Hydrodynamic instability seeding by oxygen nonuniformities in glow discharge polymer inertial fusion ablators

The U.S. indirect drive inertial confinement fusion (ICF) program uses glow discharge polymer (GDP) as one of the ablator materials for the ICF experiments at the National Ignition Facility (NIF). The performance of those implosions may be adversely affected by photoactivated uptake of oxygen impurities in the surface of the GDP ablator. These impurities can induce significant perturbations in the shock waves generated in the ablator. These perturbed shocks generate mass modulations that are amplified by Rayleigh-Taylor (RT) instability and reduce the performance of the implosions. Here we report on experiments that provide a quantitative connection between photoactivated oxygen uptake in GDP samples and modulations in shock velocities in the samples. The observed shock nonuniformity confirms the hypothesis that irregular oxygenation is a competent seed for the RT and can have a significant effect on NIF GDP implosions.

Figure 1

(a) Target schematic for the hohlraum-drive GDP targets. GDP is opaque to 400-nm probe wavelength and the detection medium is PMMA as shocks stronger than 1 Mbar are optically reflecting in the PMMA. (b) Characteristic oxygen atomic percent over a subset of the GDP foil. (c) Line profile integrated over the vertical span of (b) showing additional structure on the peaks.

Figure 2

Velocity map and horizontal line profile from x-ray driven light-exposed GDP foils at (a) 2.50 ns, (b) 2.80 ns, and (c) 3.70 ns after drive arrival. The horizontal line profiles shown on the bottom are vertically averaged over the span of the velocity maps shown and the dashed, gray line is a profile from a nonpatterned, driven sample.

Figure 3

Temporal evolution of the 100-μm perturbation. Each profile is plotted on separate, identical axes. Indicated time is relative to the drive beam.

Figure 4

Temporal evolution of the velocity amplitude for the three harmonics observed in the data.