The goal of this project was to develop a novel approach to simultaneous use of both direct and indirect drive to implode an inertial confinement fusion capsule at the National Ignition Facility. This concept is intended to diversify the program's collection of conventional hot-spot ignition designs by providing additional design flexibility. The two driving sources, direct (Nuckolls et al. 1972) and indirect (Lindl 1998) can be used in combination or as two independent systems, and can be tailored according to the target physics objectives and constraints. The primary research objective of this project was to create a simple and flexible simulation deck to test target drive and compression physics, and to produce a robust design. We evaluated two different designs: a directly/indirectly driven capsule, and an indirectly driven capsule with an uncharacteristically large fill tube, dubbed a "venting" implosion.
Although implosion performance has drastically improved (Hurricane et al. 2014) since the end of the National Ignition Campaign (Lindl et al. 2014), ignition has still eluded researchers. A combination of hot-spot asymmetries, compressibility issues, and laser-plasma interactions continues to prevent ignition. This has prompted research on a modified implosion, with the goal of identifying critical issues that prevent current designs from succeeding. Current low-convergence, indirectly driven designs are hydrodynamically stable, but suffer when researchers attempt to improve performance. Direct-drive implosions, while not thoroughly tested at ignition scale, are thought to couple more kinetic energy to the imploding capsule, but hydrodynamic stability must be controlled. Our new concept potentially offers the advantage of the symmetry of indirect drive for fuel assembly, as well as the efficiency of direct-drive shock ignition (Betti et al. 2007) in a capsule with thick fuel layers.
This is achieved by dedicating the Facility's northern hemisphere beams to imploding part of the target capsule with direct drive and using the Facility's southern hemisphere beams to implode the rest of the capsule with indirect drive (see figure). This concept allows a new class of implosion designs to be interrogated without changing NIF's current geometry. The flexibility in implosion design allows the separation of the compression and ignition stages of the implosion (similar to fast ignition). One part of the imploding capsule will be tuned for the high compression needed for robust burn propagation, and the other will be tuned to provide the spark needed to ignite the compressed hot spot. Compared to modern indirectly driven implosions, the constraints on the indirectly driven side will be relaxed—it will be driven with a lower radiation temperature, resulting in improved hohlraum control via enhanced beam propagation and improved hydrodynamic stability due to lower capsule implosion velocities. The directly driven side will use the advanced ignition concept known as shock ignition to provide the spark. While a pure highly-convergent shock ignition platform is currently not possible to field on the NIF because of the polar geometry of the laser beams, this concept is well suited for it.
Our project focused on optimizing parameters individually, followed by the optimization of the combined pieces. This occurred for each variation of the two drive combinations we investigated: (1) direct and indirect drive on either side of a gold cone, and (2) an indirectly driven target with a large fill tube. Both drive combinations presented new sets of design constraints that required their own unique solutions.
Development of a point design using the radiation hydrodynamics code HYDRA (Marinak et al. 2001) first required a stable, simple, and easily customizable mesh. While borrowing concepts from earlier work (Perkins et al. 2012), fast ignition (Tabak et al. 1994, Shay et al. 2012, Solodov et al. 2011) and impact ignition (Murakami et al. 2014), we used a different methodology. In almost all fast ignition simulations, the code is run in the arbitrary Lagrangian Eulerian (ALE) mode. This approach requires large amounts of advection near the cone-capsule interface to prevent mesh tangling caused by shear occurring between the imploding capsule and the gold cone. Here, we chose to run a fully Eulerian simulation, where mass advects through the stationary mesh, effectively eliminating tangling. The trade-off when switching to a fully Eulerian simulation is typically non-negligible when considering the number of zones required. However, this issue was resolved by implementing what is known as a treadmill-type mesh.
This project supports the stockpile stewardship goals of the NNSA and Lawrence Livermore National Laboratory and keeps the Laboratory on the cutting edge of inertial fusion science. This research also supports the Laboratory's core competency of high-energy-density science by developing solutions for achieving high yield on the National Ignition Facility.
Our research has made progress on determining the feasibility of this concept, providing future research in this area with a significant advantage. While a robust, igniting design was not achieved, the development of a robust and simple simulation deck to investigate these implosions was successful. A deck that can easily specify a large number of geometric target parameters as well as nominal values for physics options (ray tracing, radiation transport, mesh generation) has been developed and fully tested. With more time, computer resources and a better understanding of the hydro code in a regime where it is not typically exercised, an igniting and robust implosion could be designed.
This research would benefit from additional HYDRA developer expertise. Secondarily, improvements in the accuracy of the advection routines in HYDRA would be extremely helpful, not just to this concept, but to anyone using HYDRA (either in Eulerian mode or when using different ALE schemes).
Betti, R. et al. 2007. "Shock Ignition of Thermonuclear Fuel with High Areal Density." Physical Review Letters 98(15): 155001. doi: 10.1103/PhysRevLett.98.155001.
Hurricane, O. A. et al. 2014. "Fuel Gain Exceeding Unity in an Inertially Confined Fusion Implosion." Nature 506: 343–48. doi: 10.1038/Nature13008.
Lindl, J. D. 1998. Inertial Confinement Fusion: The Quest for Ignition and Energy Gain Using Indirect Drive. Springer-Verlag, New York.
Lindl, J. D. et al. 2014. "Review of the National Ignition Campaign 2009-2012." Physics of Plasmas 21(2): 020501. doi: 10.1063/1.4865400.
Marinak, M. M. et al. 2001. "Three-Dimensional HYDRA Simulations of National Ignition Facility Targets." Physics of Plasmas 8(5): 2275–80. doi: 10.1063/1.1356740.
Murakami, M. et al. 2014. "Impact Ignition as a Track to Laser Fusion." Nuclear Fusion 54(5): 054007. doi: 10.1088/0029-5515/54/5/054007.
Nuckolls, J. et al. 1972. "Laser Compression of Matter to Super-High Densities: Thermonuclear (CTR) Applications." Nature 239: 139–42. doi: 10.1038/239139a0.
Perkins, L. J. et al. 2012. "A Hybrid Indirect-Drive/Direct-Drive Target for the National Ignition Facility." American Physical Society, 54th Annual Meeting of the APS Division of Plasma Physics, October 2012. LLNL-PRES-542011.
Shay, H. D. et al. 2012. "Implosion and Burn of Fast Ignition Capsules—Calculations with HYDRA." Physics of Plasmas 19(9): 092706. doi: 10.1063/1.4751839.
Solodov, A. A. et al. 2011. "Simulations of Implosion and Core Heating for Integrated Cone-in-Shell Fast-Ignition Experiments on OMEGA." 53rd Annual Meeting of the American Physical Society Division of Plasma Physics, Salt Lake City, UT, November 2011.
Tabak, M. et al. 1994. "Ignition and High Gain with Ultrapowerful Lasers." Physics of Plasmas 1(5): 1626. doi: 10.1063/1.870664.
Nora, R. et al. 2016. "Hybrid Drive on the NIF." LLNL Summer Poster Session, August 2016. LLNL-PRES-732450.