A mechanism for reduced compression in indirectly-driven layered capsule implosions*

High yield implosions on the National Ignition Facility rely on maintaining low entropy in the DT fuel, quantified by its adiabat, in order to efficiently couple energy to the hot spot through high compression of the fuel layer. We present very high resolution xRAGE [1] simulation results that show that jetting of

material due to surface defects or voids in the capsule shell can produce weak spots in the shell at peak implosion velocity that prevent uniform compression of the fuel layer even when they do not introduce contaminant into the hot spot. This occurs despite maintaining low fuel entropy, since the formation of

the weak spots involves non-radial displacement of fuel mass. Simulations show little effect of fuel-ablator mixing on compression. We show that for low adiabat implosions, surface defects alone can decrease compression consistent with a 30% increase in fuel adiabat. This mechanism offers a potential explanation for persistently overestimated fuel compression [2] in layered implosions on the National

Ignition Facility.

[1] B.M. Haines et al., Phys. Plasmas 24:052701, 2017.

[2] O.L. Landen et al., HEDP 36:100755, 2020.