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I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I II II I I II III IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII
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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
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
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