Shock Ignition : an alternative ignition scheme for Hiper

1. Shock Ignition : an alternative ignition scheme for Hiper Marion Lafon Xavier Ribeyre Guy Schurtz Stefan Weber 6th Direct Drive Fast Ignition Workshop 11th-14th May 2008 LISBOA

2. Motivations • Baseline Hiper design is • Cone-in-a-shell target • Electron driven fast ignition • Major uncertainties remain unsolved • Electron production and transport (cf this wkshop) • High convergence cone guided implosions • High rep operation • Need for a PLAN B • Simple, spherical, scalable targets • Demonstrated control of main physical issues  SHOCK IGNITION

3. Outline • How does it work • Shock Ignition of Hiper Baseline capsule • Chronometry • 1D robustness • Absorption • 2D ignition • Plasma physics issues • Conclusion

4. Basics of shock ignition • Principle : Central Ignition threshold is lower for non isobaric fuel assembly • roughly, Eni~Eiso/F3 +DE(F) with F=Paniso/Piso • assist spontaneous hot spot ignition by driving a strong convergent shock before stagnation time • Proposed by J. Perkins , R. Betti as an alternative to fast ignition • SI experiments at Omega produced record areal densities and demonstrated dramatic yield enhancement • Numerical simulations predict high gain at the 200-300kJ level • Shock Ignition is assisted central ignition, not fast ignition

5. How does it work • Classical medium velocity, low isentrope fuel assembly • Drive the ignition shock by means of a final spike in laser intensity • The ignition shock collides with the return shock and provides the necessary amount of energy to trigger ignition from a central hot spot • RESULTS IN A NON-ISOBARIC FUEL ASSEMBLY

6. Fuel conditions at maximum density Shocked No shock

7. Rationale for shock ignition design Hot spot • Pressures must match at shock collision time in order to maximize entropy growth • Collision radius determines the hot spot mass and pressure • Shocks must collide near the inner boundary of fuel • Too large mass at given energy means low temperature and no ignition at all • Too small roR means ignition is not self sustained • The classical hot spot ignition model must be modified to account for the pressure ratio F=Psi/Piso (R.Betti &al. PRL 98,155001-2007)

8. Shock Igniting HiPER target 1044 µm DT ice DT gas 833 mm • 180 kJ compression • + • 70 kJ, 350 ps ignitor • ~20 MJ (TN) G~80 M=0.59 mg

9. Different conditions are met when varying the shock launching time 11.10 ns : Too late  Poor hot spot heating 11 ns : Right on time 10.7 ns : Too early inhibates shell compression

10. Shock launching time Too late : 0 yield Too early : slow burn On time : 20 MJ

11. Iso energy curves 250 ps confidence interval at 80 TW • Run series of 1D calculations using radial rays and total absorption at critical • Vary the launching time of the igniting shock and the laser power in spike Slope of cliff consistent with shock velocity ~I1/3

12. Laser absorption efficiency is the first identified issue Delivering 80 TW to the target requires 150-200 TW from the laser • Temperature at critical exceds 7 keV during the spike • DT absorbs poorly • Critical radius is half its initial value at spike launching time

13. Absorption • Motion of critical surface suggests the use of reduced focal spots (zooming ?) • An alternative is to use dedicated beams with specific RPP (bipolar shock ignition)

14. Using dedicated beam lines 2 mm • 48 compression beam lines + 12 shock ignition beam lines at 32° from polar axis • 2D ignition simulations (allDT , 200 TW, 3D rays ) Laser absorbed power 500 mm 250 mm Pressure 7 ns 10.5 ns

15. Using dedicated beam lines • 48 compression beam lines • + 12 shock ignition beam lines at θ° from polar axis • 2 D ignition simulations (allDT , 150 TW ) Pressure during shock ignition propagation Θ=33.2° Θ=0° Θ=54.7° 100 ps later Pressure becomes isotropic In 100 ps Little sensitive to ignitor incidence Whathever θ G=80

16. S.A.’s target with a wetted foam ablator gaz gaz CH(DT)6 DT 1020 µm 950 mm • Target • 1D CHIC (MG rad, 3D rays) Vi=250 Km/s, rR=1.55, rmax=570g/cc • absorption efficiency Is strongly enhanced 833 mm

17. Experimenting shock ignition on LMJ 49° Full Indirect drive geometry 32° First step 1MJ DD shock ignition geometry 59° • Use 49° and 59° cones to compress fuel (cf B. Canaud & al. + JL Feugeas, this wkshp) • 49 & 59 bracket 53.7 • exact cancellation of P2 by tuning the balance between cones 0.3% rms non uniformity on target • Max available energy is ~ 1 MJ.  300 kJ is routinely achievable - safe, low velocity , 1 < a <2 implosions • 20 beams at 32° may provide the 120 TW (max is 200) ignition pulse 59° 49° 32°

18. Shock production relies on thermal transport in a quite unusual regime • Heat conduction greatly helps smoothing out shock pressure • Probably non local • Magnetic fields likely to be self generated End of ignition pulse Start of ignition pulse gradTe

19. LPI issues

20. Hot electrons (if any) are welcome Betti & al. FSC Meeting February 28, 2007 Chicago, IL

22. a perturbated core still ignites(… with a narrower window) • According to Rochester simulations 1 … 80 2D simulations Modes l=4-100, NIF 2D-SSD Energy = 400kJ Normal Incidence Thomas-Fermi EOS No radiation 60 1D Gain Gain 40 20 0 FSC Meeting February 28, 2007 Chicago, IL 11.1 11.3 11.5 Ignitor shock launching time (ns) 1: how do they get higher gains in 2D ?

23. Summary / Conclusions • Shocks driven by a 80 (150) TW intensity peak ignite the .6 mg Hiper target proposed by Stefano Atzeni & al. with target gains up to 80. • Absorption is a critical issue, not drive symmetry • Dedicated, more tightly focused ignition beams should be used • Wetted foam ablators significantly increase absorption • Several plasma physics issues may show up. • I > 2.5x1015 W/cm2, long plasmas • Te>8 keV, 2D : transport is non local, B fields likely • Survives laser imprint in Rochester simulations • Shock Ignition is an attractive candidate for Hiper high rep operation. • LMJ ( 3 cones ID geometry) is well suited for shock ignition experimentation • Work in progress at CELIA