Fast Ignition targets and experiments

1. Fast Ignition targets and experiments Richard Stephens General Atomics US/Japan Workshop on Laser IFE La Jolla CA March 21-22, 2005

2. Acknowledgements • OFES Collaborators: LLNL, LLE, GA, UC Davis, UC San Diego, and Ohio State • Collaborations with RAL, ILE, and LULI have enabled most of the experiments K. Akli, F.Beg, M. Bonino, M.H. Chen, Z. Chen, H-K. Chung, K. Fournier, R.R. Freeman, S. Fujioka, J. S.Green, P. Gu, J. Gregori, H. Habara, S.P. Hatchett, D. Hey, J.M. Hill, Y. Izawa, M.H. Key, J.A. King, Y. Kitagawa, R. Kodama, J.A. Koch, K. Lancaster, B.F. Lasinski, B. Langdon, A.J. MacKinnon, D.D. Meyerhofer, A. Lei, S.J. Moon, C.D. Murphy, P.A. Norreys, H-S. Park, N. Patel, P. Patel, J. Pasley, R. Petrasso, H. Shiraga, J. Smith, R.A. Snavely, C. Stoeckl, M. Tabak, M. Tampo, W. Theobold, K.A. Tanaka, R.J.P. Town, Y. Toyama, T. Tsutsumi, S.C. Wilks, T. Yabuuchi,B. Zhang, J. Zheng

3. Fuel • PW laser to ignite dense core • Absorbed into fast electrons • Fast electrons propagate to core or create proton beam • Absorbed heat ignites spark Electrons Laser or Protons Fast ignition is a two driver process Fast Ignition Target • Drive laser to compress fuel <=1 mm=>

4. FI at NIF 100 Fast Ignition Direct Drive NIF Point Designs Target gain Advanced Indirect Drive on NIF 10 Indirect Drive 1 0.1 10 Driver energy (MJ) FI advantage is in potentially higher gain • rR~ 3 g cm-2 required for burn, ~0.3 g cm-2 for ignition • Central Hot Spot Target: • low density hot spot takes half the target volume • ~600 g cm-3 needed in cold fuel • ~1/2 the drive energy needed for ignition • Fast Ignition Target: • Ignites dense fuel - no low density region • ~200 g cm-3 gives the same rR • ~ 1/10 as much fuel needed in ignition spark Need homogenous core, efficient transport, minimal ignition volume

5. Fuel Fuel Electrons Laser Laser or Protons Those requirements pose challenges to our capability • Assembling compact fuel mass • Understanding of high intensity and HED science • Fast electron generation • Fast electron transport & energy loss • Developing integrated solution

6. density Be+Cu C1H1 250 µm DT ice Au cone Au cone r=0 r=3.e-5gcm-3 150 µm Reentrant cone FI targets for OMEGA were produced from NIF scale designs NIF cryo-ignition target • Cryo-FI targets predicted to collapse to rR ~2 gm cm-2 • Scaled to hydrodynamically equivalent plastic shell for x-ray drive (0.5 mm od) or laser drive (1 mm od) • Back-lit x-radiograph should show structure, density, collapse timing 2 mm X-ray driven target Tested X-ray and Laser drive target concepts

7. Core exhaust flattens cone Core exhaust flattens cone Cone contaminates core Cone contaminates core <==200 mm==> <==200 mm==> Reentrant cone geometry exhibits core-cone interactions X-ray drive Laser drive <=1 mm=> LASNEX calculation of cone heating: ~40 kJ/cm2 From hohlraum M-line emissions 9 kJ/cm2 From thermal electron bremstrahlung during drive pulse

8. Shell exhaust stream impacts cone tip 200 mm • Exhaust pressure pushes back the Au vapor layer unstably • Visible in backlit radiograph • And in emission using MIXS Shot #30577

9. LASNEX Cone tip collapses under the gas flow • Cone ledge gives fiducial for measuring tip position • Expt & simulation show 30 mm collapse before stagnation • That is more than the allowed tip thickness • Tip is pushed back before ignition pulse Gas flow force and timing can be adjusted by design

10. Cool density Hot Designs are not yet optimized: core cone distance is large or cone collapses Symmetric drive drive asym: 10% P1 • Adding asymmetry may improve compaction • Changing scale also changes fuel shape and cone interaction • These examples all have <rR> ~ 2 g cm-2 • Scale 1 - 250 eV drive • Scale 1.6 - 190 eV drive 250 µm Concepts eliminating hot core have been proposed 500 µm

11. Conclude that we qualitatively understand these targets, but … • Simulations do not include mixing - a large effect • Must control interaction of core with cone to permit ignition pulse • Have not fully explored trade-offs FI target designs require improved design and integrated analysis

12. Physics of energy transport by MeV electrons is very complex Hybrid PIC model ( Paris) L Gremillet G Bonnaud, F Amiranoff POP 9,941,(2002) • Input current >> Alfven limit - return current compensated • Return current Ohmic E field slows input electrons • Azimuthal B field pinches input electrons dB/dt =curl(E) • Resistive Weibel filamentation instability • Entry surface collisionless Weibel - transport barrier • Entry surface dB/dt= (gradN)XgradT- radial ExB drift

13. RAL data 70J,0.8 ps MC model Cone angle 40o Min diameter 80 mm Initial spread is a worry -- starting spot size is ~ 5X laser spot size Al Cu Al Ka image data at 30J and 70J show deep transport into aluminum 180 mm

14. Recent 300J (RAL PW) data show similar transport cone angle in Al 120 mm Target 1mm x1mm 67Al/7Cu/26Al mm 5ps ,330J 68eV XUV 68 eV image shows Symmetrical rear surface heating Cu Ka image shows electron beam and uniform electron reflux in oblique view of square target. Cu Ka 88 mm

15. Temperature, eV Depth, mm Peak temperature vs depth in Al is obtained from the Ka image brightness • Temperature seems to drop quickly near front surface

16. Single hit CCD spectra of Cu Kshell show hot layer emits thermal Hea and Lya at highest intensities (kT~ 4keV) • thermal lines • dominate front • view spectrum • not prominent • in rear view • 1mm Al coating • suppresses • thermal lines C. Stoeckl et al. 46th APS DPP 2004

17. We have used new diagnostics to look at electron generation process in more detail • High 256 eV XUV imaging • Sees through blowoff • Freezes expansion PW Laser e- Target 100x100x (1 to 10 )mm • Fluorescence emission spectrum shows temperature

18. XUV images at 270eV and 68 eV diagnose isochoric heating of low mass targets (rear surface) Target 100x100x5 mm Cu- laser 425J , 0.6 ps 100 mm 100mm 68 eV 270 eV Images at same scale show freezing of expansion in 270 eV image

19. Emission Abs. coeff Emissivity Spectroscopic modeling gives information from line ratios on temperature and temperature gradients - kT>2keV in Cu from Lya/Hea ratio Hea • kT =2 keV in solid Cu • 5 mm thick • Opacity is up to 500 • Line ratios strongly • dependent on opacity • and temp gradients • Satellite lines brighter than • resonance lines • - opacity or gradient effect ? Lya Opacity =1 in 5 mm Cu Photon energy eV

20. 256 eV XUV image Cu K shell spectrum Front Back Ka Lya Hea These diagnostics shows strong temperature gradients in 5 mm thick targets • 0.5 mm Al/5 mm Cu target • 500 mm x 500 mm size • 0.5 ps, 300J irradiation Peak temp 2x general temp -> strong heating from initial beam 3:1 front:back intensity ratio in Hea -> strong axial temp gradient

21. That disappear in thinner targets Homogeneous reflux heating to about 4 keV seen in 2 mm thick Cu target irradiated with 0.5 ps 300J pulse 256 eV 68 eV 1.5:1 front:back intensity ratio in Hea -> small axial temp gradient 200 mm 200 mm Electrostatic confinement allows control of heat deposition

22. Joint work with ILE Osaka on creating extreme energy density in thin fiber targets is published in the Dec 2004 issue of Nature

23. Cu R Al Al 50 mm 0.5-ps z 10 mm Electron refluxing and Cu-Ka image from low mass foil target is simulated with LSP Example of data Ka intensity

24. R 200 mm Al 0.5-ps Cu z LSP is also being used to model cone/fiber electron transport and isochoric heating Example of data Example of data eV Ka image 1mmx10 mm fibre

25. Summary • Reentrant cone targets have core-cone interactions • Hot, low density core must be managed • Electron transport unchanged for 300J pulses • Bottleneck for electron injection? • New diagnostics give more detail about transport • Ka emission spectrum • 256 eV XUV imaging • Electrostatic confinement can be used to control electron density • 4 keV temperatures in solid density foil • LSP is being developed to do integrated modeling • Adding electron injection and Ka packages These capabilities are necessary for integrated hot-plasma experiments