Precision Measurements of Hohlraum L-shell Preheating in Tungsten-based Double Shells and their Consequences for Shape and Stability Control

To approach the ideal, one-dimensional thermonuclear burn performance of double shell Inertial Confinement Fusion (ICF) capsules, detrimental sources of implosion asymmetry must be controlled [D.S. Montgomery et al., Phys. Plasmas 25 (2018)]. These asymmetries span a broad spectrum from low-modes, born during early stages of the outer shell implosion, to high-mode instability growth from surface roughness [P. Amendt et al., Phys. Plasmas 10 (2003)]. Hard x-ray preheat is hypothesized to be a controlling factor in how modes across this range transfer and grow between shells. 

In low-gas-fill, low-laser-backscatter hohlraums at the National Ignition Facility (NIF), significant levels of Au M-shell and L-shell radiation are produced as potential sources of preheat. Current double shell implosions use Al ablators to block M-shell from penetrating the capsule leaving L-shell as the primary source of preheat to the high-atomic-number interior pusher. Because L-shell is produced only at multi-keV, non-LTE plasma conditions, it is exceedingly difficult to calculate. To this end we have made the first measurements of the Au L-shell symmetry environment in NIF hohlraums, which used dual-axis VISAR interferometry [H.F. Robey et al., Phys. Rev. Lett. 108 (2012)] to record tungsten pusher motion to within 5%. The multi-Mbar tungsten shock induced by the L-shell radiation was found to be 60% stronger at the pole relative to the equator, which with improved hohlraum modeling we can now quantify its impact on the mode 2 shape of the pusher and DT fuel and account for it by tuning laser power balance. These measurements have also placed more rigorous constraints on Rayleigh-Taylor instability (RTI) growth calculations in double shells. This is of particular importance in designing effective engineered density gradients to control RTI in the presence of changing pre-collision conditions due to preheat.