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Experimental Characterization of the Fast Ignition Electron Source

Experimental Characterization of the Fast Ignition Electron Source. Cliff Chen. March 12, 2010. US-Japan Workshop, San Diego, CA. LLNL- PRES-425394. Acknowledgements.

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Experimental Characterization of the Fast Ignition Electron Source

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  1. Experimental Characterization of the Fast Ignition Electron Source Cliff Chen March 12, 2010 US-Japan Workshop, San Diego, CA LLNL-PRES-425394

  2. Acknowledgements P. Amendt, K. Baker, C.D. Chen, B. Cohen, D. Clark, D. Hey, L. Divol, D. Ho, D. Homoelle, N. Izumi, A.J. Kemp, M.H. Key, D. Larson, B. Lasinkski, S. Le Pape, Y. Ping, A.J. Mackinnon, A. MacPhee, H. McLean, D. Meeker, M. Tabak, R. Town, H. Shay, D. Strozzi, S.C. Wilks T. Ma, D. Higginson, B. Westover, H. Sawada, T. Yabuuchi, M.S. Wei, T. Bartal, S. Chawla, C. Murphy, F. Beg K. Akli, R. Stephens W. Theobald A. Link, E. Kemp, A. Krygier, L. Van Woerkom, R.R. Freeman, R. Fedosejevs, Y.Y. Tsui, H. Friesen, H. Tiedje F. Perez, S. Baton *Funded through DOE Office of Fusion Energy Science: Advancing Research in HEDLP Program, and ACE Program

  3. In FI the core is heated to 10 keV using an intense particle beam generated by an ultrahigh power laser • Short-pulse laser must heat core to 10 KeV: Energy~ 20 kJ • Dz ~ 40 µm • t~ 20 ps Key Source Characteristics • Electron Spectrum • Range of hot electrons • Wavelength scaling, ponderomotive steepening • Coupling Efficiency • coupling into cones, prepulsedepenency • Divergence Angle • Transport efficiency to core S. Atzeni, POP 8, 3316 (1999)

  4. The short-pulse laser conditions have been carefully characterized in order to understand the physics Intensity distribution from low power focal spot consistent with full energy equivalent plane measurement 150 J 7 μm FWHM Low power Full energy Prepulse ASE ~ 13 mJ Spike ~ 8 mJ 700 fs FWHM Autocorrelator 16-bit CCD

  5. Bremsstrahlung and Kα measurements are used to infer the electron spectrum and coupling in planar foils Bremsstrahlung measurements in the 1-5 MeV range would significantly reduce the uncertainty • Raw Data fit with 2-T distributions, Thot up to 10 MeV, Rcold:hot = 0.1 to 1000 • Many distributions generate the same Bremsstrahlung profile in the <500 keV range • Total Conversion Efficiencies are bounded between 20-40% from 1019-1020 W/cm2

  6. Shots on conical targets underscore the importance of controlling the preformed plasma 100 mJprepulse vs 7.5 mJprepulse *Macphee, PRL 2009

  7. Cone-wires demonstrate the influence of the laser contrast on the forward coupling efficiency Recent 1ω high contrast experiments at Trident show coupling in line with 2ω LULI results *Courtesy of T. Ma

  8. 1 mm The divergence angle is measured in buried cone targets, which emulate fast ignition relevant plasma conditions FI cone in coronal plasma Spherical crystal imaging of Cu Kα emission 0826s4 1mm

  9. The cone geometry has a significant effect on the electron divergence angle FWHM fluorescence diameter RAL flat Titan 30 m dia cone tip Titan 90 m dia cone tip Titan 30 m dia - first try Monte-Carlo model Electrons spreadiing more rapidly with small cone tip

  10. PIC & transport modeling are crucial for interpreting experimental data and scaling to FI relevant conditions Hybrid transport modeling for shifting of the Bremsstrahlung spectrum LSP and ZUMA Simulations for Cone Wires Isotropic electron divergence X-ray emission falloff Spot HWHM 45°! *F. Perez, LULI + = Depth

  11. Summary • Experimental characterization of the electron spectrum, conversion efficiency, and divergence are all important but require significant modeling for interpretation • The spectrum and conversion efficiency are being studied with Bremsstrahlung and KA emission of planar targets and also with cone wires • Divergence is studied in a buried cone geometry and needs modeling for interpreting the experimental data. Qualitatively, divergence looks much bigger with 30 degree cones vs 90 degree cones

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