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17Lawrence Livermore National Laboratory

ResearchHighlights

AStheworld’spopulationcontinuestogrowatabriskpace,moreandmorepeopleaspiretothelifestyleandenergy

consumptionlevelsofindustrializedcountries.Theworld’senergyneedsareincreasingrapidly,butsuppliesofthemostconvenientformsoffuelarefinite.Commercialfusionpowerhasthepotentialtofillthegapbetweensupplyanddemandbyprovidinganaffordable,virtuallylimitlesssourceofenergy.Butfirst,thetechnologymustpassanimportanttest:demonstratingthatself-sustainingfusionreactionscanbeachievedandreproducedinthelaboratory.

Researchteamsworldwidehavebeenworkingonthisscienceandengineeringgrandchallengefordecades.ExperimentsattheNationalIgnitionFacility(NIF),theworld’slargest,mostenergeticlaser,arethenextmajorsteptowardmakingfusionaviableenergysource.In2009,amultidisciplinaryteamofscientistsfromLawrenceLivermoreanditspartnersontheNationalIgnitionCampaign(NIC)—LosAlamosandSandianationallaboratories,theLaboratoryforLaserEnergeticsattheUniversityofRochester,GeneralAtomics,MassachusettsInstituteofTechnology,Commissariatàl’ÉnergieAtomiqueinFrance,andAtomicWeaponsEstablishmentintheUnitedKingdom—conductedaseriesofNIFexperimentstovalidateandoptimizethetargetdesignforupcomingignitionexperiments.Thesehohlraumenergeticsshots,togetherwithlasercommissioning(seeS&TR,April/May2010,pp.4–11)andfuelingtargetswithtritium,arethefinalpiecesneededtodemonstratethatthegiantlaserisreadyforthefirstcredibleignitionexperiments.

NIFexperimentsaredesignedtoachieveignitionthroughindirect-drive,inertial-confinementfusion.Ratherthandirectinglaserbeamstohitatargetfordirect-driveignition,NIF’s192beamsarefocusedthroughaholeinthetopandbottomofagas-filled,goldhohlraum—acentimeter-sizeenclosurethattrapsradiation.Inthecenterofthehohlraumisa2-millimeter-diameterpelletfilledwithnuclearfuelinbothgasandsolidforms.(SeeS&TR,July/August2007,pp.12–19.)Duringashot,laserbeamsstriketheinternalwallsofthecryogenicallycooledhohlraumandareconvertedtoxraysthatirradiateandheattheouterlayerofthefuelpellet.Asthislayerexpandsandablates,itrapidlycompressesthepellet’score,drivingittotemperaturesandpressuresgreaterthanthoseintheinterioroftheSun.Theseextremeconditionscausethefuel’snucleitofuseandreleasefarmoreenergythanwasnecessarytoinitiatethereaction.

TheNICteamdividedthehohlraumshotseries,aprecursortoignitionattempts,intothreesetsthatprogressivelyscaleup

Targets Designed for Ignition

A postshot photograph of a bent heat shield surrounding a spent target

illustrates the energy concentrated on the target during a hohlraum

energetics experiment at the National Ignition Facility (NIF).

thecomplexity,laserpower,andhohlraumsizeofexperiments.Inthefirstset,researchersactivated20majoroptical,x-ray,andneutrondiagnosticinstruments,whichtogetherprovidemorethan200streamsofdataforeveryexperiment.Thegoalforthefirstsequencewastoassessdiagnosticperformance,sothehohlraumsinthese24shotswereeitheremptyorfilledwithgasandkeptatroomtemperature.

Oncethediagnosticswerebroughton-line,theexperimenterscouldgatherdatainsubsequentshotstodeterminewhetherthehohlraumproducestheenvironmentneededforignition.Thesecondshotsequencefocusedoncryogenicsandthethirdonhohlraumenergetics.These45shotsusedhohlraumscooledto–253°C,similartotheanticipatedignitiontargetsbutwithdummycapsules.Inthecryogenicsseries,researchersassessedwhetherthehohlraumcouldbekeptattheoptimumtemperatureforfuelcapsuleperformanceandwhethermechanicalcomponentsassociatedwithtargetcoolingwouldcauseundesirablelaserlightreflections.Theenergeticssequenceexaminedthex-rayenvironmentwithinthehohlraumtoensurethateachshotwouldachievetherequiredtemperatureandsymmetry.

EvenSmallTargetsProvideGrandDataThefirstexperimentsintheenergeticsserieswereconducted

withscaled-downtargetsandlaserenergiesrangingfrom500to800kilojoules,lowerthanresearchersexpecttouseforignition.Theseriesculminatedina1-megajouleshotonafull-scale,1-centimeter-longhohlraum.Afteranalyzingresultsfromtheseshots,theNICteamnowpredictsthat1.2to1.3megajoulesofenergymustbedeliveredtothetargetforsuccessfulfusionexperiments.According

Heat shield

Target

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18 Lawrence Livermore National Laboratory

Resultsthroughouttheenergeticsshotserieswereevenbetterthanscientistshadpredicted.Dataconsistentlyagreedwithpreviouscomputermodelsandcalculations.BecauseofNIF’sreliabilityandflexibility,thecryogenicsexperimentswentsmoothly.InearlierexperimentswithlaserssuchasOMEGAandNova(theLaboratory’spredecessortoNIF),obtainingconsistentresultsandreliabledataonthistypeofexperimentwasachallenge.WithNIF,thecryogenichohlraumsheatedalmostaseffectivelyasemptytargets.

Anothersurprisefindingwasthatsimplermaterialsseemedtoworkaswellasorbetterthancombinationsofmaterials.Justasheliumprovedtobeabetterhohlraumgasthanthehelium–hydrogenmixture,ahohlraumlinedwithgoldperformedaswellasonewithamorecomplexgold–boronlining.BecauseeachNIFshotissuchanelaborateenterprise,physicistswelcomeevenminorreductionsincomplexity.

TuningBeamstoBalancePowerImplosionbynatureisanunstableprocess.Toimprovethe

chancesforsuccessatcreatingfusion,scientistsworkdiligentlytomaketheconditionsaspreciseandconsistentaspossible.Forexample,thefuelcapsulemustabsorbenergyinanevenandcontrolledfashiontocreateasymmetriccompression.WithNIF,laserbeamsenterthroughholesinthetopandbottomofthehohlraumandformtwo“cones.”Theouterconeshineslaser

Hohlraum Energetics S&TR June 2010

toLivermorephysicistSiegfriedGlenzer,conductingtheearlyportionofthecampaignwithlowerenergiesandsmallertargetswasprudent.Byscalingtheenergyandhohlraumsize,researcherscanobtainimportantinformationonhohlraumphysicsandmakerelevantmeasurementswhileminimizingrisk.

“Wewanttobegoodstewardsandnotdamagethefacilityifsomethingunexpectedhappened,”saysGlenzer,wholedtheenergeticsexperiments.“Ourteamworkedhardtomakethemostoftheseexperiments.Thequalityofeachshotwasoutstanding,andwetriedtolearnthemaximumamountoneveryone.Theresultsgaveusconfidencethatwe’remakingstridestowardgettingtherighthohlraumwiththerightperformanceforignition.”

LimitingEscapedLightOneuncertaintythattheNICteamsetouttoresolveiswhether

laser–plasmainteractionsinthehohlraumwouldsapenoughenergytothwartpelletfuelcompression.Theseinteractionscouldcauselaserlighttoscatterawayfromthehohlraum,aprocessthatisdifficulttomodelandthuspredictaccurately.

“Backscattercanreflectasubstantialamountoflaserlight,whichreducestheenergythatiscoupledtothetargetinthefirstplace,”saysGlenzer.“Itcanalsoaccelerateelectronsinaplasma,causingthecapsuletopreheatandmakingithardertocompress.”Agasofchargedparticles,plasmaisaninevitableresultoftheextremetemperaturesandpressureswithinthehohlraum.However,iftheplasmaredirectsvaluableenergyawayfromthefuelpellet,itinterfereswiththecreationoftheuniformx-rayfieldneededtoevenlycompressthetarget.

Livermore’sNathanMeezan,leadtargetdesignerfortheenergeticsshotseries,says,“Laser–plasmainteractionswereamajorconsiderationwhendesigningtheexperiments.”Andtheresultsyieldedsurprisinginformationaboutbackscatter.Inroom-temperatureshotsonhohlraumsfilledwithpentanegas—controlshots,ofasort—verylittlebackscatteroccurred.Thephysiciststhenswitchedtoamixtureof80percentheliumand20percenthydrogen,twogasesthatdonotfreezeatcryogenictemperatures.PreviousexperimentswiththeOMEGAlaserattheLaboratoryforLaserEnergeticsindicatedthatthismixturewoulddampenatypeofbackscatterknownasstimulatedBrillouinscattering.ButontheNIFshots,thehelium–hydrogencombinationproducedsignificantRamanbackscattering,anunexpectedeffect.

CalculationsmadebyLivermorephysicistDeniseHinkelshowedthatapureheliumgasfillwouldreduceRamanscattering.Totestthatprediction,theNICteamfilledafuelcapsulewith100percentheliumbutmadenootherchangestothetarget.TheRamanbackscatterdecreasedbymorethanafactoroftwo.StimulatedBrillouinscatteringturnedouttobeoflittleconcernforthistargetdesign.Infact,overall,theplasmadidnotcauseasmuchinterferenceasprojected.Onsubsequentshots,lessthan5percentofenergywaslostbecauseofbackscatter.

(above) In this cutaway drawing,

NIF’s 192 laser beams enter holes

on both sides of the hohlraum

and strike its inner walls, forming

an x-ray field that heats the fuel

capsule at the center. (right) Results

from a NIF experiment show the

laser beams intersecting at a

hohlraum entrance hole. This region

is a key area in tuning laser light to

achieve symmetric compression.

5 micrometers

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19Lawrence Livermore National Laboratory

Hohlraum EnergeticsS&TR June 2010

addsthatcombiningNIF’slong,plasma-producingpulseswithlargerhohlraumdesignsmadethiseffectpossible.

TheNICteamcontinuestorefinethehohlraumdesignandperformance.Toreachthepeakradiationtemperaturesof300electronvolts,ormorethan3millionkelvins,requiredforignition,researchersmuststillpairaslightlylargerhohlraumwithsomewhathigherenergylevels.Sofar,shotshavepeakedat285electronvolts.NICscientistsmustalsoprovethattheycanachievethenecessarysymmetrytocompressthefuelcapsulebyaboutdoubletheamountachievedtodate.Upcomingexperimentswilltestignition-sizedhohlraumsandtargetcapsulesfilledwithignitionfuel.Additionalneutrondiagnosticswillbeactivatedaswelltoexaminethephysicalprocessesoccurringinsidethecapsule.

ExcitementandexpectationscontinuetobuildthroughoutthefusionenergycommunityworldwideastheNICteamreleasesresultsfromtheenergeticsshotseries.ThemanagementteamattheLaboratory’sNIFandPhotonSciencePrincipalDirectoratehasseenamarkedincreaseinapplicationsfromotherresearchorganizationsproposingexperimentsattheLivermorefacility.AndtheshotserieshasalreadyinspiredthefirstsetofastrophysicalexperimentsonNIF,usingthehohlraumdesignedfortheenergeticssequence.Thankstoateameffortandspectacularresults,NICscientistsaregainingconfidencethattheycanmeettheNICchallenge—producingtheimplosionconditionsneededtodriveignition.

—Rose Hansen

Key Words:Brillouinscattering,cryogenics,energetics,fuelcapsule,hohlraum,inertialconfinementfusion,laser–plasmainteraction,NationalIgnitionCampaign(NIC),NationalIgnitionFacility(NIF),Ramanscattering,targetdesign.

For further information contact Trish Damkroger (925) 422-5816

(damkroger1@llnl.gov).

lightonthewallnearthecapsule’stwopoles,andtheinnerconeshineslightnearthecapsule’sequator.(SeeS&TR,July/August1999,pp.4–11.)

Earlyresultsfromtheenergeticsshotsindicatedthatthecapsulewascompressedintoapancakeshape,ratherthanauniformlyroundsphere.Thatis,lessenergyreachedtheequatorialregionsofthepelletthanthetopandbottom,likelybecauselightwasscatteringwithinthehohlraum.Theinnerconeneededmorepowertomakeupforscatteringlossesandavoidapancakeshape.Shiftingthepowerbalanceofthetwoconesrequiredchangingthecolor,orwavelength,ofbeamsformingtheoutercone.Glenzerexplains,“Bychangingthewavelengthsoflaserlight,wecancontrolwherethelightgoes.”

Thisnewtechnique,calledwavelengthtuning,hadbeencalculatedayearearlierbyLivermorephysicistPierreMichel.Itusesthegratingeffectthatoccurswhentheoverlappingbeamsformingtheinnerandouterconesenterthehohlraumandinteractwiththeplasmatheycreate.Tuningthesebeamswithrespecttooneanothercontrolsthepowerdistributioninthehohlraumbecausethegratingeffectredirectslight,justasaprismsplitsandredirectssunlightaccordingtoitswavelength.

Withthistechnique,saysMeezan,“Thelasercandeliveralotmorepowertopartofthehohlraumwithouthavingtoincreaseproduction.”Infact,itdeliveredatleast25percentmorepowertotheinnerconeandeffectivelyredistributedpowerafterbeamsleftthelaser.ThissortofprecisiontuningispossiblebecauseofNIF’sflexibilityandallowsshotdesignerstomakethemostefficientuseoftheavailablebeamenergy.

FusingItAllTogetherGlenzerandMeezanbothnotethatanexcitingaspectofthe

energeticsshotserieswasbringingtogethersomanyaspectsofignitionexperimentsforthefirsttime.TheshorterpulsesandlowerpowerlevelsontheOMEGAlasersimplycannotmatchthoseofNIF.WithNIF,researcherscanstudyallthevitalaspectsoflaserandtargetphysicsinthesameexperiment.“EventhewavelengthtuningisenabledbyNIF’sscale,”saysGlenzer.He

X-ray emission

images from a NIF

shot demonstrate

how wavelength

tuning transforms

compression,

changing the fuel

capsule from a

pancake shape to a

sphere that is much

more symmetric.50 micrometers

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