Generation and transport of high intensity laser-generated hot electrons in cone targets

1. This work was performed under the auspices of the U.S. DOE under contracts No.DE-FG02-05ER54834, DE-FC0204ER54789 and DE-AC52-07NA27344. Generation and transport of high intensity laser-generated hot electrons in cone targets Richard Stephens General Atomics, San Diego, CA USA XXXth European Conference on Laser Interaction with Matter Darmstadt, Germany 31 August - 5 September, 2008 IFT\P2008-072

2. Collaborators K. Akli M.S. Wei, J. King, S. Chen,T. Ma, T. Bartal, S. Chawla, F.N. Beg J. Pasley, J. Green, K. Lancaster, P. Norreys RAC R. Mason Y. Sentoku, B. Chrisman L. Van Woerkom, R. Freeman, A. Link, D. Offerman, V. Ovchinnikov, D. Schumacher A. Mackinnon, M. Key, A. Macphee, A. Kemp, H. Chen, P. Patel, D. Hey, S. LePape, M. Tabak, R. Town, S. Wilks C. Chen Y. Tsui W. Theobald

3. Goal to understand the physics of laser plasma interaction with cone tip • Electron generation • Electron energy spectrum • Coupling to forward direction How to optimize for Fast Ignition • Preliminary conclusions put requirements on laser and cone • Electron energy follows Beg scaling - coupling is good at high laser intensity • High contrast will be required for ignition pulse (<< 1 J prepulse) • Most of the laser light should be focused to the tip - useful electrons are only generated there • Electron directionality should be improved

4. Surrogates used to examine cone physics • 1D wire geometry for current diagnosis and analysis • Exptl: Easy view of electrons (Cu-K fluorescence) • Modeling: 2D effects can be neglected • Flat planes for cone reflectivity • Thin cones for electron generation • Buried cones for directionality

5. Multiple diagnostics needed for current measurement Cu Kα line Increasing λ Cu Kα signal No. of Pixels Pixel Value Kα image of Cu wire 1mm cone 40µm Ø wire HOPG spectrometer (accurate) 52° X-ray Imager 29.4° Single hit Spectrometer (absolute) 29.8° CPA laser Targets: Hollow Al cones joined to 1mm long Cu wires of diameters 10, 20 and 40µm Resolution = 20µm ≈ wire diam’s so, images vertically integrated into 1D emission profiles. 500J (RAL) 150J (Titan), 1ps @1053nm focused at f/3 into the cone

6. Electron coupling efficiency scales as the cross sectional wire area Kα emission profiles 10µm Ø, η = 1.0±0.5% 10-4 10-5 10-6 10-7 40µm Ø, η = 15±8% Kα (J/sr/µm) ~1mm ~1mm Cone tip region 20µm Ø, η = 5±3% 10-4 10-5 10-6 10-7 Peak=1.5E-5 J/sr/µm 10-4 10-5 10-6 10-7 Peak=7.2E-5 J/sr/µm Peak=4.5E-6 J/sr/µm Copper wire Cone tip region • Consistent -previous integrated experiments showed ~20% coupling • R. Kodama et al., Nature 418, 936 (2002), M.H. Key et al., PoP 15, 22701 (2008) • Electron penetration scalelengths (1/e ~ 100µm) indicate strong resistive inhibition! (collisional stopping alone gives scalelengths ~ 1 mm)

7. Comparison of emission profile with 1-D numerical model gives hot electron temperature Comparison with modeling (40µm diameter wire case) • Experimental scalelengths and Kαyields matched by varying Thot and η • in a numerical model that calculates observed fluorescence (blue). • Temperature-corrected absolute Kα/sr/µm profile generated•(green) • Analytical Bell’s† model profiles generated using numerical Thot and η values (red) The emission profiles were compared to 1-D numerical and analytical transport models Scalelength ~ 100µm *K. U. Akli et al., Phys. Plasmas 14, 023102 (2007) †A. Bell et al., PPCF 39, 653 (1997) *S. Wilks et al., PRL 69, 1383 (1992) • Resulting Thot (~670 KeV) values significantly lower than that predicted by ponderomotive scaling*: Te(MeV) ~0.511{(1+I18)1/2-1}

8. Indeed, recent experiments confirm low hot electron temperature *C. Chen et al., presented at 17th High Temperature Plasma Diagnostic Conf., Albuquerque, NM May 2008 • Single temperature fit shows hot electron temperature follows Beg’s • scaling*: T(MeV) = 0.215(I18)1/3 • More sophisticated modeling is underway to understand the underlying physics *F.N. Beg et al., Phys. Plasmas 4, 447 (1997)

9. Model predicts intense laser pulse moderates electron energy pulse • Simulations§ show high intensity beams compress the laser plasma interface • So electrons escape EM field before attaining ponderomotive energy† • I~1020W/cm2 => 1 MeV e §B. Chrisman et al., PoP 15, 056309 (2008) †M. Haines et al., submitted to PRL (2008) => Good coupling to compressed target

10. Cu K imager 75° Spectralon 25 m Cu K imager Used flat geometry to investigate cone wall LPI Titan laser • f/3 ~1020 W/cm2, 1 psec, ~150 J, focused to 10 m Flat Cu foil • Incident at 28° and 75° from surface normal - s-polarized • Target 0.5x0.5 mm2 x 25 m thick Light reflection and electron generation detection • Detect reflectance by light scattered off Spectralon™ surface • Detect electrons using Bragg mirror to image K reflectance • Count electrons with single hit ccd

11. 60 50 40 30 0 40 80 120 160 200 240 20 10 0 Reflectivity 50% at glancing incidence S pol - 10 mAl/25mCu/1mmAl P pol - 10 mAl/25mCu/1mmAl • focus light directly to cone tip S pol - 25mCu Specular Reflectivity, %  = 75° • R Apparently decreases with increasing power • No difference between S- & P- polarizations  = 28° Laser Peak Power, TW

12. 1011 Yield (Ph/J/sr) 1010 109 1017 1018 1019 1020 1021 Peak intensity (W/cm2) Electron generation is low at glancing incidence • generate electrons only at cone tip Slabs 28° incidence 10X Slabs 75° incidence

13. 1011 Yield (Ph/J/sr) 1010 109 1017 1018 1019 1020 1021 Peak intensity (W/cm2) But in a cone it is 3x higher Cones Are these extra electrons useful? 3X Slabs 28° incidence Slabs 75° incidence

14. 750 m Log intensity scale Electron paths affected by prepulse 5.5 mJ 100 mJ 1 J Linear scale 20080819s1 20080819s2 20070830shot6 • Prepulse generated plasma fills cone tip • Electrons move freely in that conducting volume • Surface electrostatic fields confine them to the cone Are these electrons useful?

15. not all cone electrons are useful Preplasma in cone reduces coupling Use buried cones with wires to look at electron coupling • Thick walls mimic blow-off plasma 12 m wall 25 m wall 250 m wall (mJ)

16. Use buried cones with layers to look at electron directions • Fluorescence shows wider spread of electrons than for flat surfaces This is ~50% wider than for flat geometry

17. Can electron spread be controlled? • Experiments and models show lateral cooling in wires • Electrons diverge only a few degrees leaving long wire • losing energy at surfaces by accelerating protons • ILE double wall cone? • Magnetic collimation? • Double pulse produces guiding field? Z.L. Chen et al., PRL 96, 084802 (2006) R. Kodama et al., Nature 432, 135 (2004) A. Robiinson et al., PoP 14, 083105 (2007))

18. laser plasma interaction with cone tip must be carefully controlled • TeI1/3 • Laser intensity > 1020 W/cm2 for acceptable electron energies • Glancing Reflectivity ~ 50%, electron production low • Laser has to be accurately focused to the cone tip • MJs of prepulse reduce coupling, increase spreading of electrons >>>Tentative, for 1 ps pulses - need further study with longer pulses<<< • Suggests a requirement that Eprepulse ≤ 10-7Elaser • Spreading is rather large even with minimal prepulse • Investigate alternative cone designs