Multimode Hydrodynamic Instability Growth of Preimposed Isolated Defects in Ablatively Driven Foils

The Nike KrF laser facility was used to study the evolution of isolated defects with characteristic sizes of $<1$ to 10s of $\mu \mathrm{m}$ in laser-accelerated plastic foils. The experimental platform permitted, for the first time, the systematic study of localized perturbation growth, which is inherently multimode, through ablative Richtmyer-Meshkov and Rayleigh-Taylor stages and into the strongly nonlinear regime. Initial target defects were relatively large amplitude, but spatially localized, and emulated tent, fill-tube, and other nonuniformities that are present in inertial confinement fusion capsules. Face-on x-ray radiography indicated initial growth of the perturbation in both depth and width, followed by its apparent closure due to oblique spike growth. Hollow jetlike profiles of laterally expanding, rising, Rayleigh-Taylor bubbles were observed on the rear surface of the target from each isolated defect. Radiation hydrodynamic simulations provided insight into the mechanism of the closure and other features of the bubble and spike evolution specific to isolated defects.

Figure 1

Streaked areal density from (a) experiment and (b) simulation for a $20\mu \mathrm{m}$ deep by $25\mu \mathrm{m}$ wide defect. Plasma density maps of two defects, size as indicated, from fastrad3d (c) at specified times showing formation of the rising bubble and the oblique spikes as well as the lateral bubble expansion responsible for the expanding light area in the streaked image.

Figure 2

Streaked areal density (a) of a target with grooves of dimensions $0.1\times 1$, $2.5\times 6$, and $8.5\times 8.5$ deep ($\mu \mathrm{m}$) $\times$ wide ($\mu \mathrm{m}$). Linear plot (b) of the target areal density perturbation amplitude along the isolated defect centerline. The experimental data (solid line) is compared to RM and RT growth fitting (dotted). (c) Linear RM and RT fitted growth rates as a function of inverse defect width along with simulated fits (dashed). (d) Schematic of the RT growth in a thin layer: Ott’s model [28] for single-mode nonlinear (S-NL); multimode linear (M-L); and multimode nonlinear (M-NL) for an isolated 2D perturbation.

Figure 3

Side-on image at 2 ns of three machined defects (a) showing rear surface jets. Initial isolated defect feature size and location are indicated. A narrow ($500\mu \mathrm{m}$ wide) target shows (b) the machined defect produced a large indentation in the accelerated target at 4 ns. fastrad3d simulations show the same indentation and qualitative appearance in the transmitted backlighter signal (c) corresponding to the simulated plasma density map (d). The laser enters from the right side of the images and self-emission is evident in (b).