High-Intensity Laser–Plasma Interactions - PDF document

High-Intensity Laser–Plasma Interactions in the Reuxing Limit P. M. Nilson University of Rochester Fusion Science Center and Laboratory for Laser Energetics 49th Annual Meeting of the American Physical Society Division of Plasma Physics Orlando, FL 12–16 November 2007 10%Normalized K Intensity MTW 1 J, 1 psVULCAN PW 500 J, 1 ps FSC The laser-to-electron energy-conversion efficiency is h L " e = 20% for laser intensities I � 10 18 W / cm 2 E16140 • Comparison a yields from small-mass targets with a productioninfers h L " e = 20% ± 10%for1-ps I � 10 18 W / cm 2 • Targetaffects L " K and M " K electron transitions • K b / a signals are consistent with numerical target-heating calculations for h L " e = 20% ± 10% over a wide range of target volumes Summary FSC Solid-density material heated to � 200J,1-ps Collaborators W. Theobald, J. Myatt, C. Stoeckl, M. Storm, O. V. Gotchev, C. Mileham, R. Betti* † , D. D. Meyerhofer* † , and T. C. Sangster University of Rochester Laboratory for Laser Energetics *also at Fusion Science Center for Extreme States of Matter and Fast-Ignition Physics University of Rochester † MechanicalEngineeringPhysicsDepartment University of Rochester FSC Outline E16141 • High-temperaturematter • Small-massfoiltargets • Electronre�uxing • K a experiments • Bulktarget-heatingexperiments • Futureexperiments FSC Fast-electron reuxing in small-mass targets allows tohigh-energy-density E16142 • Re�uxingby Debye sheath eld effects 1,2 • Majorityelectrons arestoppedtarget • Providesgeometry fortestinglaser-coupling, electron-generation, and target-heating models 3,4 FSC 10Fastest electrons 2 to 20 20 to500 1 S. P. Hatchett et al. , Phys. Plasmas 7 , 2076 ( 2000 ) . 2 R. A. Snavely et al. , Phys. Rev. Lett. 85 , 2945 ( 2000 ) . 3 W. Theobald et al. , Phys. Plasmas 13 , 043102 ( 2006 ) . 4 J. Myatt et al. , Phys. Plasmas 14 , 056301 ( 2007 ) . The laser-to-electron energy-conversion efficiency h L " e is inferred from the absolute K a yield E16143 • Energeticelectronscreate vacancies ( E k c 9 keV ) • K-shellfrom cold bulk material during the fast- electron lifetime FSC Wharton et al ., Phys. Rev. Lett. 81 , 822 ( 1998 ) . Stephens et al. , Phys. Rev. E 69 , 066414 ( 2004 ) . J. D. Hares et al ., Phys. Rev. Lett. 42 , 1216 ( 1979 ) . W. Theobald et al ., Phys. Plasmas 13 , 043102 ( 2006 ) . energy levels8.05 keV8.91 keV Reuxing in small-mass targets allows a number of K-photonproduction TC7474b 1 H. O. Wyckoff, ICRU Report 37 , Intern. Comm. on Radiation Units and Measurements, Inc., Bethesda, MD ( 1984 ) . 2 Kolbenstvedt,J.Phys. 38 , 4785 ( 1967 ) . • K-photongeneration calculated as in an innite medium • Relativistic cross sections 2 included • Classicalslowingdown approximation ( CSDA ) 1 • Fluorescenceprobabilityfor cold matter is corrected for �nitetemperature Electron energy (MeV) Ionization cross section (barns)Range (nm)Solid density copper400300200010–310–210–110010110310210–110–2100 1 MeV~1 mm Relativisticcorrections FSC energy levels8.05 keV Ionization Depleted8.91 keV Target bulk-heating affects L " K and M " K electron transitions* E16143a • Inelasticelectron–electron collisions heat the target • Collisional thermalbackground occurs T e � 100 eV causes significant • Targetinferred from b / a FSC *J. Myatt et al ., Phys. Plasmas 14 , 056301 ( 2007 ) . *G. Gregori et al ., Contrib. Plasma Phys. 45 , 284 ( 2005 ) . b / a ratio is sensitive to the bulk-electrontemperature E16147 1.000.00200400Temperature Decreasing target volumeNormalized K Cold material value • In b / a c 0.14 • For T e = 400 eV, the completelydepleted • K b / a variation withtemperature can be studied experimentally various mass targets ( for xed laser conditions ) FSC experimentswereperformed Multi-Terawatt ( MTW ) Laser Facility at LLE E16144 • Laserintensities I 2 × 10 19 W / cm 2 • Coppertargets • Targetvolumes V � 20 × 20 × 2 n m 3 stackSingle-photon-OTRimagerparabolaMultiple-channelelectron spectrometerX-ray-pinholecamera 20 nm OAP Spider-silkmounted 20 × 20 × 2 n m 3 target FSC single-photon-countingx-raymeasures absolute a b yields TC7471a Cu-foiltargetX-ray radiationLaser collimator 100200arbitrary units300400Photon energy keVX-ray spectrum 30 nm0406022 7.5 150-Cu foil filter K-edge Tho *Stoeckl et al ., Rev. Sci. Instrum. 75 , 3702 ( 2004 ) . FSC a productionrequiresfast-electron spectrumintensitytospeci�ed E16316 • Anexponentialfast-electronspectrum • Thefast-electron-temperature intensitygivenby ponderomotive T e ( MeV ) = 0.05 I 18 1 / 3 …for I 10 18 W / cm 2 …for I � 10 18 W / cm 2 P. Gibbon and E. Förster, Plasma Phys. Control. Fusion 38 , 769 ( 1996 ) . S. C. Wilks et al ., Phys. Rev. Lett. 69 , 1383 ( 1982 ) . T e ( MeV ) = 0.511 [( 1 + I 18 m n m / 1.37 ) 1 / 2 – 1 ] 2 FSC The laser-to-electron energy-conversion efficiency h L " e inferred a production E16145 FSC 10%Normalized K Intensity P. Gibbon and E. Förster, Plasma Phys. Control. Fusion 38 , 769 ( 1996 ) . S. C. Wilks et al ., Phys. Rev. Lett. 69 , 1383 ( 1982 ) . a yields are consistent with the reuxing electron model assuming h L " e = 20% and I � 10 18 W / cm 2 E16150 • K a production insensitive to fast- electron energy spectrumrange for I � 10 18 W / cm 2 • Con�rmsprevious observations from Vulcan PW* FSC * W. Theobald et al. , Phys. Plasmas 13 , 043102 ( 2006 ) . 10%30%Normalized K signal Intensity MTW 1 J, 1 psVULCAN PW 500 J, 1 ps Cu targets: 500 × 500 × 20 n m 3 b / a variations with target heating were observed experimentsVulcanLaserFacility* E16317 • Largescatter • Scaletargetsto ( J, ) accordingly Volume Noise level0.150.100.00 100 TW *W. Theobald et al ., Phys. Plasmas 13 , 043102 ( 2006 ) . effectbulktarget spectrumLaserFacilityobserved E16040 10987 Energy (keVEnergy keV 100150100Number of pixels KaKbKaKb FSC Cu target: ( 500 × 500 × 50 ) n m 3 Cu target: ( 20 × 20 × 3 ) n m 3 Laser: 5J, Intensity: × 10 19 W / cm 2 Threeregimesareobserved spectratoincreaseenergy E16041 • Coppertarget:J, • Intensity: × 10 19 W / cm 2 • Region1: cold material limit • Region2: a yield constant, b yield falls • Region3: a b yields decrease Energy in K, Klaser energyRegion 3Region 2Region 1 Target volume KaKb Increasing energy density FSC A 3.5 × reduction of K b / K a for target volumes V = 10 – 6 mm 3 is consistent with bulk-electron temperatures T e c 200 eV E16148 FSC 1.01.2Normalized KRegion 3Region 2Region 1 Target volume Cold material limitTemperature Increasing energy density 200400100 temporalvariations mustconsidered b / a E16038 FSC * D. Welch et al. , Nucl. Inst. Methods Res. A 464 , 134 ( 2001 ) . ** Prism53711 1.0Probability multiplier 50010001500Temperature pKapKb PrismSPECT** calculatesion-population distribution • 3-D LSP * calculates target heating • Fast-electronsource prescribedvarying energy • Sametargetvolumes interaction timescales are modelled ( no scaling ) • AssumesThomas–Fermi model • Calculates�elds consistently • Emissionprobability calculated using the local temperature of emission A comparison of K b / K a to LSP calculations give h L " e c 20% consistent with those from tting the absolute K a yield E16314 • Providesself-consistency check on h L " e • Con�rms dominant physics in the simple reuxing K a production model are correctly accounted for set for comparison to future experiments at higher energy densities FSC 0.00.2 Temperature 1.01.2Normalized KRegion 3Region 2Region 1Target volume 200400100 10% material Increasing energy density OMEGAallowtotemperatures T e � 1 keV using reuxing in small-mass solid targets E16315 • OMEGA kJ,10 • Relevantto backlighter • Relevantto ignition • Study h L " e to 10-psregime • Benchmark FSC D. D. Meyerhofer ( TO6.00001 ) E16140 FSC Summary / Conclusions The laser-to-electron energy-conversion efficiency is h L " e = 20% for laser intensities I � 10 18 W / cm 2 • Comparison a yields from small-mass targets with a productioninfers h L " e = 20% ± 10%for1-ps I � 10 18 W / cm 2 • Targetaffects L " K and M " K electron transitions • K b / a signals are consistent with numerical target-heating calculations for h L " e = 20% ± 10% over a wide range of target volumes Solid-density material heated to � 200J,1-ps