Inferring high-Z pusher morphology near stagnation in Double Shell implosions from x-ray self-emission

The Double Shell (DS) platform [1] on the National Ignition Facility (NIF) is used to investigate volume burn. DS capsules consist of two shells, an outer low-Z ablator (Al) and an inner high-Z pusher, with a foam between the shells to mediate the transfer of kinetic energy from outer to inner shell, which subsequently compresses and ignites the DT fuel. Observations of the pusher morphology near stagnation would provide crucial information about implosion quality; however, the pusher morphology near stagnation is difficult to determine from radiography as its outer radius is ~100 µm with an areal densities up to a few g/cm².

The x-ray self-emission observed in DS implosions may offer a path to infer pusher morphology at stagnation. We follow the approach taken in previous experiments on the NIF [2] and OMEGA [3], albeit in a much different areal density regime (~1 g/cm² vs. ~1 mg/cm²). For DS, the magnitude of measured x-ray self-emission is strongly dependent on the minimum areal density of the pusher along a line of sight. When combined with diagnostics that provide the 4π-averaged pusher areal density, the x-ray self-emission data may allow the modulation in the pusher morphology at stagnation to be inferrred. The inferred pusher morphology will provide insight into the impact that various degradation mechanisms have on implosion performance.

[1] D. Montgomery et al., Phys. Plasmas 25, 092706 (2018).

[2] L. Pickworth et al., Phys. Rev. Lett. 117, 035001 (2016).

[3] V. Smalyuk et al., Phys. Rev. Lett. 87, 155002 (2001).