Lawrence Livermore National Laboratory

Targeting Improved Fusion Performance

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Two of the National Ignition Facility’s latest experiments aimed at improving performance through target design changes recently were reported in Physical Review Letters. The first experiment used larger hohlraums filled with gas at a lower density in an effort to demonstrate better performance than the hohlraums previously used for the high-performing “high-foot” implosions on NIF.A Depleted Uranium HohlraumA NIF depleted uranium hohlraum target installed in the cryogenic target positioner for a high-yield “high-foot” implosion experiment. The new hohlraum improved radiation symmetry and coupling of laser energy to the hohlraum, and reduced production of hot electrons that preheat the fuel. At the same laser energy, high-foot implosions driven with the improved hohlraum have achieved a 1.4× increase in stagnation pressure with an accompanying 50 percent increase in fusion yield compared to an experiment using the smaller hohlraum.

In a second effort, using less than half of NIF’s laser energy, experiments with target capsules lined with a liquid deuterium-tritium (DT)-saturated polymer foam instead of the usual layer of hydrogen “ice” achieved neutron yields comparable to many full-laser-energy NIF experiments. The liquid-hydrogen experiments demonstrate the ability to vary the convergence ratio of a layered implosion in a predictable manner through a change in the initial cryogenic temperature of the target. The new platform also enables designs which should allow more direct contact of dopants with the fuel.