Assessing the prospects for achieving double-shell ignition on the National Ignition Facility using vacuum hohlraums

The goal of demonstrating ignition on the National Ignition Facility [J. D. Lindl et al., Phys. Plasmas 11, 339 (2003)] has motivated a revisit of double-shell (DS) targets as a complementary path to the cryogenic baseline approach. Expected benefits of DS ignition targets include noncryogenic deuterium-tritium (DT) fuel preparation, minimal hohlraum-plasma-mediated laser backscatter, low threshold-ignition temperatures $(\approx 4\mathrm{keV})$ for relaxed hohlraum x-ray flux asymmetry tolerances, and minimal (two-) shock timing requirements. On the other hand, DS ignition presents several formidable challenges, encompassing room-temperature containment of high-pressure DT $(\approx 790\mathrm{atm})$ in the inner shell, strict concentricity requirements on the two shells $(<3\mu \mathrm{m})$ , development of nanoporous ($<100\mathrm{nm}$ cell size) low-density $(<100\mathrm{mg}∕\mathrm{cc})$ metallic foams for structural support of the inner shell and hydrodynamic instability mitigation, and effective control of hydrodynamic instabilities on the high-Atwood-number interface between the DT fuel and the high-$Z$ inner shell. Recent progress in DS ignition designs and required materials science advances at the nanoscale are described herein. Two new ignition designs that use rugby-shaped vacuum hohlraums are presented that utilize either 1 or $2\mathrm{MJ}$ of laser energy at $3\omega$ . The capability of the National Ignition Facility to generate the requested 2 MJ reverse-ramp pulse shape for DS ignition is expected to be comparable to the planned high-contrast $(\approx 100)$ pulse shape at $1.8\mathrm{MJ}$ for the baseline cryogenic target. Nanocrystalline, high-strength, Au-Cu alloy inner shells are under development using electrochemical deposition over a glass mandrel, exhibiting tensile strengths well in excess of $790\mathrm{atm}$ . Novel, low-density $(85\mathrm{mg}∕\mathrm{cc})$ copper foams have recently been demonstrated using $10\mathrm{mg}∕\mathrm{cc}$ $\mathrm{Si}{\mathrm{O}}_2$ nanoporous aerogels with suspended Cu particles. A prototype demonstration of an ignition DS is planned for 2008, incorporating the needed novel nanomaterials science developments and the required fabrication tolerances for a realistic ignition attempt after 2010.