X-ray shadow imprint of hydrodynamic instabilities on the surface of inertial confinement fusion capsules by the fuel fill tube

Measurements of hydrodynamic instability growth for a high-density carbon ablator for indirectly driven inertial confinement fusion implosions on the National Ignition Facility are reported. We observe significant unexpected features on the capsule surface created by shadows of the capsule fill tube, as illuminated by laser-irradiated x-ray spots on the hohlraum wall. These shadows increase the spatial size and shape of the fill tube perturbation in a way that can significantly degrade performance in layered implosions compared to previous expectations. The measurements were performed at a convergence ratio of ∼2 using in-flight x-ray radiography. The initial seed due to shadow imprint is estimated to be equivalent to ∼50–100 nm of solid ablator material. This discovery has prompted the need for a mitigation strategy for future inertial confinement fusion designs as proposed here.

Figure 1

Experimental platform. Outer (red) and inner (blue) laser quads; two outer quads above and below the fill tube are repurposed to excite the x-ray backlighter. Inset: x-ray radiograph of fill tube.

Figure 2

Optical density modulation in (a) spatial coordinates, (b) $r$,θ coordinates, (c) lineouts along the equator, pole, and θ at $r=45\mu \mathrm{m}$. Inset: Radiograph for a CH capsule showing expected tent location. The dashed lines in (b) correspond to the ray-traced orientation of the fill tube shadows.

Figure 3

Laser pulse shape for the inner and outer quads. Hohlraum $T_r$ (Dante) reaches ∼100 eV during the toe and rises to 140 eV in the foot. Insets at time 0 at 6.7 ns show the relative capsule radii, initially and at the time of the backlit image.

Figure 4

Left: Fill tube shadow orientation (dashed lines) for the outer quads (red), lower intensity inner quads (blue), and missing backlighter quads (white). Right: Shadow cast by inner (Q26T) and outer (Q32B) laser spots. The length of the umbra for the outer quads scales as ∼2× the fill tube diameter, suggesting a 100-μm effective shadowing diameter for the expanded tube, prior to the capsule convergence by ∼2.

Figure 5

Surface roughness and noise for HDC capsule at CR ∼2. There is no evidence of native roughness seeds with suitable frequency and amplitude to generate the spoke feature.