UCRL-JC-138315. The Potential of Fast. Ignition and Related. Experiments with a. Petawatt Laser Facility. U.S. De artment of Energy. ~. -I Lawrence. ] Livermore.
Preprint UCRL-JC-138315
The Potential of Fast Ignition and Related Experiments with a Petawatt Laser Facility
M.H. Key, E.M. Campbell, T.E. Cowan,S.P. Hatchett, E.A. Henry, J.A. Koch, A.B. Langdon,B.F. Lasinski, A. MacKinnon, A.A. Offenberger,D.M. Pennington,M.D. Perry, T.J. Phillips, T.C. Sangster, M.S. Singh, R.A.Snavely, M.A. Stoyer, M. Tsukamoto,K.B. Wharton, S.C. Wilks U.S. De~artment of Energy
-I Lawrence
~ |
] Livermore This article wassubmittedto National on Cost-Effective Steps to Fusion Power, Laboratory 1999 Symposium Washington,D.C., March 25, 2000
//
April 6, 2000
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THE POTENTIAL OF FAST IGNITION AND RELATED EXPERIMENTS WITH A PETAWATT LASER FACILITY M. H. Key, E. M. Campbell,T. E. Cowan,S. P. Hatchett, E. A. Henry J. A. Koch, A. B. Langdon,B. F. Lasinski, A MacKinnon A. A. Offenberger*,D. M. Pennington,M. D. Perry, T. J. Phillips T. C. Sangster, MS. Singh, R A. Snavely, M. A. Stoyer, M. Tsukamoto**,K B. Wharton, S. C. Wilks. LawrenceLivermore National Laboratory, P.O. Box 808, L-473 Livermore CA 94550 USA *VisitingfromDepartment of Electrical Engineering,Universityof Alberta,Edmonton, Alberta,T6G2G7, Canada **VisitingfromJoining&Welding ResearchInstitute, OsakaUniversity,lbaraki, Osaka567, Japan ABSTRACT Amodelof energy gain inducedby fast ignition of thermonuclearburn in compressed deuterium-tritiumfuel, is usedto showthe potential for 300xgain with a driver energyof 1 MJ, if the NationalIgnition Facility (NIF)wereto be adaptedfor fast ignition. Thephysics of fast ignition has beenstudied using a petawatt laser facility at the LawrenceLivermore National Laboratory. Laser plasmainteraction in a preformedplasmaon a solid target leads to relativistic self-focusing evidencedby x-ray images.Absorptionof the laser radiation transfers energyto an intense source of relativistic electrons. Goodconversionefficiency into a wide angular distribution is reported. Heatingby the electrons in solid density CD2produces0.5 to lkeV temperature, inferred from the D-Dthermo-nuclearneutron yield. 1.
INTRODUCTION Theconceptof Fast Ignition (FI) [1] is of importancein inertial fusion energy(IFE) research
becauseit offers the possibility of significantly highergain than can be obtainedin indirectly driven [2] or directly driven [3] inertially confinedfusion (ICF). Highergain wouldallow powergeneration with lowerdriver efficiency or lowerrecirculating powerfraction and FI is, therefore, attractive for conceptuallaser driven IFE powerplants. Figure [i] comparesthe calculated gain for ICFtargets of a
size appropriate to the 1.8 MJNational Ignition Facility (NIF) nowbeing constructed at the Lawrence LivermoreNational Laboratory. TheNIFignition target designs for indirect drive have been developedover decadesof research and there is a soundbasis for the predicted gain factor of 12 to 20. Theresearch base for direct drive with predictedgains at NIFscale of 20 to 70 is less mature,but is, nevertheless, substantial. Thegain of about 300 indicated for FI at NIFscale is basedon a simple modeldiscussedhere in Section 2. Thepotential of FI is evident but its feasibility dependson the newerphysicsof the interaction of intense laser radiation with plasmain the relativistic regimeand remainsto be proven. 2 FAST IGNITION MODELAT THE SCALE OF NIF Theessential idea of fast ignition is to pre-compressthe fusion fuel by conventionallaser driven methods,then to ignite the fuel with a separate short duration high intensity laser pulse. A pre-cursor hole boring pulse creates a channelin the plasmaatmosphereenablingthe ignitor pulse to penetrate close to the densefuel. Absorptionof the ignitor pulse at the critical density interface generates a beamof relativistic electrons, whichtransports energyto the densefuel and heats the ignition spark. Theenergyrequired to be delivered to the ignition spark by the electron beamhas been determinedfrom numericalsimulations[4] and can be written in the form 80 (100/p) 1.8 kJ wherep the fuel density in gcm-3. The sameanalysis showsthe requirements for temperature kT=10keV and density radius product pr = 0.5 g cm-2. Reducingthe density, therefore, proportionally increases both the spark radius and its inertial confinement time, the latter being proportionalto the radius. Assuming that the laser focal spot area is proportionalto the ignition spark area, the intensity in the focal spot is proportional to 9. Theelectrons are required to havean energydeposition range equal to the spark Or. A limit of about 1 MeV is, therefore, imposedon their energy.Thelaser intensity x (wavelength)2 ori~2 determinesthe ponderomotive potential and thus the closely related energy the electrons [5]. I~2 2. is required to be approximately2.5x1019Wcm-2gm
2
Figure 1 showshowthese constraints can be met at the scale of NIFfor a fuel density of -3 with 10%of the NIFbeamsadapted for chirped pulse amplification (CPA)[6]. Conversion 200gcm at 60%efficiency to 0.53 lam wavelengthis assumedin order to help meetthe IL2 constraint, giving a total short pulse energyof 200 kJ. Thebeamswouldbe focusedto a clustered array of 25 Jam -3 density; giving the IL2 value shownin figure 1. diameterspots at 20 ps pulse duration for 200 gcm It is further assumedthat hole boring brings the ignitor beamsto within about 100 gmof the dense fuel wherethere is 30%conversionto electrons. Energytransfer by the electron beamto the ignition spark with self inducedmagneticcollimation [6] is assumedto be 66%efficient with a two fold increase in electron beamdiameter betweenthe optical focal spot and the ignition spark. Compression of the fuel is assumedto be with 8%hydrodynamic efficiency typical of direct drive, and with a ratio c~=2of internal energy to the Fermidegenerate minimum. Thegain is computedby specifying the fuel density and varying the fuel mass. Summing the required primarylaser energy delivered to ignitor beamsand the laser energyneededfor fuel compressiongives the total laser energyinput. The fusion burn output is calculated fromthe burn efficiency pr/(pr+Tgcm-;)and the fuel mass. For example,if 1 MJof laser energyis used to compressthe fuel, the fuel massis 4.1 mgwith a diameter -2 andthe burnefficiency is 0.33. Thegain i.e. ratio of burnyield to laser of 340btm. the 9r is 3.4 gcm energyis, therefore 330. It is necessaryfor obtaininghigh gain that the total laser driver energybe well abovethe ignition thresholdand this sets a lowerlimit on the densityas illustrated in figure 1 becausethe thresholdenergyincreases as p 4.85. Theupperdensity limit arises fromI~. 2 in the focal spots, which increases approximately linearly with density as indicated in figure 1. In this scenario there is an operating windowcentered at about 200 -3. gcm The higher gain obtained in fast ignition arises becauseweassumea high compression efficiency 11=8%, characteristic of direct drive, a low adiabat ratio c~=2and a low density
-3, wherethe energy used to compressthe fuel dominatesthe total input energy and scales p=200gcm as r/czp 2/3. A lowfuel adiabatis easier to achievein fast ignition becausethe fuel densityandpressure are reducedand there is no needfor a central hot spot with its associatedsensitivity to hydroinstabilities. Lowervalueof 9 relative to direct drive is the primarysourceof the higher gain for FI shownin figure 1. This simple modelhas manylimitations. Therequirementin fast ignition research is to substantiate that the postulatedbehaviorscan be obtained, in particular the efficient transfer of energyfromthe ignitor laser beamto the ignition spark. It is apparentthat if they can be obtained,then fast ignition has a uniquecapability for high gain of considerablepotential importancefor IFE. 3.
THE PETAWATT LASER at
LLNL
Onebeamline of the 10 beamNovalaser using disc amplifiers of up to 31.5 cmdiameter, wasadaptedfor chirped pulse amplification (CPA)[8] to generate up to 0.8 kilojoule in a stretched pulse. Petawatt powerwasobtained by recompressingthe stretched pulse to a minimum duration of less than 500 ps, using a pair of 1 meter diameterreflection gratings. A computercontrolled deformablemirror (DM)improvedthe wavefrontas illustrated in figure 2. A typical imagewithout the DMafter a 7 hour cooling delay, showssignificant thermal distortions. Theimprovedfocal spot shownwith use of the DMwasreproducible for firing intervals as short as 1 1/2 hours. Analysis of the ccd imagesgavea determinationof the spectrumof intensity. Typically, peakintensities reached -2 for f/3 focussing whenthe laser wasoperated at a powerof 1 PW.20%of values up to 3 xl02° Wcm -2. Thefraction of the powercontainedup to the energywasdelivered at intensities above1020Wcm the first minimum around the central focal spot was 30%.The Strehl ratio was5%and the fwhmof the central focal spot was2.5 timesthe diffraction limit. Therewaslaser energyincident on the target prior to the arrival of the short-pulse, due to amplified spontaneousemission(ASE)and small fractional leakage of the short-pulse through Pockelscell gates in the pulse generator system. ASEbeganapproximately4 ns prior to the short 4
pulse at a roughlyconstant intensity and its energywastypically 3 xl 0-4 of the mainpulse energy. A leakage pulse an’Ned2 ns before the mainpulse and its minimizedenergy was 10-4 of the main pulse. 4. Relativistic channeling TheASEand leakage pre-pulse created a preformedplasmafrom solid targets prior to the arrival of the mainpulse. Interaction of the mainpulse with this plasmaparticularly by relativistic self-focusing wasimportantin the conversionof laser energyto electron energy. Short-pulseoptical interferometry was used to measurethe density structure of the pre-pulse inducedplasmaand these results have been comparedwith numericalsimulations in 2 dimensionalcylindrical symmetryusing the hydrodynamiccode Lasnex[9]. There was good consistency of measuredand computedresults with the approximatelyexponentiallyfalling density profile havinga scale length of 40 micron.
Evidenceof relativistic channelformation[10] is shownin figure 3. Thelaser wasfocusedat variable distances in front of andbehindthe surface of a planar Autarget at normalincidenceat energyof 550 J in 0.5 ps. Theirradiated area on the target (assumingthe propagationof the laser to be unaffected by the pre-formedplasma)is shownby the superimposedcircles. For focusing 300 gm in front of the target, the x-ray pinholecameraimageof the front surface of the target (filtered to record x-rays of about 5 keVenergy), showsan emissionzone, whichis approximately20 btm in diameter and muchsmaller than the nominal 100 microndiameter irradiated area. The imagehas structure, suggestingone strong filament and 2 subsidiary filaments. Displacementof the target to the plane of best focus showsan essentially invariant x-ray image. Whenthe laser was focused 200 btm behind the surface, the x-ray imagewasdiffused and of weakintensity extending over morethan 300 btmdiameter. It had one10 gmdiameterbright spot attributable to a small local region of the undergoingrelativistic self-focusing. Theyield of photo-neutrons[ 11 ] wasessentially invariant at 2 to 3 xl 0s for focusingin front of the target while focusingbehindthe target surface gavea dropin 5
yield to 107. Theneutron data support the scenario suggested by the x-ray imagesby implying invarianthigherintensity for focussingin front of the target attributable to relativistic filaments. 5.
CONVERSION EFFICIENCY
AND ANGULAR DISTRIBUTION
OF THE
ELECTRONS Conversionof up to 30%of the incident laser energyinto electrons of meanenergyexceeding 600 keVhas been inferred from measurementsthe K~fluorescence of a buried layer of Moin Cu "2 in 0.5 ps pulses targets [12]. Thesemeasurementsmadeat peak focused intensity of 2x1019Wcm were obtained with 15 J laser pulses using a smaller scale prototype of the PWbeam.Increasing conversionat higher intensities wasnoted in scaling studies [ 13]. The total yield of bremsstrahlungx-rays from lmmthick Autargets at photon energy >0.5 MeVwas determined for 1PWlaser pulses using an array of 150 filtered thermoluminescent(TLD) detectors covering the forwardhemisphere.11 J of X-rays equivalent to 2.9 %of the laser energy was recorded. This was estimated to correspondto 40 to 50%conversionof laser energy to electrons by using MonteCarlo modelingwith the Integrated Tiger Series code (ITS) [14]. X-raybremsstrahlungfromrelativistic electrons has a cone angle of 1/7 where7 is the relativistic factor. Consequently,measurements of the angular distribution of the x-ray emissionat photon energies in the MeVregion are almost equivalent to direct measurement of the angular distribution of the electron flux in the target. Thearray of thelrno-luminescent(TLD)detectors produceda hemispheremapof x-ray emissionwith a broad angular range with a full width at half maximum (fwhm)of 180° and a variable direction of the peak Higherenergyx-ray photonsinteracting with high z target materials induce photo-neutron emission(g xn) and nuclear activation through nuclear giant resonances [11,15]. The x=l process in Auwas used to mapthe angular pattern of higher energy photons of about 15 MeVwherethe cross section peaks. 31 Audiscs covering the forward hemispherewere used as activation detectors. There
wasa characteristic 100° fwhmof the angularpattern illustrated in figure 4, but the direction of the peak was randomover an 80° range. 6. MechanismsInfluencing the Electron Source Possible causes for the stochastic directional behaviorand the wide beamdivergenceof the x-ray emissionandthe electron sourceinclude a hosinginstability of the relativistic filament at near critical density, whichhas beenseen in particle in cell (PIC) modeling[16] and Weibelinstability breakingup the relativistic electron current into multiple off axis filamentswhichhas also beenseen in PIC modeling[17]. Refraction and multiple filamentation in the structure of preformedplasmamay also contribute. 7. Heating by the Electrons Thewideangle pattemof the electron source is disadvantageousfor fast ignition, but maybe mitigated by self inducedmagneticcollimation of the electron beamin solid density matter suggested by recent theoretical work[6,18] and optical probing experiments[20]. Specific calculations [18] for intensity of 2 x 10 ~9 Wcm-2 in a 15 pmfocal spot~ showcollimated heat flow with temperature reaching 3 keVon axis and 1 keVout to a radius of 15 gmpenetrating through 0.1 mmof solid CD2. This kind of heat transport is highly desirable for fast ignition. Experiments within buried layers of A1in CHhavegiven interesting indications of heating to about 300eVin a collimated pattern [20]. Evidenceof temperatureof 500 ev to 1 keVdue to heating by relativistic electrons was obtained from observation of thermonuclearD-Dreactions in solid CD2. Thermonuclearfusion reactions in CD2generate neutrons from DDfusion with an energy of 2.45 MeVand an energy spread in keVof 82(kT/1 keV)1/2. Suchnearly mono-energeticthermal neutron emissionis very different from the photo-neutronemissiondiscussed earlier or neutron generation by accelerateddeuteronsstriking static deuterons[21 ], or (p, n) processfromfast protonsstriking the target chamberwalls, all of whichproduce neutron energy spectra that are manyMeVwide. Figure 5 showsa neutron spectrumrecorded by a large aperture neutron scintillator array (Lansa) comprised
960photo-multiplier/scintillator detectors located 21.5 m fromthe target at an angle that is backward at 700 from the laser axis. Thetarget had 10 gmCHover 100 gmCD2and wasirradiated at 5 ps with 180J. The striking feature of the spectrumis a narrowpeak at 2.45 MeVsuperimposedon a broad background.Statistical analysis showsthat this peakis at 4 standarddeviations relative to the statistical noise in the spectrally broadbackground signal and can therefore be attributed to thermonuclearreactions. The peak implies emission of 6 x 10 4 thermo-nuclearneutrons. This experimentwasat the statistical limit for detection of the narrowthermalpeakin the spectrally broad backgroundof neutrons producedother processes and in multiple shots and it was seen only occasionally. Such randombehavior is consistent with the numberof thermonuclearneutrons being close to the statistical noise level of the broadspectrumandwith a strong (T5) scaling with temperature.The yield of neutronssuggests heating to a temperatureof 0.5 to 1 keV.
CONCLUSIONS Fast ignition has beenshownto havea significant potential advantageof higher gain, which at the NIFrelevant level of 1MJdrive reaches 300x. Experimentswith the PWlaser have shown evidenceof relativistic self-focusing with efficient conversionto a rather wide-anglebeamof relativistic electrons. Observationof D-Dthermonuclearfusion suggests heating of solid CD2to temperatures of 500 eVto 1 keV.
*Work perfomledunderthe auspicesof the U.S. Department of Energyby the LawrenceLivemaore National Laboratoryunder Contract No. W-7405-ENG-48.
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9
,I .... I,,,,I .... i .... I,,,,i .... I .... I .... I
Fig. 5. Neutron energy spectrum showing counts per 0.06 MeVbin as a function of neutron energy in MeV. The narrow peak at 2.45 MeVis 4(7 above the statistical noise and is attributed to thermonuclear fusion of deuterium.
