NRL Experiments in Support of High Gain Target Designs

1. NRL Experiments in Support of High Gain Target Designs James Weaver Laser Plasma Branch Naval Research Laboratory on behalf of Nike Laser Group Presented at the High Average Power Laser Program Workshop Washington, DC - March 3, 2005

2. Contributors Steve Obenschain, Andrew Schmitt,Victor Serlin, Yefim Aglitsky, Max Karasik, Yung Chan, Denis Colombant, Jaechul Oh, Edward Mclean, John Stamper, Jacob Grun, Charles Manka, Wallace Manhiemer, David Kehne, Andrew Mostovych,John Gardner, Michel Busquet, Marcel Klapisch, Robert Lehmberg, Alexander Velikovich, Nathan Metzler, Giora Hazak, Avi Bar Shalom, John Seely, Uri Feldman, Charles Brown, Josef Oreg, Jason Bates, Lee Phillips, Glenn Holland,Drew Fielding, Bruce Jenkins, Laodice Granger, Zeb Smythe, Steve Terrel, Jonathan Picotta, John Hardgrove, Nicholas Nocerino, Del Hardesty, Dennis Brown, Steve Krafsig, John Bone

3. NRL Nike laser group has extensive experimental program in support of ICF Physics • Observation of hydrodynamics: Rayleigh-Taylor instability, • Richtmeyer-Meshkov instability, imprint growth, target acceleration • Fundamental equation of state of shocked materials • Benchmarking of atomic physics codes • Cryogenic foam target development • Studies of laser-plasma instabilities • Advanced diagnostic development This talk will focus on the two most recent experimental campaigns related to high gain pellet design: Spike-Prepulse & High-Z Overcoats

4. NRL Nike KrF Laser Facility • KrF excimer laser operating at 248 nm with 1-2 THz bandwidth • Angular multiplexing through large electron pumped amplifiers provides 3 kJ for planar target experiments • Focal profile of beam on target is smoothed by the Induced Spatial Incoherence (ISI) technique. • Typical laser pulse has 4 ns long low intensity foot and a higher intensity 4 ns long main pulse • Up to 44 main beams can be overlapped on the target, focal spot FWHM of 0.75 mm and a flat central region 0.4 mm in diameter. • Up to 12 beams are used for backlight x-rays, focal spot FWHM of 0.4 mm, 0.2 mm flat top.

5. NRL Nike laser provides highly uniform target illumination Essential for well controlled hydrodynamic and shock experiments. S. P. Obenschain, et al., Phys. Plasmas 3 (5), 2098 (1996)

6. NRL BACKLIGHTER LASER BEAMS MAIN LASER BEAMS 2D IMAGE RIPPLED CH TARGET BACKLIGHTER TARGET Si Sample RT Data Time Nike is well optimized to study hydrodynamics in planar geometry 1.86 keV imaging QUARTZ CRYSTAL Nike Target Chamber STREAK CAMERA Y. Aglitskiy, et al. , Phys. Rev. Letters, 86, 265001 (2001)

7. NRL no spike, gain <1 with spike, gain = 160 Stabilizing Pulse Shape 1000 100 10 1 800 m Power (TW) t3 t2 t1 0 10 20 time (nsec) 400 m High gain target utilizing spike prepulse and zooming NRL FAST Code 2 D simulations Total Laser Energy 2.5 MJ Pellet Design spike A.J. Schmitt, et al., Phys. Of Plasmas, 11 (5), 2716 (2004).

8. NRL Spike prepulse creates sloped density profile in front of main shock 0.3 ns, 5 TW/ cm2 spike 4 ns, 50 TW/ cm2 main pulse 0.3 ns, 5 TW/ cm2 spike Density (r) Position (mm) Position (mm) Main pulse catches spike at rear surface Spike pulse creates density profile N. Metzler, et al., Phys of Plasmas, 9 (12), 5050 (2002). N. Metzler, et al., Phys of Plasmas, 6 (12), 3283(1999). A. Velikovich, Phys. of Plasmas, 10 (8), 3270 (2003).

9. NRL Spike prepulse causes a delayed onset of mode growth

10. NRL Spike Prepulse Capability of Nike Laser • Fast pulse Pockels cell driver and optics added to foot beam optical path • Spike pulse with ~ 300 ps FWHM created on all main beams • Spike intensity up to 20% of main peak, maximum delay up to ~ 4 ns Spike Pulse in Nike front end Pulseshape after final amplifier Signal (arb. units) Normalized Signal Time (ns) Time (ns)

11. NRL Observation of shock propagation from spike prepulse Comparison of observed shock propagation and analytic prediction Laser interferometer provides time history of spike shock propagation through target Analytic prediction Space (mm) Estimated shock velocity Time (ns) Data verify the desired shock motion has been achieved Shock velocity is proportional to fringe shift Jaechul Oh, Andrew Mostovych, et al.

12. NRL 3%, 4 ns foot 10% spike 2 ns early Mode Amplitude (mm p-to-v) 10% spike 3 ns early Time (ns) Initial target data qualitatively confirm predictions for mode growth 40 mm thick CH, sinusoidal ripple l= 30 mm, A = 0.25 mm • Mode amplitude growth is observed to be delayed • Spike results appear insensitive to spike-main delay Much more data left to analyze from this campaign…….

13. NRL High gain target designs have been achieved with combination of zooming and high-Z overcoats

14. NRL High-Z layer helps to reduce imprint Pure CH CH + ~380 Å Au High Z layer is ablated early in the pulse and provides additional laser smoothing. Early laser imprinting is reduced. Multiple Beams (40) Single Beam Au overcoats of sufficient thickness reduces observed imprint growth S. P. Obenschain, et al., Phys of Plasmas, 9 (5), 2234 (2002).

15. NRL Time (ns) High-Z layers suppress imprint without blowing up the target 800Å Pd withon rippled CH: Imprint suppressed,still have ripple growth Plain CH:lots of imprint 910 Å Pd: Imprint suppressed Space (µm) Space (µm) Space (µm)

16. Au density(mg/cm3): NRL foot pulse time (ns) foot pulse foot pulse 16sep04_7 17sep04_8 29sep04_9 space (µm) space (µm) space (µm) Evolution of high-Z layers has been observed side-on Initial Authickness: 590Å 400Å 200Å Max Karasik, et al.

17. NRL NRL/LLE Collaboration on High-Z Layer Targets LLE • Two basic questions: • How robust are these results? Can we reproduce them at another laser facility? • Will these layers actually improve pellet performance? Omega Laser Facility - 30 kJ glass laser, 351 nm wavelength - 60 beams available for spherical implosions - Advanced pulse-shaping capabilites, pulse lengths are typ. 1-2 ns - Beam smoothing (~1% level) achieved with SSD and RPP - Extensive suite of diagnostics available for target experiments

18. NRL Late time radiographs of accelerated CH targets LLE T~3nsec 30m CH 30m CH with 250Å Au 200 mm Thin Gold layer is effective in reducing laser imprint

19. NRL Recent implosion experiments at LLE demonstrate improved neutron yield for pellets with an outer coat of Pd LLE Pd overcoat 1500 Å Al Intermediate layer 3 atmos. D2 fill Meas. Neutron Yield 20 mm CD shell 860 mm diameter 1D Calc. Yield Pd Thick Å YOC Laser Pulse Shape Peak ~ 1015 TW/cm2 1 ns 1 ns

20. Summary • NRL has broad experimental program to explore physics relevant to high gain target for ICF: • Nike KrF laser has unique properties that enable high quality experiments • Advanced diagnostics are available at the laser facility to create a detailed • and precise understanding of target physics • Two designs for high gain targets are being explored: • Experiments using spike prepulse on the Nike laser have verified our • understanding based on simulations and analytical calculations • High-Z overcoats of Au and Pd have been shown to reduce imprint • growth in detailed studies of planar targets and have been shown to • increase neutron yield in recent implosion experiments