Dynamics and variability in near unity gain inertial confinement fusion implosions on the National Ignition Facility

The ability to robustly achieve net energy gain >1 from fusion plasmas is a grand scientific challenge and is being pursued via multiple approaches by different institutions around the world. At the National Ignition Facility, a record fusion energy yield of 1.35 MJ, corresponding to a target gain of 0.7 and capsule gain of 5, was produced using the indirectly driven inertial confinement approach on August 8^th 2021 (shot N210808) [1].

A series of four experiments with input conditions as close to N210808 as possible were fielded to assess the platform robustness. These experiments achieved fusion yields between 250 kJ and 700 kJ. This talk will describe these results and our current understanding of the variability and degradation mechanisms.

Two main sources of degradations have been identified that can explain the observed variability:

- Low-mode hot-spot asymmetry predominantly explained by residual laser power imbalance and capsule asphericity. The resulting shell asymmetry lowers stagnation pressure and confinement [2].

- Mix induced by fill tube and ablator imperfections. Visible ablator jets and meteors in the DT fuel increase radiative cooling [3].

Absolutely calibrated (imaging, time-resolved, spectrally resolved) x-ray and neutron diagnostics are fielded on the NIF along multiple line of sights for each experiment. This allows for a three-dimensional reconstruction on the DT fuel and mix mass around peak burn. Nuclear yield variations are then well reproduced by numerical modeling including these measured imperfections. These detailed measurements over a wide range of performance give new insights into the dynamic of burning expanding plasmas driven by strong alpha heating.

These experiments, all measured to be above the simplest static threshold for ignition [4], provide a framework to assess optimizations toward a higher performing, more robust design.