Terry Kammash - NASA's Institute for Advanced Concepts

Ultrafast Laser-DrivenPlasma for SpacePropulsionTerry Kammash, K. Flippo † , T. Lin,A. Maksimchuk, M. Rever,S. Banerjee, D. UmstadterFOCUS Center / Center for Ultrafast Optical Science, University of Michigan, Ann Arbor, MI 48109-2099, USAY. SentokuGeneral Atomics, San Diego, CAV. Yu. BychenkovP. N. Lebedev Physics Institute, Russian Academy of Science, 117924 Moscow, RussiaLasers supported by the National Science Foundation FOCUS Centerand the U of M Center for Ultrafast Optical Science, and funding from NASA Institute For Advanced Concepts

Accelerator SetupCR-39DetectorTarget NormalForward DirectionProtonBeamLaserForwardDirectionCUOS T 3 Laser Parameters:• Ti:Sapphire / Nd:Glass• 1.053 mm (ω o ),527nm (2 ω o )•up to ~12 TW•5 J•400 fs• Contrast: 10 -5 :1 @ ω o ,10 -7 :1 @2 ω o•2x10 18 -2x10 19 W/cm 2Incident Laser SpotFWHM = 4.3 um

Front Surface Deuteron Acceleration•Activation of 10 B to 11 C is achieved only by illuminating deuterons on thefront surface.•No activation when deuterons were on the back surface, or withoutdeuterons (i.e. no production of 11 C detected from 11 B (p,n) 11 C reaction).•Deuterons have about ½ the E maxof the measured protonsCounts/2 min1000100I las =6x10 18 W/cm 2Detection efficiency 15%Decay for 11 CLaserCDMylar filmDeuteronsBoronsample10B11Cn100 10 20 30 40 50 60 70Time after shot (min)10B(d,n) 11 C reactionK. Nemoto, S. Banerjee, K. Flippo , A. Maksimchuk, D. Umstadter App. Phys. Lett, 78, 595 (2001)

Radioisotope Activation withProtonscollimator &shieldLaserCountstarget1000800600400200protonsSingles Spectrum11B (p,n)11CNaI PMTSampleNaI PMT••• • •• •• •• ••• •• • ••• • ••• • •••• • • • •• ••• •• •• •• • •• • • • • •• • • •• • • •• •• • • • ••••• • •••• • • •• •• • •• •• • • ••• • ••• • ••••• •• • •••••••••0.511 MeVSortwindow•••••••• •• • • • •• ••• • • • • ••• • • • • • • •• • • •• • • •••••0 • ••0 50 100 150 200 250 300Channelto MCAto MCACount (0.511 MeV)Count (0.511 MeV)30001000100100010010••••• • • • • • •• • • • • • • •• •• • • • • • ••100 200 400 600 800 1000• • •• ••••Laser Induced1111B (p, n) CTime (sec)Laser Induced63Cu (p,n) 63Zn•• • • ••• • • • • •• • • • • •• •• •• • • • • • ••••• •• • • •••• • • • • • • ••• • ••• •• • • • • • ••••••• •••• • •• • •••t ~ 38 min•• ••• •••10 2000 4000 6000 8000Time (sec)t = 20 min

Material Effect on ProtonProductionConductorAluminum• ρ~2.7 g/cm 3• σ=3.6×10 7 Ω -1 m -1• Z=13InsulatorMylar (polyethylene terephthalate C 10 H 8 O 4 )• ρ~1.2 g/cm 3• σ=10 -12 Ω -1 m -1• Z=4.3e -p + e - p +p +Ee -Elaserp + p + p +BtargetlaserBtarget

Beam Profile Dependenceon Initial Target CompositionRCF (a,c,e,g) / CR-39 (b,d,f,h) detector stack images for 13µm Mylar, 10 µm silicon,12.5 µm aluminum, and 12.5 µm copper targets. All pairs are single shot except (c) and(d) which were two separate shots. RCF records protons between ~0.2 and ~2 MeV, CR-39records protons between ~2.5- ~4 MeV. Except (d) which recorded between ~1.2 MeV and 3 MeV

(a) 4 µm, (b) 12.5 µm, (c) 25 µm, (d) 50 µm, and (e) 75 µmBeam Profile Dependenceon Target Thickness(a) 6 µm, (b) 13 µm , (c) 25 µm, (d) 50 µm, and (e) 100 µmCall out: White arrowspoint to beamfilamentation, mostlikely a manifestationof the Weibel,instability.

Comparison withSimulationImages:Contrast enhanced RCF images of proton beam profiles after a drift of 5 cm from targetexit from experiments with 13 microns of Mylar (a) top left, and 12.5 microns ofaluminum (b) bottom left. Compare an electron beam profile from a simulation (c) by L.Gremillet, et al. [Phys. Plasmas 9, 941(2002)], showing the transverse electron profilejb/enc at 20 microns inside a silica target for a propagating monoenergetic electron beamof energy 500 keV, after 405 fs of propagation, which is also the beam duration. Imagereproduced with permission.Observed profilesSilicae-beam simulation

Magnetic Field from Simulation vs.Proton Beam ProfileAnd as shown byM. Honda, J. Meyer-ter-Vehnand A. Pukov, PRL 85 2128(2000) the ions can follow theelectron filaments in as littleas 60 fs.E field configurationplot from the simulationat 405 fs. Notice thesimilarities in thesimulation slices toproton beam images inrow (I) of the previousslide.e-beam induced B fieldevolution is very similar tothat of the proton beamprofile seen from Mylarpreviously.

Electron Distribution From AlTargetX-ray Film Line OutTargetHolderShadowX-ray Film0°TargetTop ViewHolderProtonslaserX-ray Film

Protons From Front Surface1614Maximum Proton Energy [MeV]1210864200 50 100 150 200E imax ~ 13 µmTarget Thickness [microns]

Simulation of proton beamSentoku’s[1] recent 1-D PICsimulations predict a 5 MeVbeam from the front surfacefor a 400fs laser pulse, withabout 13 MeV from the rear.This agrees well with theobserved 4 MeV trend, and amaximum of about 12 MeV.[1] Y. Sentoku Phys.Plasmas 10 2009 (2003)

Deuteron AccelerationDeuteron coatingd + p +Preliminary ResultsNo deuteron coatingp +Where do highest energy deuterons come from?1. The BACK of 12.5um Al2. The FRONT of 6 um Mylar3. The FRONT of 13 um Mylar4. The FRONT of 12.5 um Al5. The BACK of 13 um Mylar

Proton Energy Scaling with PulseDuration and Intensity14.5 MeVCurrentT-cubedSystem> 30 MeVFutureHERCULESSystemFrom Y. Sentoku, T. E. Cowan, A. Kemp, and H. RuhlPhysics of Plasmas 10, 2009 (2003)

Peak Proton Energyvs. Spot Size6000f/3.3 off-axis parabola5000f/1.5 off-axis parabolaPeak Proton Energy [keV]400030002000Power Scaling FitE =190.87 E = 190.87d x d 1.7404 1.704For intensities of ~2.5 x 10 19 W/cm 2For intensities of ~1.4 x 10 19 W/cm 2100003 3.5 4 4.5 5 5.5 6 6.5 7Spot size diameter [microns]

Spot Size Comparison120Total Intensity vs. Diameter for f/1.5 Paraboloid4.3 FWHM Spot Size100Total Energy [%]8060402040% in FWHMIntensity [a.u.]6000050000400003000020000100000Profile of 4.3µm FWHM Spot-15 -10 -5 0 5 10 15Radial Position [µm]00 5 10 15 20 25 30 35 40 45 50Spot Size Diameter [um]

Spot Size Comparison120Total Intensity vs. Diameter for f/3.3 ParabaloidFWHM Focal Spot of 6.4 Microns 8-17-01100Total Energy [%]8060402035% in FWHMIntensity [a.u.]6000050000400003000020000100000Profile of 6.4 µm FWHM Spot-20 -10 0 10 20Radial Position [µm]00 5 10 15 20 25 30 35 40 45 50Spot Size Diameter [um]

Material Effects on Plume ProfileProton Beam Images Using a CCDTargetPlaneDarkSideIlluminatedSideLaserPropagationDirectionProton Beam isEmitted Normal to Target25 um AlTarget25 umMylarTarget25 umMylarTargetwith 2.4Torr He4 Al Target,4MeV beamNo backfilledgas,200 mTorrambient4 um AlTargetwith 2Torr H 2

Plume Evolution in 1 Torr H 2Ambient Backfill12.5 mm Al25 mm Mylar+1 ms +4 ms +1 ms +4 ms +14 + ms 65.5us~32000 m/s~31000 m/s3.194 cm3.069 cm2.222cm2.138 cm1 cm

Target GeometryRadius ofcurvature~ 0.2 mm>1.4 MeV, 55º div.@ 1.5x10 19 W/cm 2>2 MeV, 38º div.@ 1.2x10 19 W/cm 2LaserCurved TargetGeometry25 µm AlTargetRadius ofcurvature~ 0.5 mmTargetHolderProtons> 1.4 MeV, 44º div.@ 1.6x10 19 W/cm 2> 3 MeV, 28º div.@ 1.2x10 19 W/cm 2

Target GeometryWire orientation:~100 Micron Half Wire Cross-sectionsFocus onflat surfaceProtonsCR-39Focus onround surfaceProtonsProtonsLaserLaserFlatWire positionRoundBeam Images:Focusing on flat surface(840) creates an ionbeam, while focusing onthe round side producesa cylindrical-like spraylaserlaser

Target Surface GeometryUse a material which will “trap” the laser light, toenhance the generation of hot electrons.Electron Microscopyof LaserBlackMurnane et al. APL 62 (1993) used gratings and clusters,Kulcsar et al. PRL 84 (2000) used metallic “velvet”.Both showed enhanced X-ray yield from enhance electronheating from efficient coupling.100 µm2 µmLaser Spot Size ~ 6 micronsLaserBlack ® is > 96% absorptive at 1 mm.Results:•30 mm Laserblack target ~8.2 MeV•Enhancement in the numberof maximum energy protons•Beam profile does not suffer,regardless of which surfacehas been coated, i.e. noimprinting even from rear-side>1.3MeV31º div.

Proton RadiographyThin Film TargetT-cube LaserProton BeamMeshRadiochromic FilmApproximate RegionSampled by Beam1 mmArea of Image at RightThe possibility exists to use thelaser produced proton beam forvery small scale imaging or evenlithography.The image on the left is a 5xmagnified proton radiographcaptured on RCF of a mesh with10 micron wires and 30 microngrid spacing.1 mm51.8 lines high

Future Laser DevelopmentOscillatorCleaner(10 6 contrast)RegenerativeAmplifier4-passAmplifier2-passAmplifierHigh-PowerAmplifierEnergy Pulse width RepetitionRate1 nJ 10-15 fs 80 Mhz1 mJ 15 fs 10 Hz100 mJ 350 ps 10 Hz1-1.5 J7-10 J350 psCurrent Hercules350 ps10 Hz0.1 Hz50 J 350ps 0.1 HzCompressedOutputN/AN/AN/A20-30 TW@ 25 fs100-200 TW@ 25-40 fs1 PW @ 30-40 fs

Proton Acceleration Summary• Simulation and experiment support proton acceleration at thelaser-irradiated side of the target of a 4 MeV beam, on the backof the underdense plasma under these conditions.• And a 12 MeV beam from the rear-surface of Al due torecirculation sheath enhancement.• Beam spectrum has bands of energies due to “ion fronts.”• Beam profile smoothes out as initial target conductivityincreases.• Filamentation and structures similar to the electron simulationby Gremillet et. al have been observed.• Demonstrated beam profile modification with modest geometry,and enhancement of number at the maximum energy achievedby initial target geometry and surface conditions• CR-39 response is highly non-linear when scanned optically.• By using a highly absorptive material we have increased thenumber of maximum energy protons without sacrificing beamquality. No imprint of LB on beam profile, unlike Roth et. al• New 30 fs laser has produced 10 21 W/cm 2 on target in a 1 micronspot, expect high efficiency acceleration

Ion Acceleration Physics RelativisticElectron Cloud (Beam) Model One-DimensionalPoisson’s Equation∇·E=-4 πen bWhere:e=electron chargen b =beam electron densitydRCan readily show:E z =2πen b hWhere:h=thickness of electron cloudR=radius of electron cloudd=diameter of electron cloudE z

Physics ContinuedEnergy conservation for electrons in cloudPE=KEPE≈πe 2 n b h 2KE=( γ b -1)m o c 2where γ b =Relativistic ParameterHence:h=√(γ b -1)m o c 2 /πe 2 n b = =√(γ b -1)/πr e n bWhere: r e =classical electron radiusr e =e 2 /m o c 2 =2.8×10 -13Substituting into exp. for E z we getE z =2c√πm o (γ b -1)n b

ExampleWe begin withγ b =10n b =10 19 cm -3h=10µmE z =913GV/mOver a distance of h=10 µm, the electronacquires an energy ofE b =9 MeV

ContinuedThe Ion EnergyE i =ZE b =ZeE z hE i =9MeV (Z=1)Mean Ion Velocity V i is given by½m i V i2 =ZeE z hAnd the ion acceleration time t i ist i =h/V iort i =√m i /Ze 2 n b

Two Asymptotic Regimes forIon Acceleration1. “Isothermal” expansion relevantto long pulse lengths i.e.τ>t i (t =1ps) iIons acquire exponential distributionin velocitydn i /dv ~ exp-( v/C S )Where C S =√ZT e /m i = ion soundspeed

Two Asymptotic Regimes forIon Acceleration2. “Adiabatic” regime corresponding toshorter, sub picosecond pulses i.e.τ

Ion VelocitiesMaxium Ion Velocities:Isothermal v max =2C S ln(d/h)Adiabatic v max =2√2C S ln(d/h)Note in both instances:Ion Acceleration is more efficientwhen(d/h)>>1i.e. for larger focal spots

Relationship Between Ion Energy,Laser and Target ParametersConsider power balance between laser andejected electrons:[n b (γ b -1)m o c 2 ]c=ηIWhereη=Efficiency of energy transferRewrites asε e =ηI/n b cAlso electron must exceed Coulomb Energy topenetrate the target i.e.n b = ε e /(πe 2 hR)

Relationship Between Ion Energy,Laser and Target ParametersCombining we get:ε e =√πe 2 IRh η/cSince h≈λ = laser wave length, thenε e =√πe 2 IRh η/cAndε i =Z ε eIf we express intensity I in units of 10 18W/cm 2 and R and λ in microns thenε i =Z ε e ≈ √ηIRλ MeV

An ExampleI=10 21 W/cm 2η=0.10R=2.5 µmThenε i =14 MeV

ThrustF=N i M i ωV iM i = ion mass (proton) = 1.6 ×10 -27 kgω = representation rate ≈ 1kHzV i = ion velocity (14 MeV) = 5.2×10 7 m/s

Plasma Expansion in VacuumIon acceleration timet i =h/v i = 19×10 -15 secPulse length (projected)τ=30×10 -15Thenτ>t i Expansion is Isothermalv i max = 2 C S ln(d/h)C S = √ZT e /m i =3×10 7 m/secv i max = 10 8 m/secV i initial ≈ 5×10 7 m/sec

Specific ImpulseNote improvement in energy transferefficiency for increasing (d/h), namelyfor larger aspect ratiosd/hln(d/h)V imax (m/s)Max I sp (s)51.619.7×10 79.7×10 7102.313.8×10 713.8×10 7503.9123.5×10 723.5×10 71004.6127.6×10 727.6×10 7

Accomplishments Thus Far1. Generate a Relativistically ConsistentMathematical Expression for the energyof the ejected ion as a function of laserand target parameters, i.e.E i =z √ηIRλwherez = ion chargeη = energy conversion efficiencyR = radius of focal spotλ = laser wave length

Accomplishments Thus Far2. Experimentally validatedE i ~ √IEi~ √λ3. Indirectly established relationshipsrelating E i to R and dependence on η.More work is needed in this area!Just purchased 5 parabolic mirrors toinvestigate thoroughly dependence ofE i and total number of ejectedparticles on R.

Accomplishments Thus Far4. Experimentally established dependence ofE i on target thickness “t”, optimized t≈10λ5. Experimentally established conditions forfilamentation instabilityP =5P c =5[17(ω o /ω p ) 2 GW]4Tc/ω p a 0 ≤2Rc = speed of lighta 0 =8.5×10 -10 λ [µm] I 1/2 [W/cm 2 ]ω p =plasma frequencyR= radius of focal spot

Accomplishments Thus Far4. Experimentally established energy of ionsejected from front and rear surfaces oftarget which appear to agree well withsimulations5. Established dependence of proton beamprofiles on materials, surface conditionsand geometry6. Carried out designs of space NuclearReactor for use in LAPPS. Likely candidatesare gas-cooled Cermet reactors usingUranium, Plutonium or Americium as fuel.