Diagnosing the impact of low-mode asymmetries in ignition experiments at the National Ignition Facility

Ignition via inertial confinement fusion (ICF) requires implosions to achieve high hotspot energies and pressures that are inertially confined by a dense shell of DT fuel. This requires high inflight shell velocity, good energy coupling between the hotspot and imploding shell, and high areal-density at peak compression. Three-dimensional (3D) asymmetries seeded by imperfections in the drive symmetry or target [MacGowan et al., HEDP 40 100944 (2021)] can grow during an implosion and damage both the coupling of energy to the hotspot and confinement of that energy. Recent high-yield experiments at the NIF have shown evidence that low-mode asymmetries are a key degradation mechanism and contribute to shot to shot variability. Experimental and theoretical evidence show experimental signatures of asymmetry (e.g. the observed hotspot velocity, and areal density asymmetry) change with increasing implosion yield given the same initial seed. Likewise, the level of performance degradation as inferred by the observables also changes. Understanding how these asymmetries respond to differences in performance helps untangle how they impact variability and performance at high yield.