthe facility.

More About the Main LaserComponents

AmplifiersFigure 4 is a top-down view of a

glass amplifier. The NIF amplifiersare constructed from slabs ofneodymium-doped phosphate glassset vertically on edge at Brewster’sangle to the beam. At this angle,horizontally polarized beams havevery low reflective losses whilepropagating through the plates. Theglass slabs are 46 ¥ 81 cm to give aclear aperture of 40 ¥ 40 cm from thebeam’s point of view.

Figure 5 shows how the amplifierslabs are grouped together into largearrays. Such grouping reduces thenumber of required parts and floorspace, hence the cost of the facility.Long xenon flashlamps extendvertically across a stack of four slabs,an arrangement resulting in a lengththat is convenient for the pulsed-power system that drives theflashlamps. The width of 12 slabs isconvenient for design of themechanical structures.

The NIF amplifiers are suspendedbeneath a support frame in order toprovide access from the bottom toreplace slabs, flashlamps, and blast

NIF Facility Description E&TR December 1994

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Neodymium-doped glass slabs set at Brewster’s angle to the beam

Glass blast shield

Xenon flashlamps

Reflector

Beam propagation

Figure 4. Top view of a large glass amplifier using slabs of neodymium-doped glass set atBrewster’s angle to the beam and pumped by xenon flashlamps. The glass blast shieldsprotect the slabs from acoustic disturbances and dirt generated by the flashlamps.

shields. It is extremely important tominimize contamination of theamplifiers by particles that otherwisemight burn into the glass surfaceswhen illuminated by the intenseflashlamp or laser light. Access fromthe bottom of the amplifiers ensuresthat service personnel and equipmentare always below and downwindfrom sensitive surfaces, so that dirtfalls to the floor and not into openamplifier structures. Figure 6 shows a servicing cart in the process ofinstalling a stack of four laser slabsinto the amplifier structure.

The pulsed-power system for theNIF uses advanced self-healing energystorage capacitors developed for use inStrategic Defense Initiative projects.These capacitors store energy at aboutfour times the density and half the costper joule of conventional capacitors,such as those used in the Nova laserfacility at LLNL.

The number of amplifier slabs andtheir distribution among the threeamplifiers were chosen to maximizethe output power of the laser over thedesired range of pulse shapes. Forshort pulses, the limit is set bynonlinear effects in amplifiers 2 and3. For long pulses, the limit switchesover to nonlinear effects in amplifier1 and the limit set by the total amountof energy stored in the slabs. Wemight have eliminated amplifier 3and placed 9 or 11 slabs in theamplifier 2 position. However, thepolarizer coating suffers opticaldamage at a rather low fluence(energy per unit area). Such damagelimits the laser output fluence tosignificantly less than we can runwith the configuration shown inFigure 3. Notice that amplifier blocksare restricted to have an odd numberof slabs (see Figure 4) so that gaingradients in the end slabs cancel.

Spatial FilterThe NIF has two spatial filters in

each beamline. In essence, a spatial

E&TR December 1994 NIF Facility Description

11

Amplifier glass slab

To target

Amplifier column

Figure 5. The maincavity amplifierassembly is typical ofall the NIF amplifiers.Notice that a columnconsists of anamplifier that is 4beams high. Groupsof columns that are12 beams wide forman amplifierassembly.

Slabcassette

Amplifier servicing cart

Figure 6. To facilitate maintenance, the slab and flashlamp cassettes shown at the top will bechanged from underneath an amplifier column using a special cart. This approach will allowthe critical amplifier components to be protected from the laser bay environment at all times.

filter—or image relay pair—is a pairof lenses separated by the sum oftheir focal lengths. A parallel beamincident on one lens comes to afocus in the center and emerges as aparallel beam at the other lens.

Spatial filters serve severalfunctions in large lasers. For NIF,we positioned a pair of lenses sothat an image of the very cleanintensity profile injected from thepreamplifier reforms near theamplifiers and at the frequencyconverter. Diffraction causesintensity noise to grow in lasersystems, but this growth is reset tozero in the vicinity of an image ofthe original input profile. Inaddition, nonlinear effects in thelaser cause small-scale intensitynoise in the laser to grow, but this

small-scale noise comes to a focusdisplaced to the side of the mainfocus in the spatial filter. Thisdisplaced focus means that we canplace a small pinhole at the focalplane that blocks this noise whileallowing the main beam to gothrough. In addition, the focal planeinside the spatial filter gives us aconvenient location for injecting theinput pulse from the preamplifierwithout requiring any additional,expensive, large-aperture opticalcomponents.

Because the intensity near the focalplane is very high, the spatial filters inlarge lasers such as NIF must beoperated in a vacuum. Figure 7 showsthe large transport spatial filter vacuumvessel for NIF. The cavity spatial filteris similar, but somewhat shorter.

Pockels Cell and PolarizerA Pockels cell uses electrically

induced changes in the refractiveindex of an electro-optic crystal, suchas KDP (potassium dihydrogenphosphate or KH2PO4) to rotate thepolarization of light. When combinedwith a polarizer, the Pockels cell canserve as an optical switch that directslight into one or the other of twopossible paths, and it is used for thisfunction in the NIF laser.

Conventional Pockels cells requirea crystal that is roughly the samethickness as the beam diameter. Acrystal this thick is completelyimpractical for the NIF’s 40-cm beam.Instead, the Pockels cell used in theNIF laser is a new type developed atLLNL. As shown in Figure 8a, itcontains a thin plate of KDP placed

NIF Facility Description E&TR December 1994

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17.3 meters

65 meters

Figure 7. Spatialfilters in large lasersmust contain a vacuum.Shown here is thetransport spatial filtervacuum vessel for NIF.The basic functions ofthe transport spatialfilter and cavity spatialfilter are similar, buttheir lengths andinternal componentsare different. Eachvessel contains 48 beams. The entirestructure is supportedfrom the floor byreinforced concretepillars. The pulse-generation system islocated below thecenter section. Laserdiagnostic units aresupported by a frameattached to the filtertop. Access to internalmechanisms is viadoors on the vesselsides.

between two gas-discharge plasmas.The plasmas serve as conductingelectrodes, which allow us to chargethe surface of the thin crystal plateelectrically, but they are so tenuousthat they have no effect on the high-power laser beam passing through the cell. Figure 8b shows a plasmaelectrode Pockels cell of this sortwith a clear aperture of 35 cm. Thedevice is now operating in ourBeamlet Demonstration Project atthe full laser fluence proposed forthe NIF.

The polarizer for the switch is amultilayer dielectric coating set atBrewster’s angle to the beam. These

thin-film polarizers are difficult tomanufacture, but research supported by LLNL at commercialmanufacturers has improved theirprocess control and demonstrates thatlarge polarizers meeting the NIFspecifications are now available.

Deformable MirrorThe NIF laser must have beam

quality high enough that it can placeall of the energy from each beamletinto a circle of about half a millimeterin diameter at the center of the targetchamber. It is possible to purchaseoptical components finished wellenough to achieve this goal, but the

cost of fabrication is higher than wewould like.

We can use less expensivecomponents if we design the laser toinclude an adaptive correction systemthat compensates for distortions in the beam. An adaptive system alsoallows us to compensate for otherimportant distortions, such as thermalgradients. Recent advances inadaptive optics in the Atomic VaporLaser Isotope Separation program atLLNL, and at several commercialcompanies and government researchfacilities, show that the cost of thedeformable mirror, sensor, andprocessor technology required to

E&TR December 1994 NIF Facility Description

13

Switchpulser

Switchpulser

Cathode

Side 1

Plasma

Dischargecurrent

Electronemission

Side 2KDP

Plasma

Dischargecurrent

KDPchargingcurrent

z (cable)

Switchpulser

Cathode

Anode

Electronemission

Anode

Figure 8. (a) The Pockels cell optical switch. This optical switch uses plasma as transparent, high-damage-threshold electrodes to chargethe potassium dihydrogen phosphate (KDP) crystal. (b) A large-aperture, plasma electrode Pockels cell installed in the BeamletDemonstration Project. Tests show that this device meets the requirements for timing, efficiency, and stability needed for NIF.

(a) (b)

implement such a system has fallen to the point that adaptive correctionsystems are very desirable for the NIFfacility. Figure 9 shows a typicaldeformable mirror that useselectrostrictive actuators to bend the mirror surface to compensate for wavefront error. This mirror is installed on the BeamletDemonstration Project, where we arestudying its performance. The NIFwill use a similar, but larger, mirroras mirror LM1 in Figure 3.

Frequency ConverterA neodymium glass laser generates

light at a wavelength of about 1 micrometer in the infrared region.However, we know that inertial

fusion targets perform much betterwhen they are driven with ultravioletradiation. The NIF laser will convertthe infrared (1.05-µm) light toultraviolet (approximately 0.35 µm)using a system of two nonlinearcrystal plates made of KDP, the sametype of crystal that is used in thePockels cell.

Figure 10 shows the arrangementof the two crystal plates. The firstplate converts two-thirds of theincident 1.05-µm radiation to thesecond harmonic at 0.53 µm. Thenthe second crystal mixes thatradiation with the remaining 1.05-µm light to produce radiation at 0.35 µm. This process has a peakefficiency greater than 80%, and the

efficiency can exceed 60% for thecomplex pulse shapes used to driveignition targets.

Target Area and TargetDiagnostics

Figure 11 shows an end view ofthe NIF target area. From thisperspective, we can see the beampaths from the laser output throughthe turning mirror array to the targetchamber. The target chamber is a10-m-diameter aluminum sphere.The beams enter the chamber in twoconical arrays from the top and twofrom the bottom through final opticspackages mounted to the targetchamber. Figure 12 shows a final

NIF Facility Description E&TR December 1994

14

7 cm

Figure 9. The cavity mirror farthest from the target area (LM1 inFigure 3) is a deformable mirror used for performing wavefrontcorrections of the beam. Electrostrictive actuators bend the mirrorsurface to compensate for wavefront error. This photograph showsa 70- ¥ 70-mm deformable mirror currently used on the BeamletDemonstration Project. The NIF will use a mirror that is similar tothis one, but larger.

Extraordinaryaxis

Extraordinaryaxis

Ordinaryaxis

KDPcrystal

Ordinaryaxis

Polarization(input radiation)

Second-harmonicgeneration

Third-harmonicgeneration

Figure 10. The NIF configuration for frequency conversion to thethird harmonic using two KDP crystal plates. The NIF lasergenerates light in the infrared region (this 1.05-µm wavelength lightis shown as red in the drawing). However, inertial fusion targetsperform better with ultraviolet radiation. This 0.35-µm wavelengthlight is called the third harmonic (shown as blue). The first KDPcrystal (left) converts two-thirds of the input radiation (red) tosecond-harmonic radiation (green). The second crystal mixes theremaining input radiation with the second harmonic to producethird-harmonic radiation (blue). Peak conversion efficiencies canexceed 80%.

optics assembly subsystem.Diagnostic instruments, such as x-ray spectrometers, microscopes,and cameras, are mounted aroundthe equator and at the poles of thetarget chamber.

Figure 13a is a scale drawing of atypical fusion ignition target that willbe used with this chamber. Thetarget is a metal cylinder—typicallymade of gold or lead—about 6 mm indiameter and 10 mm long. It containsa plastic fusion capsule about 3 mmin diameter. The capsule is chilled to a few degrees above absolute zero

and is lined with a layer of soliddeuterium–tritium (DT) fusionfuel. The hollow interior containsa small amount of DT gas.

Figure 13b is an artist’s 3-Drendering showing how laserbeams deposit their energy on theinside surface of the metal cylinderor “hohlraum” where the energy isconverted to thermal x rays. The x rays heat and ablate the plasticsurface of the ignition capsule,causing a rocket-like pressure onthe capsule and forcing it toimplode.

Figure 14 is an x-ray image of atarget shot from the Nova lasershowing the glowing spots whereNova’s ten beams strike the insidesurface. In this image, artists drew inthe laser beams and the outline ofthe cylinder, which are invisible in an x-ray photograph. For thisexperiment, the metal wall was madethin enough that some of the x raysleaked through to be photographed.Thicker walls are used for mosttargets. In the language of fusionresearch, this target is called an“indirect-drive” target because the

E&TR December 1994 NIF Facility Description

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Switchyard building

Switchyard mirror

Personnel access area

Final optics assembly

Target chamberBase mat

Lower mirror support frame

Target diagnostics

Domed roof Target area building Upper mirror support frame

Turning mirrors

Laser ports

Figure 11. Cut-away view of the NIF target area showing the major subsystems. The laser beams focus energy onto a target located at thecenter of the target chamber, which is housed in a reinforced-concrete building. Target diagnostics mounted on the chamber will collect theexperimental data.

laser beams do not strike the fusioncapsule directly. NIF can also study“direct-drive” targets in which thereis no hohlraum and the beams dostrike the capsule directly.

To illustrate the type of diagnosticswe use to study fusion targets, Figure 15 shows a sequence ofpictures of a fusion capsule implosionfrom a direct-drive target on theOmega laser at the University ofRochester. Here, the fusion capsule issimilar to, but smaller than, the oneshown in Figure 14. (Indirect-drivetargets give similar pictures, but thehohlraum and other complicationsmake the pictures less clear.) Theimages are from an x-ray framingcamera microscope (that is, thesequence of frames was taken by avery-high-speed motion picturecamera). Each frame lasts for 50 picoseconds (50 trillionths of asecond).

NIF Facility Description E&TR December 1994

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Bellows assembly

Debris shield

Focus lens

KDP frequency- conversion crystals

Calorimeter

Target chamberinterface spool

Figure 12. The finaloptics assembly is a single, integratedstructure that ismechanicallysupported by andfastened to the targetchamber. Thefrequency-conversioncrystals (see Figure 10) are shownat the top. Thissystem will convertfour infrared beamsto the third harmonic,focus the beams ontoa target, and providebeam smoothing andcolor separation. Thecalorimeter is used toobtain energymeasurements on theincident beam.

0.35-µm laser beamsin two cone arrays

Shield

Capsules

Figure 13. (a) Scale drawing of atypical fusion target. The outer metalcylinder, usually made of either goldor lead, is about 6 mm in diameter.Inside is a plastic fusion capsule thatis about 3 mm in diameter. Thecapsule is lined with a layer of soliddeuterium–tritium (DT) fusion fuel,and the hollow interior contains asmall amount of DT gas. Laser beamsenter the target in two conical arrays.The outer and inner cones are shownat the top and bottom of the target.(b) An artist’s 3-D rendering of thelaser beams depositing their energyon the inside surface of the hohlraumwhere they are converted to x raysthat heat the target internally,causing it to implode and ignite.

(a) (b)

In Figure 15a, the 0.25-mm-diameter capsule lights up brightly asthe laser beams first strike its coldsurface. In the next several frames, thesurface of the capsule blows outward,and the gas fill is accelerated towardsthe center, compressing to a very highdensity. In Figure 15g, the gas beginsto collide with other imploding gas atthe center of the capsule and comes toa stop. As it stops (or stagnates), thetemperature shoots up to a value atwhich fusion reactions can begin. Thehigh-temperature region at stagnationcan be seen as a bright x-ray spot inFigures 15g through 15j. If this werean ignition target shot using the NIF,the hot gas core or “spark plug”would ignite the surrounding,relatively cold DT layer, producingroughly 10 megajoules of fusionyield. The hot target plasma thenexpands and cools in Figures 15k and 15l.

Ten megajoules is not anenormous amount of energy. Itroughly corresponds to the heatreleased in burning an 11-ouncewater glass full of gasoline (about312 cm3). However, the energy in afusion target is produced and radiatedaway in less than a nanosecond froma volume with a diameter of onlyabout a fifth of a millimeter. To dothat, the target material must reachconditions that are found in natureonly deep within stars and other hotcelestial objects or deep withinnuclear weapons.

In Figure 15, the drive on thetarget was deliberately distorted, sothe implosion formed a little to theright of center from the camera’spoint of view. A perfectly centeredimplosion gives higher temperatures

and better target performance.Nevertheless, these pictures are good illustrations of how targetexperimenters study the effects ofnonideal conditions, such asnonuniform drive pressure, andcompare test results to theirtheoretical models.

NIF Beamlet DemonstrationProject

Glass lasers are a popular andwell-known technology, but the NIFlaser design is significantly differentin many ways from existing largeglass lasers (see the box on p. 18). Tobe quite sure that the NIF system willfunction as we project, it is prudent totest some of the differences well inadvance of construction. In 1992, we

E&TR December 1994 NIF Facility Description

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Figure 14. X-ray image of a target shotfrom the Nova laser. The glowing spots arewhere Nova’s ten beams strike the insidesurface of the target’s thin, cylindricalmetal wall. This target is called “indirectdrive” because the laser beams do notdirectly strike the inner spherical capsule,which contains DT fuel. The outer shell and beam paths, invisible in an x-rayphotograph, are drawn by an artist to showwhere they were located in the originaltarget.

(a) (b) (c) (d)

(e) (f) (g) (h)

(i) (j) (k) (l)

Figure 15. Sequence of pictures of fusion-target implosion taken with an x-ray framingcamera microscope. Each frame lasts for 50 ps and is a positive image, so bright areas areregions of bright x-ray emission. (a) Laser beams strike the 0.25-mm-diameter cold targetshell and start the implosion. (b) through (g) The outer surface of the shell blows off, andthe inner part of the shell and gas implode. In (g), the gas fill collides with itself near thecenter of the implosion, and the rapidly increasing temperature produces a bright spot of x-ray emission. The central hot spot develops through (j) and then begins to expand andcool in (k) and (l).

How the NIF Differs from Other Glass Lasers

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The neodymium glass laser, invented in 1961, was one of the first types of laser to be developed.Researchers quickly realized that it could be scaled upto large beam apertures and extreme peak power. Manylaboratories soon began developing glass laser hardwarefor research on nuclear fusion and other high-energy-density physics.

To reach high energy and power, large glassamplifiers are required. However, it is difficult andinefficient to generate a high-quality output beam fromlarge amplifiers. The design that evolved was calledthe single-pass master-oscillator/power-amplifier(MOPA) chain.

As shown in the illustration below, a MOPA designuses a small oscillator to produce a pulse of a fewmillijoules with a beam diameter of a few millimeters.After passing through other components, such as apreamplifier, the pulse is divided and makes a singlepass through a chain of amplifiers of graduallyincreasing size. The amplifiers are separatecomponents rather than being grouped in large arrays,as in the NIF design.

The MOPA is a low-risk, but expensive, approach toconstructing large glass lasers. Limitations includeconsiderable setup time and cost; very long propagationpaths; the need for critical alignments and physicalchanges to many different components; and thefabrication, assembly, and maintenance of a largenumber of parts.

The Nova laser at LLNL is the largest operatingglass MOPA fusion laser. It contains five sizes ofamplifiers with a total of 41 slabs of glass in each often laser chains. Compared to the size of the Novafacility, the compact design of the NIF multipass

system allows us to put a laser with a typical outputthat is 40 times larger than Nova’s into a building onlyabout twice the size (although the Nova laser does notcompletely fill the building).

For truly large lasers, such as NIF, the MOPA designhas another important disadvantage. The NIF fusion-ignition targets require pulses with a length of 3 to 5 ns.At this pulse duration, we can extract most of the energystored in the laser glass. When we do, the tail end of apulse has a much lower gain than the front end, adifference we call saturation pulse distortion (SPD).MOPA designs require larger and more expensiveamplifiers and preamplifers under these conditions.Multipass lasers, such as NIF, solve the problem of SPDwithout the need for larger preamplifiers. In contrast,Nova uses much shorter pulses and does not extract asmuch of the stored energy.

Even when the Nova laser was designed in 1978, weknew that a multipass design would be potentially muchless expensive to build. However, the necessarycomponent development that was still required meantadditional risks and possible delays. The NIF laserdesign is a result of development efforts started manyyears ago for components such as advanced oscillators,amplifiers, and Pockels cells. Today, our BeamletDemonstration Project integrates all of these newdevelopments. This effort is showing that thetechnology has now progressed to the point that a large,multipass glass laser can be built with low risk.

Master oscillator

Beam- splitters

Laser chains – single-pass amplifiers of increasing size

(more laser chains)

Nova building without lobby

Proposed NIF building

established the BeamletDemonstration Project (or theBeamlet for short) to test some of thenew features. The Beamlet has nowbeen completed and is currentlyoperating reliably up through thefrequency converters at fluence andintensity levels projected for NIF.

We built a single prototype ratherthan a large array of beams because itwas clearly much too expensive toconstruct a full NIF beamline with 4 ¥ 12 amplifier blocks and multiplebeamlets. We did, however, build themain laser amplifiers as an arraystacked two high and two widebecause it was important to begin to

understand the engineering of largeamplifier arrays. To reduce costs,only one of the four apertures in thearray contains high-quality laserglass. The other three aperturescontain an inexpensive glass thatabsorbs flashlamp light in a way thatresembles the laser glass. In addition,the amplifiers and other laserhardware rest on the floor rather thanhanging from a support frame as theywill in the NIF design. Componentsresting on the floor were moreconvenient for the room we hadavailable, although the system ismore difficult to keep clean than thatin the NIF design.

The NIF laser design has evolvedover the past two years as we try tooptimize its cost and match itsperformance to the wide range ofpossible experiments that might beconducted. This evolution has led to afew other small differences betweenthe Beamlet and the NIF designpresented in the Conceptual DesignReport and discussed in the first partof this article. For example, the beamapertures for the Beamlet are slightlysmaller than those for NIF, and thelaser is shorter. We used 16 ratherthan 19 slabs of glass, and these slabsare distributed in a ratio of 11-0-5(eliminating amplifier 2) rather than

E&TR December 1994 NIF Facility Description

19

Amplifier 3

LM2

Power supply(in basement)

Transport spatial filter

Beam dump

Laser diagnostics instruments

LM1

Masteroscillator

Amplifier 1

Cavity spatial filter

Deformable mirror

Preamplifier

Pockels celland polarizer

Frequencyconverters

Figure 16. The Beamlet is a scientific prototype of the NIF multipass laser. This demonstration unit, which is now operating at LLNL, usesessentially the same technology as that for NIF but with one output channel rather than 192.

9-5-5 in the three amplifiers shown inFigure 4. The NIF distribution givesbetter performance for pulses of 4 to5 ns and longer, whereas the Beamletdistribution is better for short pulses.The input pulse from the preamplifieris injected into the cavity spatial filterrather than the transport spatial filter.This design makes the front endslightly larger because the pulse doesnot see the small-signal gain ofamplifiers 2 and 3. However, thedesign simplifies alignment if the input pulse is used for laseralignment, as it is in Beamlet (but notNIF). Otherwise, the Beamlet has allthe NIF components and featuresshown in Figure 4.

Figure 16 is a drawing of theBeamlet facility. The master oscillatorand preamplifier are the sametechnology as that for NIF, but withonly one output channel rather than192 . The laser pulse reflects from adeformable mirror (see Figure 9) andenters the cavity transport spatialfilter. We use a deformable mirror inthis position rather than in the LM1position described for NIF becausethis smaller mirror could be adaptedfrom an existing design at very lowcost. The pulse then passes throughamplifier 1 and reflects from mirror

LM1, returns through the amplifierand cavity spatial filter to the Pockelscell and LM2, makes a second roundtrip through amplifier 1, and returnsthrough the Pockels cell to reflectfrom the polarizer, just as describedfor NIF.

The output pulse then reflects fromthree turning mirrors that are used tofold the Beamlet optical path in twoso that it fits in the available space.The pulse passes through amplifier 3and the transport spatial filter, thenenters the frequency-conversioncrystals. Beam splitters directsamples of the infrared and ultravioletbeams to diagnostic instrumentslocated near the frequency converters.Most of the beam energy is absorbedby an absorbing glass beam dump atthe end of the beamline.

Figure 17 is a photograph of theBeamlet amplifier. The amplifier’sclear aperture is 39 cm, or essentiallythe same as the 40-cm apertureproposed for NIF. The amplifiersperform exactly as predicted fromdesign codes (elaborate computersimulations) we developed forsmaller amplifiers.

Large amplifier apertures such asthis lead to a less expensive NIFbecause there are fewer beamlets,

and many of the costs of a systemscale with the number of beamletsrather than their size. However,amplified spontaneous emissioncauses the gain of large amplifiers todrop by a few percent at the edges ofthe aperture. We compensate for thisgain drop on Beamlet (and NIF) bymaking the input pulse more intensearound the edges.

The large plasma-electrodePockels cell switch installed onBeamlet is shown in Figure 8b. Onlyabout 30 J leak through the polarizerwhen we set this switch to reflect 6 kJfrom the polarizer, so the Pockels celland polarizer are remarkablyefficient.

Figure 18 shows the infrared outputenergy from Beamlet as a function ofthe input energy from the preamplifier.For this set of shots, we set the beamaperture to 34 ¥ 34 cm, a value limitedby the 35-cm clear aperture of thePockels cell crystal. The output energymatches the theoretical model (solidline) very well. We have fired thesystem at an output up to 13.9 kJ at

NIF Facility Description E&TR December 1994

20

Figure 17. The 2 ¥ 2 amplifier modulefor Beamlet shownduring assembly. Toreduce costs, onlyone of the fourapertures in thisarray contains high-quality laser glass.The clear aperture of39 cm is essentiallythe same as thatproposed for NIF.Such large amplifierapertures lead to aless expensive NIF.

16

Out

put e

nerg

y, k

J

Injected energy, J

3-ns pulse5-ns pulse8-ns pulse

12

8

4

00 0.25 0.50 0.75 1.00

Figure 18. Performance of the Beamletamplifier for a 34- ¥ 34-cm beam at threedifferent pulse lengths. The infrared outputenergy is plotted as a function of the inputor injected energy from the preamplifier.This plot shows that the output energymatches our theoretical model (solid line)quite well.

5 ns and at somewhat higher output forlonger pulses. At 5 ns, the fluence(energy per unit area) across the flat-top part of the beam is 14.3 J/cm2.This is about 7% above the nominaloperating point for the NIF ignitiontarget in the Conceptual DesignReport.

We recently conducted the firstseries of Beamlet frequency-conversion tests. For these tests, theaperture of the frequency-conversioncrystals limited the beam aperture to29.6 ¥ 29.6 cm. We have operated theconversion crystals up to 8.7 J/cm2

ultraviolet output in 3-ns pulses, whichis once again about 10% above theNIF nominal operating point. Theconversion efficiency from infrared to ultraviolet was just over 80% for square pulses, as anticipated. The maximum ultraviolet energydemonstrated to date is 6.7 kJ. It is

likely that we can operate atsomewhat higher fluences, but wewill not push beyond the nominal NIFfluences and run the risk of damagingoptical components until we haveconducted all of the most critical testson the system. We will certainly havehigher energies when we obtain largercrystals and expand the beam sizelater this year. We will not quite reach the nominal NIF beam aperture of 36 ¥ 38 cm because the Beamletcomponents are smaller and the beampath is shorter than for NIF.Nevertheless, we should be able torun experiments at apertures of about34 to 35 cm. We have also runcomplex shaped pulses of the sortrequired for ignition targets. In theseexperiments, we obtained conversionefficiency up to 65% and fluence andenergy similar to those for squarepulses.

It is important to have veryuniform intensity profiles in a fusionlaser. Uniform profiles minimize therisk of damage caused by intensitymaxima in the beam and help tomaintain high frequency-conversionefficiency. Figure 19 shows intensityprofiles of the infrared and ultravioletbeams from Beamlet as recorded by television cameras in the laserdiagnostics area. The beam profilesare very smooth and flat across thecenter, as desired, and they roll offsmoothly to zero intensity in a smallmargin around the edge.

Low phase distortions on the beamare also important. Phase distortionsnot only prevent the beam fromfocusing on a small spot, but theyalso degrade the process of frequencyconversion. Figure 20 shows thedistribution of Beamlet energy at thefocus of a lens (in the far field).

E&TR December 1994 NIF Facility Description

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Laser Damage in Optical MaterialsWhen a laser beam strikes a component, it can cause

optical-induced damage. The damage usually appearsonly at high fluence (energy per unit area) and is causedby a small flaw or contamination by foreign material.Flaws or contaminants can absorb enough energy tomelt or vaporize and then disrupt the surroundingoptical material. The cost of a large laser is roughlyproportional to the total beam area, so we push laserdesigns to the highest fluence that can be toleratedwithout damage to minimize the beam area and cost.

On the left is a view through a microscope of a tinylight-absorbing defect in a piece of optical glass. Thisparticular defect is about a twentieth of a millimeter indiameter, or about the diameter of a human hair, and itis 10 or 20 times larger than the typical defect thatconcerns us. When an intense laser beam strikes thedefect, its surface evaporates and explodes causing tinycracks in the glass. As shown on the right, the crackstend to grow larger with each laser shot, and thedamaged spot eventually gets large enough to disruptthe laser beam quality seriously.

In the past 15 years, the number of defects that initiatelaser damage in optical materials has decreaseddramatically. The decrease is a result of painstakingresearch at commercial suppliers and various researchlaboratories. Much of the work was funded by the LLNL laser program and other programs interested inconstructing large lasers. As a result, the NIF laser willoperate at fluence levels that would have been unthinkableat the time Nova was constructed. As one example, theaverage ultraviolet fluence at the output of our BeamletDemonstration Project is more than three times theaverage fluence that would have been safe for the opticalcomponents that were available when Nova was built.

These data reveal that about 95% of the energy is within an angle of ±25 microradians from the center of the spot. Because the NIFrequirement for ignition targets is ±35 microradians, this spot meets thatrequirement. Nevertheless, we wouldlike to improve this value even morebecause some potential experimentscould use a smaller spot.

We obtained the far-field spot inFigure 20 at the end of a sequence thatincluded four full-power Beamletshots over about 10 hours. Much ofthe remaining structure in the spot

(smaller peaks on the curve) is causedby gas turbulence in the amplifiers andbeam tubes. Beamlet’s deformablemirror, which was in operation duringthis test, can correct the long-termthermal distortions in the amplifierslabs very effectively. Indeed, this spotis smaller—by a factor of four orfive—than it would be without suchcorrection. However, the deformablemirror system cannot respond rapidlyenough to correct for gas turbulence.When the glass amplifier slabs arecold and the temperature is uniform,the deformable mirror has less effect

on the spot size, and the spot size isalso somewhat smaller than thatshown in Figure 20. We have onlybegun to study the performance of thedeformable mirror, but even theseearly results confirm that adaptiveoptics will be a highly valuableaddition to NIF.

Next Steps Toward NIF

Our Beamlet tests to datedemonstrate clearly that large,multipass laser systems can operate at the nominal operating conditionsproposed for the NIF. We will nowproceed to explore more extremeoperating conditions and todemonstrate that the proposed targetoptics will give spots of the desiredsize and uniformity at the plane of thetarget in the far field. We will alsotest many other features suggested forNIF, such as new glass compositions,changes in amplifier pumpingconditions, and alternate switchtechnology. The results from ourBeamlet Demonstration Project,along with the models and designcodes we are testing, will ensure thatwe can have great confidence in theperformance projected for NIF.

Key Words: adaptive optics; BeamletDemonstration Project; deuterium–tritium (DT) fuel;fusion energy; multipass lasers; National IgnitionFacility (NIF); neodymium glass lasers; Pockelscell; potassium dihydrogen phosphate (KDP).

NIF Facility Description E&TR December 1994

22

Figure 20. The distribution of Beamletenergy at the focus of a lens (in the farfield). This curve shows that about 95% ofthe energy is within ±25 microradians of thecenter of the spot, well within the designrequirements for NIF ignition targets.

For furtherinformationcontact John R. Murray(510) 422-6152.

40200™20™40

Centerof spot

Divergence from center, µrad

™0.1Ene

rgy,

nor

mal

to p

eak

0.3

0.7

1.1

(a) Infrared (b) Ultraviolet250

200

150

100

50

0

Figure 19. A fusion laser must have uniform intensity profiles. These intensity profilesobtained from Beamlet are smooth and flat across the center during frequency conversionfrom (a) infrared to (b) ultraviolet light. The small margins indicate that the profiles roll offsmoothly to zero intensity around the edges of the 30- ¥ 30-cm beam.

HE other articles in this issuedescribe the intended features

and capabilities of the NationalIgnition Facility and the diverse kindsof research that it will support, such asattempts to achieve break-even energyoutput through inertial confinementfusion (ICF). These other applicationsare important benefits that derive fromthe availability of NIF, but here wedescribe the value of the facilitythrough its key contribution toexperimental research in the physics ofnuclear weapons. With the moratoriumon nuclear testing and the likelihood ofa Comprehensive Test-Ban Treatymaking such testing permanentlyunavailable, NIF becomes one of a fewmeans of maintaining and advancingour understanding of the weapons nowin the stockpile.

models of performance–that is,science-based Stockpile Stewardship.The Stockpile Stewardship Programis based on several assumptions andobservations:• Nuclear weapons cannot beuninvented and will not go away,even if the U.S. were to dismantle itsentire nuclear stockpile.• U.S. defense policy will continue torely on nuclear deterrence for theforeseeable future. Therefore,maintaining confidence in thestockpile—in its safety, security, andreliability—is essential.• The moratorium on nuclear testingwill likely be followed by aComprehensive Test-Ban Treaty,which the U.S. must adhere to whileretaining confidence in its nucleararsenal.

Stockpile Stewardship

Since the Cold War ended with thedissolution of the Soviet Union, theU.S. nuclear weapons program haschanged dramatically. The U.S.brought a unilateral halt to thedevelopment and production of newnuclear weapon systems. Also, amoratorium on underground nucleartesting was implemented to furthernegotiations on a ComprehensiveTest-Ban Treaty and to encourage thebroadest possible participation in theNuclear Non-Proliferation Treaty.

A major change in the nuclearweapons program has accordinglybeen a move from nuclear test-basedweapon reliability and safety toreliance on a thorough scientificunderstanding and better predictive

NIF and National Security

Although producing total energies that are minusculecompared to those in a nuclear device, the NationalIgnition Facility will produce energy densities high

enough to duplicate many of the physics phenomena thatoccur in nuclear weapons and will thus help the U.S. to

maintain its enduring stockpile of nuclear weapons.

23

T

• No new weapons are beingdeveloped, and currently there is no known need for future weapondevelopment programs; moreover,some essential facilities of the U.S.nuclear weapons production complexno longer exist.• The U.S. nuclear stockpile willcontain fewer weapons, of fewertypes, as well as weapons that willbecome considerably older than theirdesign lifetimes; this stockpile willrequire enhanced surveillance andmaintenance to recognize, evaluate,and correct problems that may arise.• There may be a growing need toevaluate potential threats fromunfriendly foreign powers andterrorist groups.

We must continue to provide thetraining and required information fora group of scientists and engineersthat will be the stewards for thisstockpile under these conditions. Theremaining tools at their disposal will

have to be used to fill the gaps left bythe cessation of nuclear testing. (SeeFigure 1.)

Complexity of Nuclear Weapons

Maintaining confidence in thestockpile under the conditionsdescribed above is a challengebecause of the complexity of nuclear weapons design and of thephenomena that take place whennuclear weapons are operated—chemical explosion, hydrodynamicimplosion, mixing of materials,radiation transport, thermonuclearignition and burn, etc.

Nuclear testing provided apragmatic solution—integrated testsof the devices—that is no longeravailable. With the moratorium onnuclear testing, we must rely onadvanced computational modelingand non-nuclear experimentaltechniques for predictions and data.

We do not completely understandthe physical processes involved in the operation of a nuclear weapon.Indeed, a complete, detailed, andmathematically exact description of the physics would exceed the capabilities of today’ssupercomputers. We must thereforemake approximations to the physics inour evaluations of performance,although these approximationsintroduce uncertainties in ourpredictions. We must rely on ourcumulative knowledge, including pasttest data, to make valid inferences forphysics regimes that are inaccessiblewith current experimental methods.

This expertise is also the only waywe now have for the evaluation ofmany crucial issues, including:• The severity of age-related materialchanges discovered through routinestockpile surveillance.• The severity of unexpected effectsdiscovered with improved computermodels.• Whether retrofits, such as toimprove safety or reliability, willfunction properly.• Whether new technologies can orshould be incorporated in a stockpiledweapon system.

In a nuclear weapon, thephenomena occur in two verydifferent regimes of energy density. Inthe early phase of the implosion,before the development of significantnuclear yield, temperatures arerelatively low. This is the low-energy-density regime; here there isconsiderable complexity because thestrength of the materials and thechemistry of their composition play arole in how events proceed. Full-scaleassemblies using mock nuclearmaterial are used to test experimentallythe hydrodynamics of the implosionprocess at the beginning of aweapon’s operation. The study of thissubcritical regime is carried out athydrodynamic test facilities using

NIF and National Security E&TR December 1994

24

Past Future

Underground nuclear testing

Numerical simulations

Prenuclear (high-explosive) phasesimulations

Nuclear or high-energy-density laboratory simulations

System designs

Modernization

Warheads

Figure 1. Graphic comparison of the situation of the nuclear-weapons stockpile before andafter the enormous changes of recent years. The challenge is to maintain confidence in amuch smaller stockpile without nuclear testing and modernization.

powerful x-ray machines, high-speedoptics, and other methods. Currenthydrodynamic test facilities canaccess the precritical physics regime,although, without complimentarynuclear tests, these facilities must beimproved to provide much morespatial and temporal information.

After significant nuclear yieldbegins in a real device and fissilematerial is heated, we enter the high-energy-density regime. Although nolaboratory experiment can duplicatethe amount of energy released by anuclear weapon, many of the physicalconditions relevant to such a weaponcan be created in the laboratory.

Improving our predictivecapabilities for evaluating theseprocesses will be difficult. Withoutnuclear tests, we can never directlyobserve the full operation of thishigh-energy-density regime. We musttherefore improve our understandingof the relevant physics with bettercomputations and new experiments,techniques, and facilities. TheNational Ignition Facility will allowus—on a microscopic scale—to attainthe high-energy-density conditionsthat exist in weapons.

NIF and Weapons Physics

As a versatile high-energy-densityphysics machine, the NIF will enableus to gain an improved understandingof the underlying physics andphenomena of nuclear weapons, toacquire and benchmark new data toexisting databases, and to test andvalidate the physics computer codesfor ensuring future reliability andperformance. The NIF will providevaluable data for predicting theperformance of nuclear assembliesand for testing the complex numericalcodes used in weapons testcalculations.

Creating thermonuclear burn in thelaboratory will not only help us to

integrate and test all of our physicsknowledge but will also help theDepartment of Energy maintainexpertise in weapons design. Weenvisage training designers both on specific stockpile stewardshipissues and on broader NIF ignitionquestions relevant to inertialconfinement fusion.

Weapons research on NIF is drivenby a need to acquire a much moredetailed understanding of physicsprocesses at high energy densities, aswell as by a desire to achieve ignition.We have been studying theseprocesses on Nova and other lasers,but, again, NIF allows us to move ourstudies into those energy densities thatoccur in a nuclear weapon.

The maximum total energiesavailable on NIF will be an extremelytiny fraction of the yield of thesmallest nuclear weapon (see the boxon p. 26); but there are significantbenefits to generating so much lessenergy. The main event of a nucleartest is both extremely brief andextremely violent; it destroys most orall of the diagnostic and measuringinstruments. It remains for theresearchers afterwards to try to sortout all the physics and phenomenasubmerged in the event in order to analyze it. By contrast, on the NIF, we can design and performexperiments that isolate whateverphysics phenomenon is of interest.We can study the physics at relevantenergy densities without having todeal with large total energies. We canthus build an incremental, exactdescription of the cumulative physicsthat would make up a nuclear event.

In the high energy densities atwhich thermonuclear reactions occur,several distinctive phenomenapredominate: very high materialcompressions; unstable, turbulenthydrodynamic motion; highly ionizedatoms with high atomic numbers(high-Z atoms); and radiation

important in energy transport. These phenomena occur in suchastrophysical realms as stellarinteriors, accretion disks, andsupernovae (and occurred in the BigBang), but on Earth they occur onlyin machines such as NIF (andformerly in testing environmentscreated in the areas surroundingnuclear tests).

The weapon physics researchprogram at the NIF will stressinvestigations of these phenomena,including:• Material equation-of-stateproperties.• Unstable hydrodynamics.• Radiation flow, including theopacity of ionized elements and x-rayproduction.• Nonequilibrium plasma physics,including short-wavelength lasing.• Thermonuclear burn in thelaboratory.

The NIF will have the capacity fordoing systematic, well-characterizedexperiments because of the flexibilityof its multiple beam configuration.Considerable experience in doingsuch experiments very successfullyon Nova and other lasers will carryover to the NIF. Laser experiments onthe NIF, like those on Nova, can bedirectly or indirectly driven. In directdrive, multiple beams are directed atthe target. In indirect drive, a set oflaser beams is directed into ahohlraum, which is a tiny, hollowcylinder (see Figure 2) made of ahigh-Z material such as gold. Thebeams enter through the open endsand strike the inner walls, where theyare absorbed and generate x rays thatheat the interior of the hohlraum. Thetarget is then bathed in this relativelyuniform radiation field that heats it tothe desired temperature.

In either direct- or indirect-driveexperiments, a second set of laserbeams prepares a backlighter thatproduces x rays that probe the target

E&TR December 1994 NIF and National Security

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NIF and National Security E&TR December 1994

26

NIF: High Energy Density at Low Total EnergyThe NIF will provide the high energy densities that

are needed for thermonuclear reactions to occur. Highenergy density, however, should not be confused withhigh total energy. The two measures are independent:there can be high total energy with low energy density,and high energy density with low total energy. Energydensity is the amount of energy per particle, or per unitof volume.

The NIF will operate at low total energies. Highenergy densities can be produced on a small scale, andin the case of the NIF, the scale is about a millimeter.This fact is vividly evident in Figure 2, which shows aNova hohlraum compared to a human hair; the largehohlraum used on NIF will be no larger than a dime. ADT-filled capsule of the sort used for ignition studieson the NIF is very small—some 2 to 5 mm in diameter,a tiny fraction the size of a thermonuclear secondary.Thus, although the energy density—the energy perparticle—in such a capsule is comparable to that in a thermonuclear secondary, the total energy isminuscule. Ignition on the NIF will have roughly the same explosive force—that is, total energy—as a gallon of gasoline.

The accompanying figure shows the importance ofthe difference between the two measures of energy.The figure plots energy density versus total energyfor laser facilities, such as Nova and NIF, as well asthat achieved in a weapon test. Two regions areshown for NIF—without ignition and with ignition.This distinction reflects the two alternative modes inwhich NIF will be used for experiments in physicsrelated to weapons. NIF without ignition ischaracterized by the types of experiment described inthis article. These experiments do not use fuel-filledcapsules; instead, the targets are foils and othermaterials that enable us to study the behavior ofmaterials and media in the extreme conditions heatedby x rays to high energy densities.

NIF with ignition characterizes experiments inwhich the target is indeed a DT- or fuel-filled capsule,and the calculated energy densities are those predicted

to be achievable in the different regions of a burningDT capsule.1 Because the energy densities achieved inboth modes of NIF operation—with and withoutignition—show significant overlap with the energydensity regime available from weapons tests, NIF canbe used to investigate the high-energy-density physicssubprocesses that occur in that regime.

Nevertheless, physics investigations on NIF relyonly on high energy density (i.e., how dense and hotwe can make a relevant target), not on total energy.(The total energy is only high enough to heat or drivea target that is big enough to yield measurableresults.) The second, related, point is that the NIF orany other AGEX (above-ground experiment) facilitycannot conduct integrative weapon tests because itstotal energy falls many orders of magnitude short ofthat regime. NIF cannot be used as a testbed forweapon development. Our analysis of stockpilequestions will therefore rely on computer calculationsto put together different parts of the physics that westudy on NIF.

104

Specific energy density, kt/kg

Nova

Primary hydro

Criticality

NIFNIF with ignitionE

nerg

y, k

t

102

1

10™2

10™4

10™6

10™8

Weapons test

NIF energy densities will overlap those of nuclear weapons; theshaded area represents the region of high energy density. Note,however, that in measures of total energy, NIF energy regimesare well below the weapons test regime.

and go to the detector. The measuredabsorption of this well-characterizedand well-controlled x-ray sourceprovides insight into the characteristicsof the target material. The timing ofthe heating beams and backlighterbeams can be independently controlledto probe the targets under a widevariety of conditions.

Although the experimentalprogram at NIF evolves fromtechniques developed on Nova, NIF experiments will probe thequalitatively different regime ofplasmas characterized by radiationdominance at high-energy densities.The three examples of laser-drivenexperiments described here figureprominently in weapons research onthe NIF: opacity, equation-of-state,and hydrodynamic instabilityexperiments. Each of these explores a different set of fundamentalphenomena characteristic of theextreme conditions within a nuclearweapon. In all three experiments, we can analyze how far the 1.8-megajoule drive of NIF can push the energy density.

Opacity ExperimentsLoosely defined, opacity is the

degree to which a medium absorbsradiation of a given wavelength.Knowledge of the opacity of amedium is crucial to understandinghow the medium absorbs energy andtransmits it from one place to another.This knowledge is important innuclear weapons, where we carespecifically about opacities at x-raywavelengths, because this is themanner in which much of the energyin a weapon is transported.

If we are analyzing radiant energytransfer in a medium that is locally inthermodynamic equilibrium (LTE),

we need know only an appropriateaverage of the mean distances that aphoton can travel before it isabsorbed—that is, its Rosseland meanfree path between emission andabsorption. (To analyze radiantenergy transfer in a medium that isnot in thermal equilibrium, we wouldhave to retain detailed transmissioninformation for every wavelength.)To achieve LTE for opacityexperiments, we use the indirectmethod described above, creating abath of x rays inside of a hohlraum.We thus make a diagnosable plasmain equilibrium and then determine its x-ray transmission at theappropriate wavelength.

To make a good plasma, the samplemust be carefully tamped so that itretains uniform density under heatingwhile hydrodynamically expanding to the desired density duringmeasurement. The measurement isperformed by passing backlighter x rays through the hohlraum to probethe tamped target. The target mayhave sections of different thickness sothat, when analyzing the film image,we can separate the actual sampleopacity from the absorption of

radiation by other parts of theexperiment. Figure 3 shows a typicaltransmission experiment in localthermal equilibrium.2 Generally, asthe atomic number is increased, weneed to either increase the temperatureof the tamped target, or lower itsdensity, or both, in order to strip offenough electrons from the atoms inthe target to ionize the plasmas to thedesired level. Reaching suchionizations in the materials relevant toweapons requires a much morepowerful laser than Nova, as Figure 4shows.

Opacity research on the NIF willbe conducted to evaluate newmethods for predicting opacities.Such predictions are difficult,because there are many transitionsand competing ionization stages thatcan contribute to the opacity of agiven element. The electrons in atomsare arranged in shell structures ofincreasing complexity (frominnermost outward), and the shells areconventionally labeled K, L, M, N,etc. M-shell-dominated opacitiesoccur when an atom has been strippedof enough electrons to open the Mshell. Complicated configurations of

E&TR December 1994 NIF and National Security

27

Figure 2. Two views of a typical Nova hohlraum shown next to a human hair. The end-onview shows a target within the hohlraum. Hohlraums for NIF will have linear dimensionsabout five times greater than those shown.

this sort play an important role indetermining opacity; for example,the M-band opacity of materialsinvolves computing features of 108

ionic configurations. Clearly this isimpossible in any direct way;

present ideas necessarily involvepredicting key features withapproximate statistical methods.3Experiments are crucial in checkingthat these models and predictions arecorrect.

Equation-of-State ExperimentsUnderstanding the physics of

nuclear weapons requires that weanswer the practical question of howmuch pressure is developed in agiven material when a given amountof energy has been added. That is,we must determine the material’sequation of state: the thermodynamicrelationship between the energycontent of a given mass of thematerial and its pressure,temperature, and volume.

Figure 5a shows a setup for ashock breakout experiment—anexperiment for determining thethermodynamic states created by thepassage of a single shock wavethrough the subject material. Bystriking a material at standardtemperature and pressure with singleshocks of different strengths, weobtain a set of states that lie on theprincipal Hugoniot. Hugoniots notonly describe how materials behavewhen shocked; they also serve asbaselines for models of much of thethermodynamic space covered by thefull equation of state. Hugoniotexperiments present a rare case inwhich thermodynamic quantitiessuch as pressure can be determinedfrom the measurement of materialvelocities alone.

In a shock breakout experiment,4lasers create an x-ray bath inside ahohlraum. The x rays heat anabsorbing material that ablates, orrockets off, and sends a shock waveinto a flyer plate. The flyer platethen hits a target that has twoprecisely measured thicknesses or“steps.” The stepped target isobserved end on by diagnostics thatrecord the shock breakout. Bymeasuring the difference in thetiming of the shock breakout fromthe two sides of the step (Figure 5b),we can determine the speed at whichthe shock passed through the steppedmaterial. However, shock breakout

NIF and National Security E&TR December 1994

28

Film

x

Bragg crystal

Hohlraum heated by 8 laser beams

X-raysource

y

x

Figure 3. Schematic of a setup for absorption opacity experiments. The tamped targethas two thicknesses. Laser beams entering the hohlraum generate an x-ray bath to heat thetarget. Backlighter x rays have significantly higher energies than the driver x rays.

90

500 200 350Temperature, eV

Nova

NIF

Relativistic M-shell

Weapons test

500 650

Ato

mic

num

ber

60

30

0

Figure 4.Comparison ofopacity regimesachievable on Novaand NIF and inweapons tests.

experiments are difficult to interpret.For example, we must determinewhether the shock was planar: Did itstrike the surface of the stepped platewith uniform force, or did the flyerplate undergo “preheat” anddisassemble before it shocked thestep? Like opacity studies, equation-of-state studies of these microscopicquantities are not only crucial tounderstanding the effects of high-energy densities in weapons, butthey are crucial to understandinglaser-based experiments themselves.Nova allowed us to study equationsof state in the multimegabar pressureregions, but scaling equation-of-stateexperiments to pressures in theimportant gigabar region require amore powerful laser such as the NIF.

This comparison is detailed in Figure 6.

Hydrodynamic InstabilityExperiments

The third experimental exampleuses indirect drive to createhydrodynamically unstable flows athigh compressions and Reynoldsnumbers. Unstable flows in highlycompressed materials are ubiquitousin weapon physics. We typically mustdetermine the thickness of the mixinglayer between two materials causedby the passage of a strong shockwave. Much research on turbulentflows relies on the assumption thatthe flow is incompressible (like waterin an ocean). However, here we areinterested in the situation whereconsiderable compression andionization can occur at the same timeas turbulent, mixing motion.

Figure 7 shows the experimentalsetup for studying the instabilitygrowth at an interface caused by thepassage of a controlled, planar shock.

As before, the lasers heat thehohlraum to create the x-ray heatbath that drives the experimentalpackage. Here, the x rays ablate acarefully designed sleeve that drivesa shock into the instabilityexperiment. Another laser beammakes an x-ray backlighter thatallows us to “photograph” thegrowing instability. We mustcarefully check the equation of stateof the subject materials, the planarityof the shocks, etc., before we cancompare the experiment with adetailed simulation. Figure 8 shows acomparison between a preliminaryinstability experiment done on Novaand an arbitrary Lagrange-Eulerianhydrodynamics calculation of thesame setup done to test the ability ofthe code to probe large-scale slidingmotions and deformations.

The program of work in instabilityresearch involves the study ofshocked mixing layer growth, and theevolution of compressible turbulencefrom the small-amplitude, linear

E&TR December 1994 NIF and National Security

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X rays

Flyerplate

(b)

(a)

Shock breakout

Tim

e

Steptarget

Figure 5. (a) Schematic of an x-ray-drivenshock breakout experiment in colliding foils(the flyer plate and the step target are goldfoil); (b) shows the breakout timingdifference between the two sides of thestep, as captured on film.

104

104103102

Temperature, eV101

Pre

ssur

e, M

bar,

at c

onst

ant v

olum

e

103

102

10

Nova

NIF

Hugoniot

Weaponstest

1

Figure 6.Comparison ofequation-of-stateregimes in flyer- plate experimentsachievable on Novaand NIF; also roughlyindicated is theweapons-test regime.

growth regime (which is pertinent toICF implosions) to the full nonlinearevolution of turbulence. In the caseof the mixing layers, there aresuggestions for universal rules that

control the width of such mixinglayers as a function of time.5 Itwould be of great importance toweapons designers to pin downthese rules.

To explore the full evolution toturbulence from the simpler lineargrowth to the highly compressedturbulent regime, the experimentsnaturally scale to a higher-energylaser, as Figure 9 shows.1

Other Weapons ExperimentalRealms

The three types of experiments just described are among the mostfundamental that will be performed onNIF—fundamental in the sense ofprobing phenomena that are virtuallyirreducible. Several areas ofinvestigation are more integrative,probing phenomena arising fromcombinations or interactions of severaldifferent processes. These areas,described below, will also receiveintensive investigation on NIF. Theseinvestigations and those alreadyenumerated will contribute to furtherrefinement of our weapons codes.

Radiation Transport. We do notunderstand nuclear weapon processeswell enough to calculate precisely thetransfer of energy within a weapon.6This transfer is crucial, sinceinadequate energy coupling candegrade yield or cause failure. In theera of nuclear testing, this incomplete

NIF and National Security E&TR December 1994

30

FourNova

beams

Backlighterx rays

Laserbeams Beryllium tube

X-ray imagingdiagnostics

FourNova

beamsX-ray drive

Doped polystyrene

Instability surface

Carbon foam

Grid

Gold

(a) 0 ns (b) 20 ns (c) 20 ns

2 m

m

2% brominatedpolystyrene

Beryllium

0.1 g/cm3

carbon

Figure 7. Schematic of a shock-driven hydrodynamic instability experiment. Changes inthe transmission profile of the backlighter x rays reaching the diagnostics allow the mixingat the shocked interface to be measured.

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.

understanding was not a problembecause the radiative energy transfercould be determined specifically.

Weapon Output. The output of anuclear weapon includes neutrons,gamma rays, x rays, fission products,activated elements, and explodingdebris as kinetic energy. The abilityto calculate the total spectral outputof a weapon is an ultimate measure of our understanding of weaponperformance.

Role of Ignition. Ignitionencompasses two distinct weaponsphysics issues: weapons physicsmeasurements and maintenance ofcritical skills. Several broad areasappear uniquely accessible to anignition capsule:• The possible use to study onset ofDT ignition in the presence ofimpurities, which can occur from a mixof intentional contaminants placed inthe gas. The NIF will provide the onlyplace where DT burn will be studiedin detail.• The generation of x rays from thecapsules. This will challenge ourability to model and understand burn,energy balance, and transportprocesses in a highly transient systemhaving large gradients.• The NIF capsules will also providean intense source of 14-MeVneutrons. These neutrons could beused to heat material instantaneouslyto temperatures more than 50 eVwithout changing the material’svolume. This unique capability mayprove useful for other weaponsphysics studies such as equation-of-state experiments.

Designing NIF fuel capsules totailor output and explore efficientoperation is a challenge to designers,computational physicists, andengineers. Such modeling willchallenge our understanding of manyfundamental processes associatedwith weapon design and will helpkeep us intelligent in this criticaltechnology.

Non-LTE Physics and X-RayLasers. The NIF will allow us toaddress important physics issues insituations in which plasmas are not inthermodynamic equilibrium (callednonlocal thermodynamicequilibrium, or non-LTE, physics).Understanding non-LTE effects playsan important role in determining x-rayoutputs and in developingtemperature, ionization balance, andkinetics diagnostics. It is alsoessential in the development of laser-driven x-ray lasers. In the specificarea of x-ray lasers, NIF will allow usto explore x-ray laser pumpingschemes with various materials andconditions.

An important benefit of the x-raylaser research is its use as animaging system for ICF, weaponsphysics, and biology experiments.High brightness, narrow bandwidth,small source size, and short pulseduration give the x-ray laser manyadvantages over conventional x-rayillumination sources.7

Future imaging applications willinclude the study of laser-driven mass ablation on the interior walls of hohlraums, equation of state,

and perhaps the ICF implosionsthemselves. As the NIF programfor high-energy-density physicsevolves, we expect these results toinfluence the development offurther advanced diagnostics.

Code Development. To a largeextent, our weapons computercodes embody the cumulativeknowledge of weapons design.NIF data will help to improve andrefine these codes to enable moreaccurate modeling of results fromprevious weapons tests and fromweapons physics experiments pastand future. Code development forICF research has focused on theneed to calculate many coupledphysical processes innonequilibrium conditions and tosimulate all resulting experimentaldiagnostics within a singlecomputational model. The futureneed for higher accuracy andincreased engineering detail willrequire better numerical methods,three-dimensional simulations, andmassively parallel computers.These growing ICF codecapabilities will be very importantto weapons researchers for

E&TR December 1994 NIF and National Security

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Figure 9. Whereasthe moderatecompressions onNova allow us tofollow the transitionfrom linear instabilityto weak turbulence,the highcompressions andlarger scale volumeson NIF allow us tofollow this all theway to turbulent mix.

Com

pres

sion

Sho

ck p

ress

ure,

Mba

r (a

t con

stan

t vol

ume)

Sample size/perturbation wavelength

2

102 103 10410

Nova

30 kJ3–10 TW

1.2 MJ60–200 TW

NIF

3

4

5 1000

400

100

25

Compressivehydrodynamics

Weaponstest

understanding the results of NIFweapons physics experiments.

Summary

Underground nuclear tests playedan important role in advancingknowledge of the physics of nuclearweapons. This knowledge led toprogressively safer and moreeffective performance and to retrofitsfor older designs that improved theirsafety as well. Results from tests alsoenabled us to build and refine ourweapons computer codes. Theunilateral moratorium that the UnitedStates imposed on undergroundnuclear testing is likely to befollowed by a Comprehensive Test-Ban Treaty. The United States musttherefore have an alternative meansof safely and securely maintaining itsstockpile of nuclear weapons andensuring their reliability. Stockpilestewardship is one of the functions towhich the National Ignition Facilitywill contribute by virtue ofexperimental work in the physics ofnuclear weapons.

The total energy output fromthermonuclear ignition on NIF will bean extremely tiny fraction of theenergy from even the smallest nuclearweapon—indeed it will be roughlyequivalent to the output of a gallon ofgasoline. Nevertheless, experimentswill generate the same energydensities—energies per particle—thatoccur in nuclear weapons. This

combination of low total energy withweapons-regime energy density willallow us to pursue, besides ignitionexperiments, many nonignitionexperiments. These will allow us toimprove our understanding ofmaterials and processes in extremeconditions by isolating variousfundamental physics processes andphenomena for separate investigation.Such studies will include opacity toradiation, equations of state, andhydrodynamic instability. In additionto these, we will study processes inwhich two or more such phenomenacome into play, such as in radiationtransport and in ignition itself.

Weapons physics research on NIFoffers a considerable benefit tostockpile stewardship not only inenabling us to keep abreast of issuesassociated with an aging stockpile,but also in offering a major resourcefor attracting and training the nextgeneration of scientists with nuclearstockpile expertise. According to therecent JASON report on stockpilestewardship, the NIF “will promotethe goal of sustaining a high-qualitygroup of scientists with expertiserelated to the nuclear weaponsprogram.”8

Key Words: inertial confinement fusion; NationalIgnition Facility; Stockpile Stewardship Program.

Notes and References1. S. W. Haan et al., Design and Modeling of

Ignition Targets for the National IgnitionFacility, Lawrence Livermore NationalLaboratory, Livermore, CA, UCRL-JC-117034(1994).

2. T. S. Perry et al., “Opacity Measurements in aHot Dense Medium,” Physical Review Letters67, 3784, (1991); L. Da Silva et al.,“Absorption Measurement Demonstrating theImportance of ∆n = 0 Transitions in the Opacityof Iron,” Physical Review Letters 69, 438(1992); P. T. Springer et al., “SpectroscopicAbsorption Measurements of an Iron Plasma,”Physical Review Letters 69, 3735 (1992).

3. W. H. Goldstein, “A New Model for HeavyElement Opacity,” Procedures of the 8thBiennial Nuclear Explosives Design PhysicsConference (NEDPC), Los Alamos NationalLaboratory, Los Alamos, NM, LA-12305-C,364 (1992).

4. R. Cauble et al., “Demonstration of 0.75 GbarPlanar Shocks in X-Ray Driven CollidingFoils,” Physical Review Letters 70, 2102(1993).

5. D. L. Youngs, “Numerical Simulations ofTurbulent Mixing by Rayleigh-TaylorInstability,” Physica (Utrecht) 12D, 32 (1984).

6. T. S. Perry et al., “Experimental Techniques toMeasure Thermal Radiation Heat Transfer,”Journal of Quantitative Spectroscopy &Radiative Transfer 51, 273 (1994).

7. L. Da Silva et al., “X-Ray Lasers for Imagingand Plasma Diagnostics,” Proceedings of the4th International Colloquium on X-Ray Lasers,Williamsburg, VA (1994).

8. C. Callan et al., Science Based StockpileStewardship, The MITRE Corporation, JASONProgram Office, 7525 Colshire Dr., McLean,VA, JSR-94-345 (1994).

NIF and National Security E&TR December 1994

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For furtherinformationcontact Stephen B. Libby(510) 422-9785.

MERICA’S dependence onimported oil currently accounts

for a trade deficit of $60 billion peryear. As time passes, the worlddemand for energy will continue togrow, in part for demographicreasons, such as the rapidlyincreasing energy use per capita indeveloping Asian and LatinAmerican countries together withthe expected doubling of the world’spopulation over the next 50 years.Our deficit and world energydemand will also grow forenvironmental reasons, particularlyin the United States, which will needa substantial new source of energyto power zero-emissiontransportation and reduce urban airpollution, to charge batteries inelectric cars, or to produce clean-

only dependable in the limited desertregions of the world, some fusionfuels can be extracted from seawater,making them available to allcountries of the world. Fusion powerplants, if they can be developedeconomically, will also have manyadvantages over fission. Theradiation hazard presented by fusionpower plants can potentially bethousands of times smaller than thatof fission power plants, with properchoice of materials.

Two Approaches to Fusion

Fusion combines nuclei of lightelements into helium to release energyand is the same process that powers thesun. As noted, the fuel for fusion(deuterium and lithium, which can

burning hydrogen fuel by waterelectrolysis. Clearly, an alternativeenergy source is needed.

At present, there are only threeknown inexhaustible primary energysources for the future: the fissionbreeder reactor, solar energy, andfusion. All are superior to coal or oil-based power plants because theyare environmentally cleaner andecologically safer. They will releaselittle or no radioactivity per unit of power, as do coal mining andburning in the form of radon,uranium, and thorium,1 and they willemit none of the gases (carbondioxide and nitrogen dioxide) thatcontribute to greenhouse effects.Fusion, however, offers certainadvantages over fission and solarenergy. Unlike solar energy, which is

The Role of NIF in DevelopingInertial Fusion Energy

The National Ignition Facility will demonstrate fusionignition, which is central to proving the feasibility of

inertial fusion energy. It will also help us determine thefull potential of this alternate energy source.

33

A

capture a neutron to regenerate tritium)can be extracted from seawater. Themost likely fuel for any approach tofusion energy is DT (either liquid, gas,or a combination as in inertial fusionenergy targets), which is a mixture ofdeuterium and tritium isotopes ofhydrogen. This DT must be heateduntil it is hotter than the interior of the sun, but it fuses at the lowesttemperature of any fusion fuel.

To explore the feasibility ofeconomical fusion power plants, theDepartment of Energy is currentlydeveloping two primary approachesto fusion energy—magnetic fusionenergy and inertial fusion energy(IFE). Both approaches use DT fueland offer the potential advantagesdescribed above, but they must be developed more fully beforeeconomical fusion energy can beassured. Because the two approachesuse different physics and presentdifferent technological challenges, the National Energy Policy Act of19922 calls for both to be developedto the demonstration (DEMO) stage.

Magnetic fusion ignition is thegoal of the proposed InternationalThermonuclear Experimental Reactor,which uses strong superconductingmagnets to confine a low-density DT

plasma inside a large, high-vacuum,toroidally-shaped vacuum chamber.3The IFE approach to fusion, incontrast, is one of the goals of theNational Ignition Facility (NIF), thesubject of this article. This approachuses powerful lasers or ion beams(drivers) to demonstrate fusionignition and energy gain in thelaboratory by imploding and ignitingsmall, spherical DT fuel capsules(targets) to release fusion energy in aseries of pulses (see box on p. 38). Inits quest to accomplish this goal, theNIF supports a primary nationalsecurity mission for science-basedstockpile stewardship (see precedingarticle) and secondary missionssupporting energy and basic science.

The IFE Power Plant

Figure 1 is a conceptual view of ageneric IFE power plant, showing itsfour major parts—the driver, targetfactory, fusion chamber, and steam-turbine generator (balance of plant).This figure demonstrates some of theprincipal advantages of IFE as anenergy source:• The driver and target factory areseparated from the fusion chamber toavoid radiation and shock damage to

the most complex plant equipment.The separation between the driver andfusion chamber also allows a singledriver to drive multiple fusionchambers, thus permitting flexibility inthe required chamber pulse rate andlifetime and allowing for the stageddeployment of several fusion chambersto achieve low-cost electricity.4• Progress in inertial fusionexperiments on the Nova laser facilityat LLNL allows the most importanttarget physics affecting target gain tobe modeled successfully by computercodes such as LASNEX. When thesecomputer models are better confirmedby target-ignition tests in the NIF,they can be used to design targets forfuture IFE power plants.• IFE fusion chambers do not requirea hard vacuum; therefore, a widerrange of materials can be used toachieve very low activation andradioactive waste. IFE chamberdesigns that protect the structuralwalls with thick, renewable fluidflows are also possible, which willeliminate the need to replace thechamber’s internal structuralcomponents periodically.5,6

• The cost of developing IFE can bediluted by sharing NIF for defenseand energy missions.

How the NIF Can Help DevelopIFE

In 1990, the Fusion PolicyAdvisory Committee7 recommendedthat inertial fusion ignition bedemonstrated in the NIF as a keyprerequisite to IFE. In addition toignition, IFE needs development inthree major areas of technology:• High-gain, injectable, mass-produced, low-cost targets.• An efficient high-pulse-rate driver.• A suitable, long-lasting fusionchamber.A major facility following the NIF, tobe called an Engineering Test Facility

Inertial Fusion Energy E&TR December 1994

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Beams

HeatHeat

Recycled target material

TargetsDriver

Turbinegenerator

Targetfactory

Fusionchamber

Figure 1. Conceptual view of a four-part IFE power plant, showing the driver (either laseror ion particle beams), the target factory, the fusion chamber, and the turbine generator thatproduces electricity.

(ETF), is planned to test the feasibilityof these three areas of technologyintegrated together. The ETF willexplore and develop the high pulserate (several pulses per second) andoverall system efficiency needed foreconomical IFE power production.

Filling Technological NeedsTargets. The targets for IFE must

be capable of high energy gain.Energy gain is achieved when thefusion energy released from a reactionexceeds the energy that was put intothe target by a laser or ion-beamdriver. For high gain, the energyreleased from the target should bemore than 50 to 100 times greaterthan the driver energy. Tests ofinertial fusion target physics andignition in the NIF will allow us topredict confidently the performance ofseveral candidate IFE target designs.

For IFE targets to produceelectricity at competitive rates (lessthan 5 cents per kilowatt-hour), theymust be mass-producible at a cost ofless than 30 cents each. This meansnew target-fabrication techniques must be researched and developed. Inaddition, we will have to develop andtest methods of target injection andtracking for driver–target engagementsat pulse rates of 5 to10 Hz. The NIFcan test the performance of candidatemass-produced IFE targets and, atleast for a limited number of pulses ina short burst, the associated target-injection methods.

The option of using direct-drive inaddition to indirect-drive targets (seethe box on pp. 38-39) is underconsideration for the NIF. If direct-drive implosion experiments on theOmega Upgrade (an upgrade of theOmega glass laser to 60 beams) at theUniversity of Rochester’s Laboratoryof Laser Energetics are successful,this option will be exercised, and bothdirect- and indirect-drive targets willbe examined on the NIF. Figure 2

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Indirect-drive laser target

Laser-driven model of heavy-ion target

(a)

Direct-drive laser target

(b)

Figure 2. The NIF target area and beam-transport system (a) for indirect-drive experimentsrelevant 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 bereconfigured to design (b) by the repositioning of 24 four-beam clusters, making direct-driveexperiments possible.

shows the laser-beam configurationsaround the NIF fusion chamber thatwill be used to conduct indirect-drive (Figure 2a) and direct-driveexperiments (Figure 2b). Figure 2also shows examples of indirect- anddirect-drive targets that can be testedin each configuration.

Figure 3 shows a heavy-ion-driventarget for IFE (Figure 3a) comparedwith a modified laser-driven target(Figure 3b) that is designed to modelmore closely the IFE heavy-iontarget. The latter (Figure 3b)illustrates how the NIF could use alaser to test the soft-x-ray transportand plasma dynamics inside a higher-fidelity hohlraum geometry similar tothat in Figure 3a. Note that thecapsule performance and implosionsymmetry requirements for indirect-drive targets are independent ofwhether the x rays are generated witha laser or an ion-beam driver. We canalso use the NIF for special laser-target experiments that simulate manyaspects of heavy-ion targets.

Drivers. Although the key target-physics issues that NIF will resolveare largely independent of the type of driver used, it is essential inevaluating the potential of IFE todetermine the minimum driver energy

needed for ignition. Regardless ofdriver type, all IFE drivers for powerplants need a similar combination ofcharacteristics: high pulse-repetitionrates (5 to 10 Hz) and high efficiency(i.e., driver output beamenergy/electrical energy input to thedriver greater than 10 to 20%,depending on target gain). Inaddition, they should be highlyreliable and affordable whencompared with nuclear generatorplants.

The Energy Research branch ofDOE is developing heavy-ionaccelerators to meet the aboverequirements.8 Heavy-ion drivers canbe either straight linear accelerators(linacs) or circular (recirculating)beam accelerators like that shown in the box on p. 39. The DefensePrograms branch of DOE, in contrast,is pursuing advanced solid-statelasers, krypton–fluoride lasers, andlight-ion pulsed-power acceleratorsfor defense applications that may,with improvements, lead to alternateIFE drivers. Diode-pumped solid-state lasers will be able to build onthe laser technology being developedfor the NIF.9

While other DOE researchexamines the direct-drive option

and develops more efficient, high-repetition-rate IFE drivers(principally heavy-ion beamaccelerators) for power plants, theNIF can be built and achieve itsmission with current solid-state lasertechnology. Diode-pumped solid-state lasers (DPSSLs), which alsobuild on NIF laser technology, mayprove to be a backup to the heavy-ionaccelerator. Using laser-diode arraysunder development for industrialapplications, DPSSL drivers mayultimately improve the efficiency,pulse rate, and cost of solid-statelasers enough for use as IFE drivers.Figure 4 shows a schematic DPSSLdriver layout that, except for thediode pump arrays, has anarchitecture similar to that beingdeveloped for the NIF.

Fusion chambers. IFE needsfusion chambers where target fusionenergy can be captured in suitablecoolants for conversion intoelectricity. To allow high pulse rates,these chambers will have to be builtso they can be cleared of target debrisin fractions of a second. Further, theymust be reliable enough to withstandthe pulsed stresses of one billionshots (30 years of operation) withoutstructural failure. They should also

Inertial Fusion Energy E&TR December 1994

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Heavy-ionbeams

DT fuelcapsule

0.35-µm laser beamsin two cone arrays

Dopedplastic

Hohlraum

Hohlraum

Shield

ShieldCapsule

Doped gas

14–1

6 m

m

Figure 3. The NIF cantest important heavy-ion physics issues,such as soft-x-raytransport and drivesymmetry, hohlraumplasma dynamics,capsule-implosionhydrodynamics, andmix. Here the modifiedlaser-driven target in(b) shows how the NIFcould use a laser totest x-ray transport andplasma dynamics in ahohlraum geometrylike that shown in (a).

(a) (b)

use low-activation materials (such asmolten salt coolants or carbon-composite materials) to minimize thegeneration of radioactive waste.Many IFE power plant studies havealready found conceptual designs thatmeet these goals, but actual tests willbe required. What we learn from theNIF fusion chamber can provide datato benchmark design codes for futureIFE chamber designs.

Other Needs. In addition to fusionignition, the NIF will provideimportant data on other key IFEpower plant needs. These needsinclude wall protection from targetdebris and radiation damage, chamberclearing, rapid target injection, andprecision tracking. The NIF will also be used to provide data that can benchmark and improve thepredictive capability of variouscomputer codes that will be needed todesign future IFE power plants, toselect among possible IFE technologyoptions, and to improve ourunderstanding of IFE target andchamber physics.

One predictive capability that cancalculate and interpret materialresponses to neutron damage is atechnique called molecular dynamicsimulation (MDS).10 MDS calculatesresponses at the atomic level byquantifying how a three-dimensionalarray of atoms responds to knock-onatoms that impinge on the matrixfrom a range of angles and with arange of energies as a result of anincident neutron flux. Potentially,MDS capabilities may includepredicting, for a material, the numberof vacancies and interstitials that willresult from a neutron irradiationpulse, as well as the cluster fractionof defects, atomic mixing and solute precipitation, and phasetransformations. Figure 5 shows howsamples of materials exposed to thetarget neutron emission in a NIF shotcan provide data that confirm theMDS model calculations.

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Heated fusedsilica final optic Reactor

chamber

First wall

Spatial filter

PulseinjectionPolarizer

Diodes

Output pulse

Gas-cooledharmonic

conversion

Gas-cooled gain medium

Gas-cooled Pockels cell

Neutronabsorber

Dichroicmirror

Figure 4. The diode-pumped, solid-state laser driver for IFE is similar in design to thatbeing developed for the NIF. Although the NIF architecture will not include the diode pumparrays shown here, it will serve as an experience and technology base for the IFE driver. Thisfigure shows a DPSSL IFE laser designed like the NIF in that it uses a multipass laseramplifier in which the laser beam is amplified by passing back and forth between the cavitymirrors four times before a Pockels cell optical switch sends the amplified beam out to thefinal optics and the target. However, the DPSSL uses light from arrays of efficient diodelasers to pump the amplifier from the ends rather than using light generated from flashlamps on the sides of the amplifier as in the present NIF design.

100

50

0

–50

–100

Sample array

Shield

Capsule1015 n/shot

–100

2–5 cm

–50 0 50 100

Leng

th, Å

Length, Å

t = 4.61 ps

Cu 25 keV PKA (a)

Figure 5. A molecular-dynamic simulation experiment on the NIF. Samples of 2- to 5-cm-widthmaterial placed within 20 cm of a NIF yield capsule (at left) will receive a significant exposure to14-MeV neutrons (1015 neutrons per shot per square centimeter of sample area). The tantalumshield will stop most x rays. Electron microscope images of the damage sites will be comparedto MDS code predictions as shown at right. The figure shows a typical damage site in a coppersample due to primary knock-on copper atoms (25 keV primary knock-on atoms [PKA]) arisingfrom collisions of fast neutrons with copper atoms in the sample.

Inertial Fusion Energy E&TR December 1994

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An inertial confinement fusion (ICF) capsule ortarget is a small, millimeter-sized, spherical capsulewhose hollow interior contains a thin annular layer ofliquid or solid DT fuel (a mixture of deuterium andtritium isotopes of hydrogen). The outer surface of thecapsule is rapidly heated and ablated either directly byintense laser or ion particle beams (drivers), calleddirect drive (a below), or indirectly by absorption ofsoft x-rays in the outer capsule surface. These soft x-rays are generated by driver beams hitting a nearbymetal surface, a process called indirect drive (b below).

The rocket effect caused by the ablated outer capsulematerial creates an inward pressure causing the capsuleto implode in about 4 nanoseconds (a nanosecond isone billionth of a second). The implosion heats the DTfuel in the core of the capsule to a temperature of about 50 million degrees Celsius, sufficient to cause theinnermost core of the DT fuel to undergo fusion. Thefusion reaction products deposit energy in the capsule,further increasing the fuel temperature and the fusionreaction rate. Core fuel ignition occurs when the self-heating of the core DT fuel due to the fusion reactionproduct deposition becomes faster than the heating dueto compression. The ignition of the core will thenpropagate the fusion burn into the compressed fuellayer around the core. This will result in the release ofmuch more fusion energy than the energy required tocompress and implode the core.

An inertial fusion power plant would typically fire acontinuous series of targets at a pulse rate of 6 Hz. Theseries of fusion energy releases thus created in the formof fast reaction products (helium alpha particles andneutrons) would be absorbed as heat in the low-activation coolants (fusion chamber) that surround thetargets. Once heated, the coolants would be transferredto heat exchangers for turbine generators that produceelectricity. The inertial fusion power plant exampleshown below uses jets of molten salt, called Flibe,surrounding the targets inserted into the fusionchamber. The molten salt jets absorb the fusion energypulses from each target while flowing from the top tothe bottom. The molten salt is collected from thebottom of the chamber and circulated to steamgenerators (not shown) to produce steam for standardturbine generators. This particular power plant exampleuses a ring-shaped ion beam accelerator as a driver, butthere are also laser driver possibilities.

The minimum driver energy required to implode thecapsule fast enough for ignition to occur is typicallyabout a megajoule, the caloric equivalent of a largedoughnut. Since this driver energy must be delivered in a few nanoseconds, however, a power of severalhundred terawatts (1 terawatt = 1 million megawatts)will be needed. For reference, the entire electricalgenerating capacity of the United States is about one-half terawatt.

Producing Inertial Fusion Energy

(a) Direct-drive targets are directly heated and implodedby intense driver beams.

Ionparticle

orlaser

beams

Ignition fusion reaction products (alpha particles)heat the compressed fuel

core to fusion temperature causing implosion

Fuel capsule

DT fuel layer

Hea

vy-io

nbe

am

High-Z radiation case

Indirect-drivelaser target

Indirect-driveheavy-ion target

Vacuum Fuel capsuleSoft x ray

Lase

rbe

ams

High-Z radiator

(b) Indirect-drive target fuel capsules are imploded by soft x rays generated by intense lasers or ion beams at the ends of a high-Z radiation case (“hohlraum”).

(a) Direct-drive targets are directly heated and imploded (b) Indirect-drive target fuel capsules are imploded by soft x raysby intense driver beams. generated by intense lasers or ion beams at the ends of a high-Z

radiation case (“hohlraum”).

The rapid thermal motion of the deuterium andtritium nuclei will cause a significant fraction of them tocollide and fuse into helium ash before the compressedfuel mass from the implosion has had time to reboundand expand. The reaction products will fly away withseveral hundred times more kinetic energy than thethermal energy of the deuterium–tritium ion pair beforefusion occurred. If some inefficiency in coupling thedriver laser or ion-beam energy into compressing andheating the capsule is taken into account, the ratio offusion energy produced by the target to the driver beamenergy input to the target—called the target gain—canrange from 50 to 100 in a typical power plant. Once thisfusion heat is converted into electricity, the averageamount of electricity needed to energize the driverwould be 5 to 10% of the total plant output.

Inertial fusion targets are of two basic types: directdrive and indirect drive, both of which will be tested bythe NIF to determine the best target for inertial fusionenergy. A direct-drive target consists of a sphericalcapsule driven directly by laser or ion beams. So that thecapsule will implode symmetrically and achieve highgain, it must be illuminated uniformly, from all directions,

by many driver beams. In indirect-drive targets, the fuelcapsule is placed inside a thin-walled cylindrical container(hohlraum) made from a high-atomic-number material,such as lead. Here a smaller number of driver beams (witha total energy similar to that required for direct drive) aredirected at the two ends of the hohlraum cylinder, wherethe driver beam energy is converted to soft x rays, which,in turn, lead to the compression of the fuel capsule. Thehohlraum spreads the soft x rays uniformly around thecapsule to achieve a symmetric implosion.

For its driver, the NIF will use a solid-state glasslaser to deposit the externally directed energy. Thislaser will deliver 1.8 MJ of laser light energy (in pulsesspaced several hours apart) to test the minimum energyrequired for target ignition and the scaling of target gainso that any type of target optimized for future powerplants can be designed with confidence. DOE–EnergyResearch is developing heavy-ion beam accelerators asits leading candidate drivers for future IFE powerplants, while DOE–Defense Programs is developingother driver technologies for ICF research, includingadvanced solid-state lasers, that could lead to alternativeIFE drivers as well.

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Recirculating heavy-ion induction accelerator Bypass pumps

Rotatingshutter

OscillatingFlibe jets

Developing Fusion PowerTechnology

The NIF can also help developfusion power technology (FPT),which includes the technologiesneeded to remove the heat of fusionand deliver it to the power plant. Theprimary functions of such componentsin IFE power plants are to convertenergy, to produce and processtritium, and to provide radiationshielding. The dominant issues forFPT in IFE power plants concerncomponent performance (both nuclearand material) so as to achieveeconomic competitiveness and torealize safety and environmentaladvantages. In this regard, NIF willprovide valuable FPT informationgained from the demonstratedperformance and operation of the NIF facility itself, as well as fromexperiments designed specifically

to test FPT issues. NIF’s relevance to FPT has to do with both itsprototypical size and configurationand its prototypical radiation-field(neutrons, x rays, and debris) spectraand intensity per shot. The mostimportant limitation of NIF for FPTexperiments is its low repetition rate(low neutron fluence), and its mostimportant contributions to FPTdevelopment for IFE are related to:• Fusion ignition.• Design, construction, and operationof the NIF (integration of manyprototypical IFE subsystems).• Viability of first-wall protectionschemes.• Dose-rate effects on radiationdamage in materials.• Data on tritium burnup fractions inthe target, tritium inventory and flow-rate parameters, and the achievabletritium breeding rate in samples.

• Neutronics data on radioactivity,nuclear heating, and radiationshielding.

The NIF will also be able todemonstrate the safe andenvironmentally benign operationthat is important for IFE, includinghandling tritium safely andmaintaining minimum inventories of low-activation materials. It isdesigned to keep radioactiveinventories low enough to qualify asa low-hazard, non-nuclear facilityaccording to current DOE and federalguidelines, thus setting the pattern forfuture IFE plants. Similar non-nucleardesign goals will also be met for IFEpower plants if the design selected for the fusion chamber is carefullyfollowed and the low activationmaterials for it are used. The NIF will also demonstrate proper qualityassurance in minimizing both

Inertial Fusion Energy E&TR December 1994

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Figure 6. The timeline for IFE development includes ICF ignition and gain, IFE technologies, and the IFE power plant.

Defense Programs (ICF) National ignition Facility (NIF)

Energy Research (IFE) Heavy-ion driver technology (ILSE Facility at LBL)

Target fabrication andfusion chambertechnologies

Power plant demonstration Engineering Test Facility (ETF) and DEMO

Design

Tests ILSE experiments

Technology development and tests

Stage 1 Stage 2 Tests DEMOTechnologyintegration

Construct Target physicsexperiments

ETFPowerplant

Target andfusion

chamber

1995 2000 2005 2010 2015 2020 2025

Energyapplicationsdecision

Driver feasibilitydemonstration

Ignition and gaindemonstration

occupational and public exposures toradiation.

An Integrated IFE DevelopmentPlan

To capitalize on the success offusion ignition in the NIF, which isexpected to occur around the year2005, an Engineering Test Facility(ETF) will be needed. This facilitywill test the fusion power planttechnologies called for in the 1990Fusion Policy Advisory Committee7

the 1992 National Energy Policy Actof 19922 plans. A decision to moveforward with the ETF will alsodepend on the timely demonstrationof a feasible, efficient, high-repetitionIFE driver.

Figure 6 shows existing andproposed facilities in an integratedplan for IFE development. In additionto the NIF, they include:• The Induction Linac SystemsExperiment (ILSE). Plans call forthis proposed heavy-ion acceleratortest facility to be built at theLawrence Berkeley Laboratory. Itsmission will be to demonstrate thefeasibility of a heavy-ion driver forIFE by testing critical, high-current,ion-beam-induction accelerator andfocusing physics with properlyscaled-down ion energy and mass.ILSE may be built in two stages for a total construction cost of about $46 million. The ILSE experimentsshould also be completed by the year2005.• The ETF/LaboratoryMicrofusion Facility (ETF/LMF).This multiuser facility for bothdefense experiments and IFEtechnology development will be ableto produce target-fusion energyyields at full-power plant scale (200to 400 MJ) and high pulse rates (5 to

10 Hz). As indicated, it will alsodrive multiple test fusion chambersfor defense, IFE (ETF), basicscience, and materials research, usinga single driver to save costs. Its totalconstruction cost is expected to be $2 billion in today’s dollars, and itslife-cycle costs to the year 2020 areexpected to be $3 billion. Then asuccessful IFE chamber fromprevious tests will be upgraded to ahigher average fusion power level.This upgrade, which is expected toprovide a DEMO (net electric-powerdemonstration) by the year 2025, is shown as the last phase of theupgradable ETF/LMF facility.

Note in Figure 6 that the decisionto initiate the ETF/LMF facility,including selecting an ETF/LMFdriver, will be made after ignition isdemonstrated in the NIF. An ETFwith a single driver can be designedto test several types of fusionchambers at reduced power, greatlyreducing the cost of IFE developmentthrough a demonstration power plant. This parallel approach to IFE development has already been endorsed by many reviewcommittees, including the NationalAcademy of Sciences,11 the FusionPolicy Advisory Committee,7 theFusion Energy Advisory Committee,and the Inertial Confinement Fusion Advisory Committee.12

DOE–Defense Programs (using theNIF for fusion ignition and gaindemonstration) and DOE–EnergyResearch will play complementaryroles in driver development and otherIFE technologies.

Chairman Robert Conn, inreporting the recommendations of the 1993 Fusion Energy AdvisoryCommittee to then DOE EnergyResearch Director Will Happer,wrote: “We recognize the great

opportunity for fusion developmentafforded the DOE by a modestheavy-ion driver program thatleverages off the extensive targetprogram being conducted by DefensePrograms. Consequently, we urge the DOE to reexamine its manyprograms, both inside and outside ofEnergy Research, with the view toembark more realistically on aheavy-ion program. Such a programwould have the ILSE as acenterpiece, and be done incoordination with the program todemonstrate ignition and gain byDefense Programs.”8

Summary

When the NIF demonstrates fusionignition, which is central to provingthe feasibility of IFE, it will tell usmuch about IFE target optimizationand fabrication, provide importantdata on fusion-chamber phenomenaand technologies, and demonstratethe safe and environmentally benignoperation of an IFE power plant. Inaccomplishing these tasks, the NIFwill also provide the basis for futuredecisions about IFE developmentprograms and facilities such as theETF. Furthermore, it will allow theU.S. to expand its expertise in inertialfusion and supporting industrialtechnology, as well as promote U.S.leadership in energy technologies,provide clean, viable alternatives tooil and other polluting fossil fuels,and reduce energy-related emissionsof greenhouse gases.

Key Words: drivers—laser drivers, heavy-iondrivers; energy sources—fission breeder reactors,fossil fuels, inertial fusion energy, magnetic fusionenergy, solar energy; fusion chambers; fusionpower technology; International ThermonuclearExperiment; National Ignition Facility; targets—direct-drive targets, indirect-drive targets.

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References1. Sources, Effects, and Risks of Ionizing

Radiation, United Nations Scientific Committeeon the Effects of Atomic Radiation, 1988Report to the General Assembly, UnitedNations, ISBN-92-1-142143-8, 090000P(1988), Table 33, p. 114 and Table 72, p. 232.

2. The National Energy Policy Act of 1992, PublicLaw 102-486 (1992).

3. E. M. Campbell and D. L. Correll, “Lasers,”Energy and Technology Review, LawrenceLivermore National Laboratory, Livermore,CA, UCRL-52000-94-1/2, (1994), pp. 53–55.

4. B. G. Logan, R. W. Moir, and M. A. Hoffman,Requirements for Low Cost Electricity andHydrogen Fuel Production from Multi-UnitInertial Fusion Energy Plants with a SharedDriver and Target Factory, LawrenceLivermore National Laboratory, Livermore,CA, UCRL-JC-115787 (1994), to be publishedin Fusion Technology.

5. R. W. Moir, R. L. Bieri, X. M. Chen, T. J.Dolan, M. A. Hoffman, P. A. House, R. L.Leber, J. D. Lee, Y. T. Lee, J. C. Liu, G. R.Longhurst, W. R. Meier, P. F. Peterson, R. W.Petzoldt, V. E. Schrock, M. T. Tobin, and W.H. Williams, “HYLIFE-II: A Molten-SaltInertial 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 InertialConfinement Fusion Reactor Concept,Lawrence Livermore National Laboratory,Livermore, CA, UCRL-LR-104546 (1990).

7. Fusion Policy Advisory Committee, Review ofthe U.S. Fusion Program, Final Report,Washington, DC (1990).

8. “Findings and Recommendations for the Heavy-Ion Fusion Program,” Report of the FusionEnergy Advisory Committee (FEAC) to DOE

Energy Research Director William Happer(April 1993).

9. C. D. Orth, S. A. Payne, and W. F. Krupke, ADiode-Pumped Solid-State Laser Driver forInertial Fusion Energy, Lawrence LivermoreNational Laboratory, Livermore, CA, UCRL-JC-116173 (1994).

10. J. Wong, T. Diaz de la Rubia, M. W. Guinan,M. Tobin, J. M. Perlado, A. S. Perez, and J.Sanz, The Threshold Energy for DefectProduction and FIC: A Molecular DynamicsStudy, Lawrence Livermore NationalLaboratory, Livermore, CA, UCRL-JC-114789(1993). Also see Procedures 6th InternationalConference on Fusion Reactor Materials,Stresa, Italy (1993).

11. National Research Council, Second Review of the Department of Energy’s InertialConfinement Fusion Program, Final Report,(National Academy Press, Washington, DC,1990).

12. Inertial Confinement Fusion AdvisoryCommittee for Defense Programs, letter toAssistant Secretary for Defense Programs (May 20, 1994).

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For further information contact B. Grant Logan (510) 422-9816 orMichael T. Tobin (510) 423-1168.

AST March, at the behest of theDepartment of Energy, a group

of scientists from around the worldconvened at the University ofCalifornia, Berkeley, to discusspotential scientific applications of theNational Ignition Facility (NIF). TheNIF is a 192-beam, neodymium glasslaser that the Department of Energywill use to obtain information on high-energy-density matter, which isimportant for the generation of energythrough inertial confinement fusion.This information will also be used tomaintain the skills and informationbase necessary to manage the nation’snuclear stockpile, and mostimportantly from a scientificperspective, to pursue basic andapplied research.

The objective of the gathering inBerkeley was to identify those areas

anything from a terrestrial lightningbolt (approximately 104 K) to the coreof a carbon-burning star (109 K).

In short, this versatile and powerfulresearch tool would enable scientists inthese fields to explore previouslyinaccessible regions of the physicalparameter space that could validatecurrent theories and experimentalobservations and provide a foundationfor new knowledge. Following are theareas where the NIF is expected tomake notable contributions to scienceand applied science.

Astrophysics

To obtain information about starsand other astronomical bodies, theastrophysicist produces a sampleplasma in the laboratory and studiesits physical properties. For example,

of research in which the NIF and otherhigh-energy lasers could be used toadvance knowledge in the physicalsciences and to define a tentativeprogram of high-energy laserexperiments. The scientists determinedthat the NIF as well as other high-energy lasers have effectiveapplication in areas relating toastrophysics, hydrodynamics, materialproperties, plasma physics, andradiation physics. Their determinationwas based on the wide range ofexperiments already being performedon high-energy lasers, the diverseinterests of the scientific community,and the extraordinary range of physicalconditions that would be achievablewith the NIF—densities from onemillionth the density of air to ten timesthe density of the solar core andtemperatures that would be relevant to

Science on the NIF

The National Ignition Facility will allow scientists toexplore a previously inaccessible region of physical

phenomena that could validate their current theoriesand experimental observations and provide a

foundation for new knowledge of the physical world.

43

L

to determine a star’s structurethroughout the various stages of itslifetime—that is, its mass, heat,luminosity, and pulsationalinstabilities—the astrophysicist mayrequire information on the radiativeopacity of a plasma that mimics the outer stellar envelope and/orinformation on the equations of state(how density and temperature relate tothe pressure or internal energy) of aplasma that resemble the dwarf starinterior. Furthermore, to get a betteridea of a star’s structure duringvarious stages of evolution, theastrophysicist may be interested inproducing a stellar-like plasma toinvestigate its nuclear reaction rates.The key to success in all of theseexperiments is the ability tosynthesize the very hot plasmas thatcharacterize the stellar environmentduring stages of stellar evolution. The

astrophysical community is interestedin developing this potential withhigh-energy lasers, especially withthe NIF.

Equation of StateUnder many circumstances, the

equation of state of a stellar interioris simple: most of the gas ishydrogen and other light elementsthat have lost a good portion of theirelectrons. Unfortunately, theequation of state of the star’s interioris not as simple when the star is inits later stages of evolution. Densityis quite high, and the materialbecomes strongly coupled; that is,the ions interact strongly and nolonger behave as free particles. Thisbehavior is often accompanied byelectron degeneracy. This leads tothe tendency of the electrons to fillup certain energy states in a way that

forces some of the electrons to bevery energetic, thereby affecting thepressure and internal energy.

The theory of stellar evolution isaffected by uncertainties in theequation of state in a few areas. Forexample, in white dwarfs—the“nuclear ashes,” or compressedcores—of stars that have shed theirhydrogen-containing outer layers andgone through most of their evolution,the pressure from degenerate electronssupports the material against gravity.Near the surface of the material,however, degenerate electrons losetheir dominance. The ions then takeover, setting the specific heat andestablishing the rate at which thewhite dwarf will cool, a process thattakes many millions of years.

Radiative OpacityThe radiative opacity of the

material in stellar interiors plays akey role in determining how starsevolve: what the maximum mass of astable star is, how hot and howluminous the star is while it burns itshydrogen fuel, what pulsationalinstabilities may occur. Previous torecent experiments, astrophysicistswere using a set of opacitycalculations that predicted a verynarrow range of surface temperaturesfor the hydrogen-burning phase ofstellar evolution for all stars—in otherwords, all stars were very hot at thistime despite their differences in mass.These calculations also tied pulsationinstability to stellar luminosity andmass, which resulted in the wrongpulsation periods. The solution thenwas to correct the opacities used inthe calculations.

In the last few years, a group ofphysicists at LLNL has been able toreduce the discrepancy by using anew set of opacity calculations.Although these are definitely closerto observation (Figure 1) than the

Science on the NIF E&TR December 1994

44

0.74

4 Mfl5 Mfl

6 Mfl

6 Mfl

5 Mfl

4 Mfl

7 Mfl

0.72

0.70

1.0 3.0 5.0 7.0P0

P1/

P0

Figure 1. The effects of opacity on pulsations of Cepheid stars. The stellar mass (Mfl) ispredicted from the ratio of the first harmonic, P1, to the fundamental, P0. The black lines arethe results based on the new opacity calculations; the red lines are the results based on theold opacity calculations; the dots are the observed ratios. The new calculations, whichpredict a much wider range of surface temperature for the hydrogen-burning phase ofstellar evolution, put observation and theory in agreement. Experiments on the NIF willallow us to verify these effects and confirm our theoretical predictions.

previous calculations, they stillembody many approximations. Thus,to verify opacity at the relevantconditions, astrophysicists will needto conduct direct experiments. Ahigh-energy facility like the NIF willallow them to do this.

Thermonuclear Reaction RatesAlthough astrophysicists have

been studying nuclear reactions fordecades, their experiments haverarely achieved the energies at whichsuch reactions occur in stellarenvironments. With the NIF, theywill be able to conduct experimentsthat achieve such energies.Figure 2 shows the temperature anddensity regimes attainable with theNIF and compares them to theconditions of a star as it progressesthrough each phase of evolutionarynuclear burning. The first regime,which extends up to about 14 keV,shows the temperatures and densitiesthat may be reached in a laser-heatedhohlraum or an imploding capsulewithout nuclear ignition and includesa star’s hydrogen- and helium-burning phases. The second regime,which is between 9 and 60 keV,shows the conditions that might existin a deuterium–tritium capsule afterignition and includes the temperaturesand densities achieved up to a star’scarbon-burning phase.

The nuclear cross sections dependon energy (temperature), and we arecurrently limited by conventionalaccelerator methods’ inability toprobe the relevant energy regimes ofinterest (see Figure 2). The NIF willallow us to measure the nuclearreaction rates at precisely the energiesrelevant to stellar interiors.

In a thermalized NIF capsule,where a temperature of 8 keV couldbe attained, the number of radioactive13N nuclei, which would have a half-life of approximately 10 minutes,

could be counted after the event, orscintillators could be positionedaround the target to detect their pulseof 2-MeV gamma rays during theevent. Because the events would beproduced all at once in this type ofexperiment, the usual low signal-to-noise ratio would be avoided, makingit easy to distinguish the reactionsfrom ambient room background noise.

NIF experiments may further ourunderstanding of nuclear reactionsthat explore the proton–proton chainof hydrogen-burning reactions insolar-type stars and also the carbon-nitrogen-oxygen cycle. As examples,three reactions of interest inastrophysics include the 12C(p,Ì)13Nreaction, the 3He(3He,2p)4Hereaction, and the 3He(4He,Ì)7Bereaction. The first plays an importantrole in every star’s carbon-nitrogen-

oxygen cycle; the latter two form part of the proton–proton chain ofhydrogen-burning reactions in solar-like stars.

Hydrodynamics

Hydrodynamics is the study offluid motion and the fluid’sinteraction with its boundaries. TheNIF will allow us to further ourunderstanding of the hydrodynamicsof inertial confinement fusion andshock wave phenomena in the galaxy.

Because the NIF will be capable ofdepositing a large quantity of energyin a large amount of material over along time and at high densities, it willbe able to generate hydrodynamicflow conditions that are much moreextreme than those generated by wind tunnels, shock tubes, or even

E&TR December 1994 Science on the NIF

45

10–1 10 101 102 103

Temperature, keV

1030

1020

1010

Den

sity

, cm

– 3

NIF laser only

Hydrogen-burning

Carbon-burning

Neon-oxygen-silicon-burning

Helium-burning

BurningNIF capsule

Figure 2. Temperature and density regimes attainable on the NIF overlap with theconditions of a star as it progresses through evolutionary nuclear burning. The first regime,which overlaps with a star’s hydrogen- and helium-burning phases, could be reached in alaser-heated hohlraum or an imploding capsule without nuclear ignition. The secondregime, which overlaps with the conditions achieved up to a star’s carbon-burning phase,might exist in a deuterium–tritium capsule after ignition. Experiments conducted in theseregimes could greatly enhance our knowledge of stellar evolution.

high-energy lasers such as Nova.Thus, scientists will be able toinvestigate a number of flow problemsunder previously unattainableconditions. NIF experiments designedto study these problems—for example,the growth of perturbations at a fluidinterface (unstable flow) andshock–shock boundary interactions(stable flow)—will lead to newunderstandings in fluid dynamics.

Imposed Perturbations To study the growth of an imposed

perturbation under continuousacceleration, we shock planar foils of fluorosilicone by x-ray ablation.The foil trajectory is recorded by aradiographic streak camera so that we can check the bulk movement of the sample. The image’s contrast inoptical depth is then measured as a function of time. From thismeasurement, we deduce the evolutionof the imposed perturbation.

Figure 3 shows two images of a foilfrom a perturbation experimentconducted on the Nova laser. Theimage on the left shows that foil hasbecome increasingly bright; this is the result of thinning caused by the“bubble and spike” shapecharacteristic of the nonlinear regime.The image on the right, takenapproximately 2.6 µs after the start of the drive for a duration of 100 ps,shows no transverse distortion of the foil.

We obtain quantitative data bytaking intensity traces at different timestransverse to the grooved structure, asshown in Figure 4. Note that thecurves, which represent different timesin the growth of the perturbation, areoffset for ease of comparison. At earlytimes, the growth is small and stillsinusoidal, indicating that theinstability is in the linear regime. Late

Science on the NIF E&TR December 1994

46

(a)

Pos

ition

Position Position

(b)

Tim

e

In, e

xpos

ure

50-µm increments

Burn-through

Spike

1.5

1.1

0.7

0.3

–0.1

t, ns

1.9

2.7

2.3

Non

-line

arLi

near

Bubble

Figure 3. One- and two-dimensional views of the backlight absorbed by a foil with initialperturbation. The one-dimensional view (a) shows that the end of the bright spot occurswhen the backlight ceases to radiate effectively. The face-on, two-dimensional view (b)shows little if any distortion of the foil.

Figure 4. Traces foran accelerated foilwith an imposedperturbation. Thecurves are offsetvertically to allowsimple comparison.The dashed linesindicate the backlightintensity, whichvaries across the foilas a function of time.

in time, the growth is larger anddistinctly nonsinusoidal, exhibiting the characteristic bubble-and-spikeshape. The rapid flattening of themodulations in the top two curvesresults from the burn-through thatoccurs when the bubbles break out ofthe back side of the foil. At this point,the spikes are still being ablated away;however, they can no longer bereplenished by matter flowing downfrom the bubbles.

These experiments extend from thesingle-mode example described hereto multiple-mode experiments and toburied interfaces with imposed modestructures. The limiting case of arandom set of perturbations at aburied interface requires a somewhatdifferent technique.

Impact CrateringMany phenomena in impact

cratering occur on temporal andspatial scales that are very large whencompared to those of the impactingobject; as a result, we can model the

impact as a point source of highenergy and momentum density. This modeling is usually done bydepositing focused laser energy insmall spheres of high-Z (high-atomic-number) material or by generatingprodigious shocks and post-shockpressures with flyer foils.

A study of concentrated impactson surfaces shows that scaling lawsapply to craters formed by impact andsurface energy deposition. A proof-of-principle experiment designed toexplore the effects of impact crateringon simulated soil (Figure 5) indicatesthat further research in this areawould be of great interest. The abilityof the laser to deposit large amountsof energy in a spot volume withoutresidual gases—the by-product of thesame experiment performed with highexplosive—indicates its utility as asimulation source. On the NIF, theamount of energy deposited and thein situ diagnostic potential wouldmake investigation of hydrodynamicresponse possible in real time.

Material Properties

For the last several decades,scientists have been studying thephysical properties of materials—e.g., their equations of state,opacity, and radiative transport—byconducting experiments with gasguns, high explosives, and high-pressure mechanical devices.Although these experiments haveprovided a wealth of precise data ona wide range of materials, they arelimited because they do not provideinformation on material behavior atthe extreme pressures andtemperatures of scientific interest,i.e., pressures from 1 to 100 terapascals and temperatures up to a few hundred electron volts.Although a few laser-driven andshock-wave experiments have beencarried out in this range of interest,the resulting data are quiteimprecise and do not validate anyof the theoretical models of materialbehavior.

E&TR December 1994 Science on the NIF

47

Gold hollowsphere

Soil sample

14°Laser

(a) (b)

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-mmradius 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 wasenough 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 radiationescaped the target, as indicated by the top surface of the cube (b). Although this surface is crazed and slightly bowed, the cavity and theentry 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.

With the NIF, scientists will beable to investigate material behaviorin this range and obtain the primarydata needed to test their theoreticalmodels. The experimental methodsused to obtain these data includecolliding foil experiments (for

equation-of-state data) and high-resolution x-ray measurements (forradiative opacity data).

Indirectly Driven Colliding FoilExperiments

To reach a regime of very highpressure without sacrificing laser spotsize and one-dimensionality, weemploy a variation of the well-knownflyer-plate technique. In thistechnique, a flyer—in this case a foil—stores kinetic energy from the driverduring an acceleration and rapidlydelivers it as thermal energy when itcollides with another foil. The flyerfoil also acts as a preheat shield sothat the target remains on a loweradiabat, that is, for a given pressurethe temperature is kept lower than itwould be if exposed to the driver.

In this type of experiment (Figure 6), the laser beams are focusedinto a millimeter-scale cylindrical goldhohlraum; the radiation that escapesfrom a hole in the hohlraum becomesthe x-ray drive. The hohlraum x raysablate a 50-µm layer of polystyrenewith a 3-mm-thick gold flyer foil. This foil then accelerates through avoid and, near the end of the laserpulse, collides with a stationary, two-step (two-thickness) gold target foil.The shock on the rear side of the targetfoil is then imaged with an opticalstreak camera.

Figure 6b is a typical streakcamera image of shock breakout on atwo-step target foil. The time intervalbetween the two breakout times (onefor each thickness) measures theshock speed in the target. An intervalof 57 ps between breakout on the twosteps corresponds to an average shockvelocity of 70 km/s. According to ourequation-of-state tables, this shockspeed corresponds to a density of 90 g/cm3 and a pressure of 0.74 Gbarin the gold target, which is by far thehighest inferred pressure obtained ina laboratory.

In this experiment, any spatialimbalance in the drive or anyunpredicted edge effects (forexample, those from interactionsbetween the flyer foil and sleevecontaining the target assembly) couldcause the flyer foil to tilt or curveand drive a nonplanar shock into thetarget. However, any nonplanaritywould be readily observed becauseof the relatively large diameter of thefoils; furthermore, any edge-inducednonuniformities would be minimizedbecause the step in the target is at thecenter of the foil. (See pp. 28–29 fora discussion of this experimentrelative to weapons physicsequation-of-state experiments.)

If the target foil is preheated byhigh-energy x rays from thehohlraum before the flyer foil hits it,the measurement is compromised.To test this possibility, we altered the x-ray drive in one experiment so that the overall intensity would beidentical to that in other experimentsbut the intensity of high-energyx rays (those ≥2.5 keV) would bereduced by more than a factor offive. The result indicated that themeasurement was not affected bypreheat.

X-Ray Opacity MeasurementsTo understand the plasma state

and radiative transport, we need toobtain high-quality measurements of the radiative opacity of materials.To do this, we must simultaneouslymeasure the x-ray transmission,temperature, and density of amaterial sample in a singleexperiment. These measurementshave been done successfully onNova using point-projectionspectroscopy (see Figure 7); webelieve that this technique will beeven more successful in similarexperiments on NIF because we will be able to access larger ranges ofmaterial densities and temperatures.

Science on the NIF E&TR December 1994

48

Shock breakout

Tim

e

Streakcamera

2-µm/6-µmgold

target foil

(b)

(a)

3-µm goldflyer foil

Drivebeams

1-mm-long¥ 700-µmgold sleeve

50-µm void

50-µmpolystyreneablator

X rays

Figure 6. (a) Schematic of a radiation-driven shock experiment. The x raysescape from a hole in the cylindrical goldhohlraum and ablate a 50-µm layer ofpolystyrene with a 3-µm gold flyer foil.The flyer foil accelerates through a 50-µmvoid, collides with a two-step gold targetfoil, and launches a compression wave inthe target foil. (b) A typical streak cameraimage of a shock breakout showing thetime interval between the two sides of the step.

When we use point-projectionspectroscopy on Nova, we use eightlaser beams to heat the sample. Then, apoint source of x rays is produced bytightly focusing one of the remaininglaser beams onto a small backlighttarget of high-Z material. X rays fromthe backlight pass through the sampleonto an x-ray diffraction crystal and arethen recorded on x-ray film. Other x rays from the same point bypass thesample but are still diffracted from thecrystal onto the film record. The ratio ofattenuated to unattenuated x rays provides the x-ray transmissionspectrum of the sample. Propercollimation allows a highly quantitativeanalysis of the spectrum. Backgroundfrom film chemicals, sample emission,and crystal x-ray fluorescence can all beseparately determined from the x-rayfilm record.

The sample itself must be uniformin temperature and density.Uniformity of temperature is achievedby heating the sample in a hohlraumthat does not allow laser light toreflect or impinge on it directly; thus,the sample is heated only by x rays.The hohlraum, by providing x-raydrive that volumetrically heats the

sample that is tamped, also maintainsthe relatively high density of thesample and ensures that it is in localthermodynamic equilibrium.

The sample is tamped by plastic sothat as it expands, its density remainsconstant. The thickness of the tamperis determined by calculations, and thedensity of the sample is determinedby imaging. Usually, a second point-projection spectrometer images theexpansion of the sample. The firstpoint-projection spectrometer is usedto measure the sample’s absorption.The relative intensities of thetransitions from the different ionspecies give the ion balance in thesample, which, when coupled to thedensity measurement, gives thetemperature of the sample.

The two point-projectionmeasurements allow density to bemeasured to an accuracy of ±10% andthe temperature to an accuracy ofabout 5%. With these accuracies it is possible to make a quantitativecomparison between the experimentalresults and the theoretical calculationsof the opacity.

In one experiment on Nova, wemeasured the opacity of niobium in

an aluminum–niobium sample. Thesample contained 14% aluminum by weight for the temperaturemeasurement. Figure 8 shows thetransmission of the aluminum and the transmission of the niobium. The dotted lines overlaying theexperimental data are the calculations.In general, there is excellentagreement. This experiment is amilestone. It shows that we can obtainopacity measurements accurateenough to serve as an in situtemperature diagnostic for the sample.The accuracy of the sample’stemperature, measured to be 48 eV (±2 eV), represents a very importantadvance in measuring temperatures of high-energy-density matter. (See pp. 27–28 for a discussion of thisexperiment in relation to weaponsphysics opacity experiments.)

Plasma Physics

In the broadest sense, plasmaphysics is the scientific investigationof the predominant state of matter inour universe, plasma. The study ofplasma physics has been stimulatedover the past four decades by its close

E&TR December 1994 Science on the NIF

49

Film

x

Bragg crystal

Occludedarea:emissionand fog

Tamped opacity sample with two different thicknesses

Hohlraum heated by 8 laser beams

Backlightlaser

Point sourceof x rays

Spectrum Absorptionspectrum

y

y, ¬

x

Figure 7.Schematic of point-projectionspectroscopy for opacitymeasurements. Thelaser-producedbacklight x rays passthrough the targetand are imaged. A Bragg crystaldisperses thespectrum so that a spatially andspectrally resolvedimage is obtained.Temporal resolutionis provided bybacklight duration.

connection with the goal of creatingfusion as an energy source and withthe exploration of variousastrophysical plasmas.

The advanced experimentalcapabilities of the NIF will allow usto produce and characterize large,hot, uniform plasmas. With largeuniform plasmas, we will be able tomeasure electron and iontemperature, charge state, electrondensity, and flow velocity. In short,we will be able to perform a widerange of quantitative experiments ona medium that is a very goodapproximation of a real test bed forplasma physics. Many experimentswill be extensions of the fusionenergy experiments that have beenperformed on smaller, less powerfulhigh-energy lasers. Many others,however, will go beyond therequirements of fusion energy toexplore a range of basic topics inplasma physics, a few of which arediscussed here.

FilamentationWhen a small hot spot, or speckle,

in the laser-intensity profileundergoes self-focusing,filamentation occurs: that is, electrons(and eventually ions) are expelledfrom the filament, causing laser lightto focus more tightly. This process, in

turn, creates an unstable feedbackloop; the more tightly focused thelaser light is, the higher its intensityand the lower its electron density.Eventually, this instability issaturated by diffraction effects,thermal absorption, or parametricinstabilities.

The growth of filamentarystructures can be determined by thewidth and length of speckles in theincident laser beam. Because longspeckles are more likely to self-focusthan short speckles, filamentation canbe described by a growth rate alongthe length of a speckle, or 8f2l, wheref is the f-number of the beam (i.e.,beam focal length divided by effectivemaximum beam diameter) and l is the wavelength of the laser light. Ifthe speckle lengths are smaller thanthe scale length of the plasma, the f-number and the wavelength of theincident beam can be very powerfullevers for modifying filamentation.By using large uniform plasmas onthe NIF, we will be able to producesufficiently large filaments to studythis process over a wide range ofwavelengths and f-numbers. We willalso be able to explore this processover a broad range of experimentalparameters by varying the color and f-number of the filamentation beamand by varying the plasma conditions

(e.g., temperature, density, andaverage ion charge).

The primary diagnostic forfilamentation would be the stimulatedRaman scattering signal, which isindicative of the low density in thefilaments. That could be coupled withhigh spatial-resolution imaging, high-resolution optical probing, and astudy of the angular distribution ofscattered light.

Thomson scattering could be usedfor these investigations to makehighly localized measurements ofplasma temperature and density. Itcould also be used to measure thecoherent motion of electrons involvedin ion acoustic and electron plasmawaves. This measurement wouldprovide a temporally and spatiallyresolved measure of the coherentfluctuation amplitude in a specificdirection, as determined by thedetector angle, the scattering volume,and the scattering light source.Measurement of the backgroundfluctuation levels would provideinformation about the initial level ofthe coherent fluctuations, theiramplification, and their saturation. Italso could provide useful informationabout the coupling betweenstimulated Raman and Brillouinscattering if it were done at the samelocation and at the same time as the

Science on the NIF E&TR December 1994

50

Abs

orbt

ion 0.8

0.6

0.4

0.2

1520 1540 1560Energy, eV

1580

Experimental dataPrediction

1600 2100 2200 2400 2500 2600 2700 280023000

1.0

Figure 8. Absorption of an aluminum–niobium sample. The experimental data are in the solid black line and the opacity prediction is thedashed line. The spectrum of the aluminum-potassium alpha lines, which were verified to yield an accurate temperature, were measured onthe same experiment as the niobium spectrum.

coherent motion measurements.Developing an x-ray Thomsonscattering measurement to studycoherent plasma motion in highdensity plasmas is another excitingpossibility.

Formation of Large, UniformPlasmas

Presently, we can producerelatively high-temperature (3000-eV), millimeter-scale plasmasusing the diagnostic complement andexperiments shown in Figure 9.These large, uniform plasmas areused to study phenomena as diverseas plasma–laser interactions andnuclear reaction rates. Theexperimental geometry should bedirectly scalable to the NIF, withnine-tenths of the laser being used toform the plasma and one-tenth beingused to create interactions. Theability to produce these large uniformplasmas on the NIF will allow us tostudy fundamental aspects of ourexperiments, such as hohlraumenvironments and sidescatter, thathave been virtually impossible tointerpret quantitatively.

Short-Pulse, High-PowerExperiments

There is widespread agreementthat the NIF should include a beamline for short-pulse, high-powerexperiments. This capability isespecially important for studyingsuch basic topics as relativistic, ultra-high-intensity regimes of laser–matterinteraction; high-gradient acceleratorschemes; and fast ignition (Figure 10). It is also more amenableto detailed simulation and tosystematic exploration of linear andnonlinear behavior of plasmas.

The high-gradient acceleratorschemes employ plasmas that supportmuch higher energy fields than thoseassociated with conventionalaccelerator schemes. As a result, thedevice will be much more compact

E&TR December 1994 Science on the NIF

51

Gatedspectrometer

(SXRFC)

Laser Laser

Streaked high-resolution

spectrometer(HIX)

Streakedspectrometer

(SSC)

Gated imaging(GXI-3)

Gatedimaging(WAX)

Interactionbeam

Streaked imaging(SSC)

Gatedimaging(GXI-2)

(a) High compression implosion

(b) Channeling laser beam (c) Ignitor laser beam

Pusher

Compressedmaterial

Figure 9. Schematic of the experiments and diagnostic complement (red arrows) used to forma large, hot, uniform plasma. A gas bag (light green) is filled through two tubes in the hoop (darkgreen). The pressure is stabilized by a pressure transducer that causes the fully ionized speciesto yield electron densities of approximately 10–1 cm3. The roughly spherical plasma, which has avolume of 0.066 cm3 and a radius of 2.5 mm, is heated to a temperature of about 3000 eV byheating lasers (blue arrows). A separate interaction beam (light blue arrow) drives theinstabilities in a controlled way. This geometry should be directly scalable to the NIF.

Figure 10. Fast ignition requires high compression, two laser systems, and system diagnoses.(a) In the first step of this process, a gas-filled sphere is imploded. The core of the compressedgas is at densities of 600 g/cm3. (b) In the next step, a laser with a pulse duration of 100 ps and anintensity of 1018 W/cm2 creates a channel by pushing the critical density surface toward the core.(c) Finally, the heater, or ignitor, beam is turned on. This beam interacts with the density gradientand generates hot electrons at MeV energies. These electrons penetrate into the core of thecompressed gas and cause an instantaneous rise in the local temperature of the core.

and potentially cheaper. A number ofnovel schemes have been proposedand studied at laser powers not quitehigh enough to produce the desiredelectron velocity. If these schemesprove successful, applications totunable sources of x rays are alsoenvisioned.

Radiation Sources

The conversion of laser energy intoshort wavelength radiation is a majorgoal of many high-energy laserexperiments. On the NIF, we will be

able to convert laser energy to a widevariety of x-ray and particle sourcesneeded to address several importantquestions in basic and appliedphysics. For example, we will beable to produce intense broadbandthermal x rays from high-Z targets,coherent amplified x rays (x-raylasers) from high-gain plasmas,intense neutron pulses fromimplosion plasmas, and intensepulses of hard x rays from fastelectrons. Accurate energy spectraand absolute measurements of theconversion of laser energy into all

types of radiation and particle fluxes will play an important role in benchmarking our basicunderstanding of laser–plasmainteractions and atomic physics.

Broadband x rays generated byNIF laser plasmas will be used toproduce and characterize large,uniform plasmas relevant to inertialconfinement fusion and astrophysics.The high temperatures and densitiesproduced during implosion andsubsequent ignition will be anexcellent source of continuum x rays—those extending from thesoft x-ray region to MeV with pulsedurations of less than l00 ps.

Besides producing importantcoherent radiation sources (i.e., x-raylasers), NIF will offer a critical testof our atomic modeling, allowing usto extrapolate existing neon-like andnickel-like collisional x-ray lasers towavelengths of about 20 angstroms(Å = 10–10 m). At these wavelengths,we can use x-ray laser interferometry(the interference created by splittingand then recombining the x-ray laserbeam) to measure electron densitiesin plasmas exceeding solid densities.Also, the short-pulse capability ofthe NIF may enable us to developnew x-ray lasers that emit radiationat wavelengths shorter than 10 Å;such bright, coherent sources wouldbe very useful in characterizing solidmatter for materials science andbiophysics research.

The NIF will be able to generatemore than 1018 neutrons in a single100-ps pulse, making it very usefulfor producing uniform, high-density,low-temperature plasmas. It will beable to generate fast electrons withhundreds of kiloelectron volts inenergy—which is a potential source of high-energy x rays forbacklighting and probing plasmas.

Science on the NIF E&TR December 1994

52

Figure 11. Equivalent radiation temperature vs time for three gold hohlraum experimentsperformed on Nova. The experiments used a total energy of 18 kJ. Laser entrance holeswere in the sides of the 500- and 1000-µm-length hohlraums and through the ends of the2800-µm-length hohlraum. The curves for each hohlraum show the reproducibility of thedata. The black line indicates the equivalent radiation temperature with the M band; the redline indicates the equivalent radiation temperature without the M band. The contribution ofthe M band to the radiation temperature was greatest in the small hohlraums because of theclosure of the holes; it was very small in the large hohlraums because they were large andbecause their viewing angle did not accommodate a view of the laser-irradiated spots.

0.30

0.20

0.10

0

0.40

2-ns pulse1000 µm ¥ 1000 µm

1-ns pulse1600 µm ¥ 2800 µm

1-ns pulse500 µm ¥ 500 µm

Without M bandWith M band

0 0.5 1.0 1.5 2.0 2.5 3.0Time, ns

Tem

pera

ture

, keV

These radiation sources aresupplemented by the possibility ofusing radiation enclosures, orhohlraums (such as those shown inFigure 11), to generate radiationenvironments and x-ray drive fluxes.These sources will be able to producefar in excess of the approximately200 eV produced by hohlraumsources available on current high-energy lasers. These sources will beof higher effective temperature andalso will be able to provide uniformx-ray drive over far larger areas thanis possible with today’s sources.Thus, the advantages of using x-rayheating for the study of hot, densematter will be greatly enhanced onthe NIF.

Radiative Properties

The importance of radiativeproperties in high-energy-densityplasma derives from three factors:• First, the radiative property can bethe best indicator of the level ofscientific knowledge in a particulararea. For example, when scientistswant to develop new descriptions of atomic structure, they look attransition energies.

• Second, radiative properties serve as primary data for numerous otherstudies. For example, spectral linelists are inadequate for many of thecharge states of heavier elements.Thus, scientists measure andcategorize the energies of highlyionized species for a variety of uses.• Third, radiative properties serve asnoninterfering probes. For example,by looking at the emission orabsorption spectrum of a plasma,scientists can obtain fundamentalinformation about the plasma’sionization balance, rate processes,densities, temperatures, andfluctuation levels. The radiativeproperties are therefore a powerfuldiagnostic of the plasma state.

Experiments on high-energy lasershave done much to enhance ourknowledge of the radiative propertiesof hot, dense matter; thus, we expectthat experiments on the NIF, such asthose employing interferometry andplasma spectroscopy, will advancethat knowledge even further.

Interferometry ExperimentsFor years, optical probing of

high-density or large plasmas hasbeen difficult because of the high

absorption of the probe, the effectsof refraction, and the impossibilityof going beyond critical densities.Recently, we did an experiment to see whether an optical measuringdevice, known as a Mach-Zehnderinterferometer, and a standard 3-cm-long yttrium x-ray laser could beused to probe these plasmas moresuccessfully.

In this experiment (shownschematically in Figure 12), theoutput from a standard 3-cm-longyttrium x-ray laser was collimatedby a multilayer mirror and injectedinto the interferometer. An imagingoptic from the interferometer thenimaged a plane within theinterferometer where a plasma wasproduced. Figure 13 shows therecorded interferogram of theplasma. The fringes, or contrastmodulations, due to the plasma are clearly visible, indicating thefeasibility of this technique.However, plasma blow-off, evident in the central region of the image,completely obscures the laser,indicating that the technique still has its limits. On the NIF we will be able to push the x-ray laserinterferometer to shorter x-ray laser

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Figure 12.Schematic of theexperimental setupfor x-ray laserinterferometry.

X-ray CCDdetector

Laser irradiated target

Collimatingmultilayer

mirror

X-ray laserMach-Zehnderinterferometer

Imagingmultilayer

mirrors

wavelengths, making it an even moreimportant diagnostic tool in the studyand characterization of large-scaleplasmas.

Summary

The extraordinary range ofphysical conditions that will beachievable on the NIF will advanceknowledge in the physical sciences. Itwill give us the ability to synthesizeand analyze the plasmas thatcharacterize the stellar environmentduring its evolution. It will enable usto investigate a number of stable andunstable flow problems underconditions that cannot be obtained byconventional means, such as windtunnels, shock tubes, or other high-energy lasers. It will give us theability to investigate materialbehavior at pressures from 1 to 100 terapascals and temperatures upto a few hundred electron volts sothat we can validate our theoreticalunderstanding of material behavior atextreme conditions. We will be ableto convert NIF laser energy to a widevariety of x-ray and particle sourcesneeded to address importantquestions in basic and applied

physics. Finally, the NIF will enableus to push the x-ray laserinterferometer to shorter x-ray laserwavelengths, making it an even moreimportant diagnostic tool in the studyand characterization of large-scaleplasmas. The NIF will allow us toexplore a previously inaccessibleregion of physical phenomena thatcould validate our current theoriesand experimental observations andprovide a foundation for newknowledge.

Key Words: astrophysics; high-pressure physics;hydrodynamics; National Ignition Facility—high-energy laser experiments; plasma physics; radiationsources; radiative properties.

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1200

600

00 1000

µm2000

µm

Figure 13. Interferogram of a high-density plasma produced by x-ray laserinterferometry. The plasma is made byirradiating the surface of a mylar plasticsample (solid horizontal band) with an x-raybeam. The bright spot above the plasticsample is the self-emission of the plastic.

For furtherinformationcontact Richard W. Lee(510) 422-7209.

NVIRONMENTAL, safety, andhealth (ES&H) considerations

are of paramount importance in allphases of the design of the NationalIgnition Facility (NIF). From theoutset, maximizing public andemployee safety and minimizinghealth risk and environmental impacthave been integral parts of the designprocess. In a broader sense, inertialconfinement fusion has long-termpotential as a source of future energyfor the world. Earning the trust of thepublic and operating in a manner thatprotects the environment over timeare requirements for any futureenergy source.

This article describes the results ofenvironmental analyses as well as

dose at the site boundary, themaximum annual dose and averageannual dose to any NIF worker, andthe effects of a worst-case accidentassuming release of the maximumplanned tritium inventory. We havealso estimated the quantities ofhazardous, mixed, and low-levelradioactive wastes. Before turning tothese matters, a brief overview of thereview process will help to put ourefforts to date into perspective.

The Assessment and ReviewProcess

Safety and environmental analysisspecific to the NIF began more thantwo years ago. This work was

safety and health assessments. Thetopics fall broadly into threecategories presented in the followingorder: radiation exposure, wastegeneration, and fusion targets.

The NIF will maintain a smallinventory of less than 300 curies(0.03 grams) of tritium fuel andunburned tritium from inertialconfinement fusion experiments. Thefusion reactions in the 1-mm capsulewill release a neutron for everytritium atom consumed, about 1017

atoms for an ignition experiment. TheNIF target area employs heavyshielding, including 2-meter-thickconcrete walls and a shielded vesselagainst the radiation. We haveestimated the routine annual radiation

NIF Environmental, Safety,and Health Considerations

The proposed NIF has been rated by the DOE as aradiological low-hazard, non-nuclear facility. Our analysisto date of environmental, safety, and health issues related to

the NIF, presented in documents that are available to thepublic, shows that the system will have no significant

environmental impact and present no significant safety orhealth risk to the work force and the general public.

55

E

an outgrowth of continuingenvironmental impact and safetyanalyses of general concepts forfuture inertial confinement fusion(ICF) facilities. More than a year ago,a standing NIF working group wasformed at LLNL. The group is madeup of environmental and safetyexperts in radiation protection, safetyanalysis, environmental evaluation,laser operations, occupational safety,tritium handling, waste handling,quality assurance, and fire protection.The group meets biweekly to ensureconsistent and well-documentedevaluation of environmental andsafety aspects of the NIF design.

The NIF working group hasprepared the Preliminary HazardsAnalysis for the NIF1 as well asradiation protection,2 safety,environmental, quality assurance, and decommissioning evaluations ofthe NIF design.3 These analyses areavailable to the public as publishedreports.4

ES&H issues are extensivelyanalyzed by experts, documented, andreviewed by the public as part of theprocess established by the DOE formajor system acquisitions such as theNIF. Whereas major steps towardsafety and environmental analyseshave already been done for NIF,additional analyses will include anEnvironmental Impact Statement,Preliminary and Final SafetyAnalyses, and Operational ReadinessReviews. The DOE requires all ofthese studies to ensure that essentialaspects of the project are thoroughlyanalyzed and are completelysatisfactory before operations begin.

Radiation Doses

In our radiological assessments,the unit of measure is the rem, whichstands for roentgen-equivalent in manand is a unit of biological radiationdose. It is the amount of ionizingradiation that produces the same

damage to humans as 1 roentgen of high-energy x rays. Naturalbackground radiation from theenvironment—that is, from naturallyoccurring elements, cosmic radiation,and so forth—averages 0.3 to 0.5 rem/yr depending on where anindividual lives. This naturalbackground radiation does notinclude exposure to other potentialsources of radiation, such as that fromairplane flights and some types ofmedical diagnoses or treatments.

The routine annual dose from NIFat a site boundary 300 meters fromNIF is expected to be 0.00013 rem.Put into perspective, this valuerepresents 0.13% of the DOE andEnvironmental Protection Agencyguideline and is 350 times less thanthe annual radiation dose arising fromemissions from a 1-GWe coal-firedpower plant.

The average annual dose receivedby flight attendants is 0.5 rem, a dosenot monitored by the airline industry.In comparison, the maximum annualdose to any NIF worker will be lessthan 0.5 rem, which is less than 10%of the DOE guidance. The averagedose to any NIF worker is estimatedto be about 0.01 rem.

The maximum tritium inventoryfor the NIF will be 300 curies (Ci).This amount is the equivalent of 0.03 grams of tritium. The maximumNIF inventory is less than 3% of theroutine inventory of the NationalTritium Labeling Facility inBerkeley, California, which usestritium for tagging biomedicalsamples. One NIF target will containless that 2 Ci of tritium, one-fifth theamount of tritium in some typicaltheater exit signs of which more thanone million are sold annually.

There are no significantradioactive or hazardous effluentlevels for NIF. For example, theprojected maximum emission oftritium is less than 10 Ci/yr, theequivalent of the tritium in a single

exit sign. The dose to a member ofthe public expected from all NIFeffluents is 600 times less than thatfrom a single cross-country airlineflight.

The worst-case accidentconsidered in our safety assessmentassumes the release of all the tritium(300 Ci) in its worst biohazardousform (tritiated water) immediatelyafter a maximum-yield experiment(20 megajoules). This postulated, buthighly unlikely, accident would resultin a calculated dose of 0.056 rem at asite boundary 300 meters from NIF.This dose is 0.2% of the DOE sitingguidelines for annual exposure.

Waste Generation

NIF will generate three types ofwaste: hazardous, low-levelradioactive, and mixed (acombination of hazardous and low-level radioactive). To be on theconservative side, we estimatedhigher waste quanlities than are likelyfrom the NIF’s waste streams.Moreover, our assessment has notfully considered waste-minimizationtechniques, such as frozen carbondioxide pellet cleaning. Wasteminimization will be an importantand continuing design activity.

The annual hazardous wastestream associated with NIF will beabout 3180 kg (7000 lb) of solidwaste and 2270 L (600 gal) of liquidwaste.3 Most of this waste stream,about 2270 kg (5000 lb), will be inthe form of 20 boxes of paper soakedwith capacitor oil. Such waste issimilar to but smaller in quantity thanthat generated in the same time by anautomobile oil-changing facility. Thewaste will be routinely disposed of bycertified contractors.

Mixed waste is both radioactiveand chemically hazardous. Theannual mixed waste stream associatedwith NIF will be about 135 kg (300 lb) of solid and about 2000 L

Environmental, Safety, and Health Issues E&TR December 1994

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(530 gal) of liquid,3 which represent asmall fraction of the quantitiescurrently generated at LLNL. Thesemixed waste quantities take intoconsideration some use of frozen CO2pellet cleaning, a dry, high-pressurescouring technique thatdecontaminates objects and avoids theneed for alternate methods that usehazardous liquid solvents, therebygenerating large quantities of liquidmixed waste.

The annual solid low-levelradioactive waste stream, 400 kg (850 lb), will be a small fraction ofthe quantity currently generated atLLNL and is less than one-sixth ofthat produced by a major university’smedical center. The annual liquidlow-level radioactive waste stream(aqueous waste) will be disposed of inseveral 55-gallon drums,3 along withtwo drums of vacuum pump oil,according to applicable guidelines.

Fusion Targets

ICF targets are used in manyfacilities throughout the country.Examples of current or futurefacilities using such targets includethe Nova laser; the Particle BeamFusion Accelerator II (PBFA-II) atSandia National Laboratory inAlbuquerque, NM; the plannedupgrade of the Omega laser at theUniversity of Rochester; and theproposed NIF. The manufacture andfilling of fusion fuel capsules is aseparate and ongoing activity of thenational ICF Program that supportspresent and future facilities. Thesefunctions are carried out at severalDOE and commercial sites (forexample, at General Atomics in LaJolla, CA; the University of Rochesterin Rochester, NY; Los Alamos

National Laboratory in Los Alamos,NM; and LLNL).

The filling activity for NIF targetsrequires a total inventory of less than 5 grams of tritium. The targetmanufacturing and filling facilitieshave their own NationalEnvironmental Policy Act (NEPA)documentation and engineeredsystems to protect workers, thepublic, and the environment by safelyconfining any tritium.

Because of the existing nationalcapability, the NIF project will not include a dedicated targetmanufacturing and tritium fillingfacility. Instead, it will receiveseveral types of targets from severaldifferent sites. Targets for NIF will betransported in certified containersprescribed by the Department ofTransportation and in accordancewith the Code of Federal Regulations(Title 49, section 173).

Summary

After reviewing the PreliminaryHazards Analysis report, the DOEconcurred with the preliminarycategorization of the NIF as aradiological low-hazard, non-nuclearfacility. This means that operation ofthe NIF will have minor onsite andnegligible offsite consequences. Thehazards categorization will bereviewed in each subsequent safetyanalysis report.

The conservative safety andenvironmental analyses outlined inthis article are the first of a series ofstudies required to ensure the safetyof workers, the public, and theenvironment. The NEPA process ofthe DOE ensures joint participationby the public and those states thatmay be affected by the project. The

Environmental Impact Statementprocess will also allow participationby the public in reviewing thepotential environmental impacts ofthe NIF.

Key Words: environmental safety and health(ES&H); National Ignition Facility (NIF)—radiationdose; tritium inventory; waste stream.

Notes and References1. S. J. Brereton, Preliminary Hazards Analysis for

the National Ignition Facility, LawrenceLivermore National Laboratory, Livermore, CA,UCRL-ID-116983 (1993).

2. M. S. Singh, Radiological Analysis of theNational Ignition Facility, Lawrence LivermoreNational Laboratory, Livermore, CA, UCRL-LR-115188 (1993).

3. National Ignition Facility Conceptual DesignReport, Lawrence Livermore NationalLaboratory, Livermore, CA, UCRL-PROP-117093 (1994) and National Ignition FacilitySeting Proposal–LLNL, Lawrence LivermoreNational Laboratory, Livermore, CA, UCRL-PROP-119353 (1994).

4. To obtain copies of references 1 and 2, contactthe National Technical Information Services(NTIS), U. S. Department of Commerce, 5285Port Royal Road, Springfield, VA 22161.Reference 3 cites ongoing documents that willnot be available to the public through NTIS untilcomplete; copies, however, are available to readat the Livermore Public Library, Livermore,CA, and at the Visitors Center at LLNL.

5. S. Brereton, G. Greiner, M. Singh, and M. Trentalso contributed to this article.

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For further information contact Jon M. Yatabe (510) 422-6115 orMichael T. Tobin (510) 423-1168.

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A Tour of the Proposed National Ignition FacilityWhen built, the National Ignition Facility (NIF) will

house the world’s most powerful neodymium glass lasersystem. NIF will be 50 times more powerful than theLaboratory’s Nova laser, currently the world’s mostpowerful. The NIF will contain 192 independent laserbeams, or “beamlets,” each with a square aperture of alittle less than 40 cm on a side. For economy andefficiency, beamlets will be stacked four high and twelvewide into four large arrays. The beamlines will requiremore than 9000 large-format optics (greater than 40 ¥ 40 cm) and several thousand smaller optics. Compared tothe size of the current Nova facility at LLNL, which usesa single-pass amplifier laser architecture, the compactmultipass design of the proposed NIF system allows us toput a laser with a typical output that is 40 times greaterthan Nova’s into a building only about twice the size.This article follows the path of a photon from the masteroscillator and preamplifier, through the NIF main lasercomponents, to the target. It also highlights some of the development efforts, begun many years ago, forcomponents, such as the multipass glass amplifiers andplasma electrode Pockels cell, that allow us to design alarge, multipass glass laser economically and at very lowrisk. Results from our recently completed BeamletDemonstration Project, involving a prototype NIFbeamline, along with the models and design codes we aretesting ensure that we can have great confidence in theperformance projected for NIF.Contact: John R. Murray (510) 422-6152.

NIF and National SecuritySince the end of the Cold War with the demise of the

Soviet Union, the U.S. nuclear weapons program haschanged dramatically. A major change has been themoratorium on underground nuclear testing, which islikely to be extended indefinitely by a ComprehensiveTest-Ban Treaty. Although there are now far fewerweapons and weapon types than only a few years ago, thenuclear stockpile nevertheless remains, and U.S. policywill continue to rely on nuclear deterrence for theforeseeable future. Because the U.S. must be confidentthat the nuclear arsenal would perform reliably if needed,reliance on testing to assess weapon performance must bereplaced by reliance on thorough scientific understandingand better predictive models of performance—that is,science-based stockpile stewardship.

ABSTRACTS

The National Ignition Facility (NIF) will enable us toproduce energy densities (energies per particle) thatoverlap with the energy densities produced in nuclearweapons, yet the total energy available on NIF will be aminuscule fraction of the total energy from a weapon.This combination of low total energy with weapons-regime energy density will allow us to pursue, besidesignition experiments, many nonignition experiments.These will allow us to improve our understanding ofmaterials and processes in extreme conditions byisolating various fundamental physics processes andphenomena for separate investigation. Such studies willinclude opacity to radiation, equations of state, andhydrodynamic instability. In addition to these, we willstudy processes in which two or more such phenomenacome into play, such as in radiation transport and inignition.

Weapons physics research on NIF offers aconsiderable benefit to stockpile stewardship, not only inenabling us to keep abreast of issues associated with anaging stockpile, but also in offering a major resource fortraining the next generation of scientists who will monitorthe stockpile.Contact: Stephen B. Libby (510) 422-9785.

The Role of NIF in Developing Inertial FusionEnergy

The proposed National Ignition Facility (NIF) willprovide LLNL researchers as well as others in thescientific community committed to developing InertialFusion Energy (IFE) with the means of developing andtesting data and materials that are key to the long-termgoal of building and operating IFE power plants as clean,viable, environmentally safe sources of inexhaustibleenergy. When the NIF demonstrates fusion ignition,which is central to proving the feasibility of IFE, it will tell us much about IFE target optimization andfabrication, provide important data on fusion-chamberphenomena and technologies, and demonstrate the safeand environmentally benign operation of an IFE powerplant. In accomplishing these tasks, the NIF will alsoprovide the basis for future decisions about IFEdevelopment programs and facilities, such as the plannedEngineering Test Facility (ETF). Furthermore, it willallow the U.S. to expand its expertise in inertial fusionand supporting industrial technology as well as promoteU.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.

Science on the NIFLast March, a group of scientists convened at the

University of California, Berkeley, to discuss thepotential scientific applications of the National IgnitionFacility (NIF)—a 192-beam, neodymium glass laser thatwill be used to obtain the high-energy physics dataneeded to maintain the nation’s nuclear stockpile. Theobjective of the gathering was to identify areas ofresearch in which the NIF could be used to advanceknowledge in the physical sciences and to define atentative program of high-energy laser experiments. Thescientists determined that the most effective scientificapplications of the NIF would be in astrophysics,hydrodynamics, high-pressure physics, and plasmaphysics. In astrophysics, the NIF would give scientists theability to synthesize and analyze the plasmas that occur atall stages of stellar evolution. In hydrodynamics, it wouldenable them to investigate flow problems underconditions that cannot be obtained by the conventionalwind tunnel or shock tube. In high-pressure physics, itwould allow scientists to investigate material behavior atpressures from 1 to 100 terapascals and temperatures upto a few hundred electron volts so that they could validatetheir theoretical models of material behavior. Scientistswould also be able to convert NIF laser energy to a widevariety of x-ray and particle sources needed to addressimportant questions in basic and applied physics. With the NIF, scientists could push the x-ray laserinterferometer to shorter x-ray laser wavelengths so that itwould be a more valuable diagnostic tool in the study andcharacterization of large-scale plasmas. In short, the NIFwould enable scientists to explore a previouslyinaccessible region of physical phenomena that couldvalidate their current theories and experimentalobservations and provide a foundation for newknowledge of the physical world.Contact: Richard W. Lee (510) 422-7209.

NIF Environmental, Safety, and HealthConsiderations

To ensure the safety of workers and the public and to assess potential environmental impacts, we havecompleted the first of a series of safety and environmentalanalyses related to the proposed National Ignition Facility(NIF). On the basis of its review of the PreliminaryHazards Analysis report, the DOE has concurred with thecategorization of the NIF as a radiological low-hazard,non-nuclear facility. Our studies to date show that the NIFwill present no significant environmental or health andsafety risk. For example, the average annual biologicalradiation dose to a NIF worker is estimated to be about0.01 rem. This value is less than 10% of the DOEguideline. As part of the National EnvironmentalProtection Act (NEPA) determination process establishedby the DOE, the public will be invited to participate inreviewing environmental, safety, and health issues relatedto the NIF.Contact: Jon M. Yatabe (510) 422-6115 and Michael T. Tobin (510) 423-1168.

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INDEX

1994 IndexJanuary–February 1994 PageThe State of the Laboratory 1Economic Competitiveness 4National Security 10

Nonproliferation, Arms Control, and International Security 12Defense Systems 18Nuclear Testing and Experimental Science 24

Lasers 30Environment 36

Environmental Research and Technology Development 38Environmental Restoration, Protection, and Waste Management 44

Energy 50Biology and Biotechnology 56Engineering 62Physics 68Chemistry and Materials Science 74Computers and Computing 80Education 86Administration and Institutional Services 92Awards 98

March 1994Forensic Science Center 1Melanoma at LLNL: An Update 9Center for Healthcare Technologies 21

April 1994World’s Fastest Solid-State Digitizer 1The MACHO Camera System: Searching for Dark Matter 7

May 1994Modified Retorting for Waste Treatment 1Cleaning Up Underground Contaminants 11

June 1994The Clementine Satellite 1Uncertainty and the Federal Role in Science and Technology 13

July 1994Pathfinder and the Development of Solar Rechargeable Aircraft 1ASTRID Rocket Flight Test 11

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August–September 1994Overview

Materials by Computer Design: An Introduction 1Research Highlights

Using Computers to Build Better Computers 4Modeling Large Molecular Systems 6Massively Parallel Computing CRADA 8Designing Better Microelectronics Devices 9Polymeric Nitrogen: A Potential Compound to Store Energy 10

Feature ArticlesNanotribology: Modeling Atoms When Surfaces Collide 13Toward Improved Understanding of Material Surfaces and Interfaces 25Predicting the Structural and Electronic Properties of Scintillators 33

October 1994Feature Articles

The Industrial Computing Initiative 1Artificial Hip Joints: Applying Weapons Expertise to

Medical Technology 16Research Highlights

KEN Project: Real-World Face Recognition 22Modeling Groundwater Flow and Chemical Migration 24Gas and Oil National Information Infrastructure 26

November 1994Feature Articles

LLNL’s 1994 R&D 100 Awards 7Legacy of the X-Ray Laser Program 13

Research HighlightsElevated CO2 Exposure and Tree Growth 22Meniscus Coating 24

December 1994The National Ignition Facility: An Overview 1A Tour of the Proposed National Ignition Facility 7NIF and National Security 23The Role of NIF in Developing Inertial Fusion Energy 33Science on the NIF 43NIF Environmental, Safety, and Health Considerations 55