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Designing and Fabricating a Proton Beam Source Suitable for Fast Ignition Targets

P.Patel. M. Roth et al. Designing and Fabricating a Proton Beam Source Suitable for Fast Ignition Targets. PERSISTENT SURVEILLANCE FOR PIPELINE PROTECTION AND THREAT INTERDICTION. Richard B. Stephens General Atomics. 9 th International Fast Ignition Workshop Cambridge, MA

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Designing and Fabricating a Proton Beam Source Suitable for Fast Ignition Targets

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  1. P.Patel M. Roth et al Designing and Fabricating a Proton Beam Source Suitable for Fast Ignition Targets PERSISTENT SURVEILLANCE FOR PIPELINE PROTECTION AND THREAT INTERDICTION Richard B. Stephens General Atomics 9th International Fast Ignition Workshop Cambridge, MA 3 November 2006 ICFT/P2006-054

  2. Contributors from a large collaboration M Mauldin, E Giraldez,C Shearer M Foord, A J MacKinnon, P Patel, R A Snavely, S C Wilks, K Akli, F Beg, S Chen, H-K Chung, D J Clark, K Fournier, R R Freeman, J S Green, C D Gregory, P-M Gu, G Gregori, H Habara, S P Hatchett, D Hey, K Highbarger, J M Hill, J A King, R Kodama, J A Koch, K L Lancaster, C D Murphy,, H Nakamura, M Nakatsutsumi, P A Norreys, N Patel, J Pasley , H-S Park, C Stoeckl, M Storm, M Tabak, M Tampo, W Theobold, K Tanaka, R Town, M S Wei, L van Woerkom, R Weber, T Yabuuchi,B Zhang • This work is from a US Fusion Energy Program Concept Exploration collaboration between LLNL, General Atomics, UC Davis, Ohio State and UCSD • International collaborations at RAL have enabled the experiments • Synergy with an LLNL ‘Short Pulse’ S&T Initiative has helped the work

  3. 70 60 d = 4 mm 50 40 Eig (kJ) d = 2 mm 30 d = 1 mm 20 25 10 0 0 5 10 15 20 Tp (MeV) Proton ignition concept has evolved • Initial concept avoided complexity • External focusing surface • Simple proton transport • Velocity spread cause problems • Energy must be delivered in short time • Simple solutions … • Reduce energy spread (M. Hegelich, LANL) • Reduce separation • Introduce new problems • Protection from the imploding shell Roth et al., Phys. Rev. Lett.86, 436 (2001) Atzeni et al., Nucl Fusion42, L1 (2002)

  4. Use a reentrant cone for protection • Protects proton source from coronal plasma • Limits accelerating surface • Causes focusing edge effects • Scatters proton beam Laser

  5. Tested concept by making prototype • Cone dimensions same as for electrons • 30° full cone opening • Focusing surface same as for hemi tests (existing focal length data) • rc= 170 mm • dfocus ~290 mm • Limits accelerating area (125 mm dia) • Target Cu foil - 32 mm thick (29 mg/cm2) • Stops < 4 MeV protons

  6. Proton source area depends on energy • Accelerating electrons cool off as they travel to the edge Patel et al., Phys. Rev. Lett. 91, 125004 (2003) Hybrid PIC LSP simulation M. Foord - LLNL 100 fs, 50 mm FWHM Gaussian beam 45 J beam • 200mm dia includes most useful protons (flat foil data) Our source will have limited energy output

  7. Fusion Emission Useful for ignition Low energy protons are most important to ignition 40 Protons must deliver energy in short time for ignition • limits useful proton energy range Proton Deposition Proton Energy [MeV] 300 30 Power [TW] 200 20 10 100 Sim parameters: Proton spectrum: Tp = 3 MeV, dn/deµsqrt(e)e-e/Tp Total proton energy = 26 kJ Proton beam radius = 10 mm Source distance = 4 mm Target density = 400 g/cc 65 105 45 85 125 t [ps] Temporal et al., Phys of Plasma9 3098 (2002)

  8. Protons are not easily scattered • The cone tip can be far from the compressed core • Scattering angle µ E-2 • 3 Mev Protons ~ 5° • 15 Mev Protons ~ 1° • Broadens spot 5-10 mm 5 mm Au 1-5° 15° 200 mm End wall scattering is insignificant

  9. Prototype proton focusing cone was constructed Construction is feasible

  10. HOPG 160 m K imager Initial tests show moderate proton focusing and heating

  11. Proton heating is reasonable for conditions • Ratio of HOPG intensities gives slope temp 1-4 MeV for protons • Ka spots have 106 counts - to be compared to equivalent shots using full hemi • Focal spot is rather large - 160 mm • Could be consequence of side walls changing the proton focus.

  12. Measure focus changes by radiographing grids • Send proton beam through grid and detect with RCF stack • Magnification determines focus position, fuzziness of grid shows focus size, number of grids show source area • These experiments are in preparation Put grids in flat washers for simpler construction

  13. Backlit radiograph (8 keV) at imploded max rR 55* beams, pulse-shape “26” Will use data to design integrated experiments for Omega EP Omega EP hydro simulations (S. Hatchett) Conversion to protons, focusing/ heating? 40 µm 457 µm PW laser vacuum CD2 more compact? improve eff’y? Hi-Z mix? Blob rR ~ 0.44 g cm-2 <r> ~ 120 g cm-3 <T> ~ 0.4 keV Total Energy in blob ~ 0.6 kJ • What is signature of heating, increased emission? Ka fluorescence? X-ray scattering? neutron production? Abs spectroscopy?

  14. Laser spot size influences proton focus 50 um spot 10 um spot z=50 mm (long axis) 55 mm 60 mm z=50 mm 55 mm 60 mm • The proton focal spot radius reduces as laser focal spot increases • Trade-off between fully illuminating surface, and building edge effect

  15. 320 mm Al shell Protons X-ray phc image Tight laser spot gives ‘aberrated’ proton focus X-ray phc image Gekko PW data Laser Proton heating PW laser Cu Ka image Cu Ka image Cu Ka image X- RAL PW data 20mm heated spot

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