Reduced instability growth with high-adiabat high-foot implosions at the National Ignition Facility

Hydrodynamic instabilities are a major obstacle in the quest to achieve ignition as they cause preexisting capsule defects to grow and ultimately quench the fusion burn in experiments at the National Ignition Facility. Unstable growth at the ablation front has been dramatically reduced in implosions with “high-foot” drives as measured using x-ray radiography of modulations at the most dangerous wavelengths (Legendre mode numbers of 30–90). These growth reductions have helped to improve the performance of layered DT implosions reported by O. A. Hurricane et al. [Nature (London) 506, 343 (2014)], when compared to previous “low-foot” experiments, demonstrating the value of stabilizing ablation-front growth and providing directions for future ignition designs.

Figure 1

(a) Schematic diagram of instability growth at several different radii showing density contours from hydra simulations. Also shown is a simulated stagnated hot spot showing the consequences of instability growth. (b) Schematic of an indirectly driven hydrogrowth radiography target. The capsules are machined with sinusoidal perturbations and backlit using a V foil x-ray backlighter.

Figure 2

(a) Laser pulse shape and (b) radiation drive temperature measured on two NIF shots, one shot with the low-adiabat (or low-foot, black curve) and the other with the high-adiabat (or high-foot, red curve) drive.

Figure 3

(a) Slit image radiograph of side-by-side, preimposed, single-mode ripples of $\ell$ = 60 and 90, for the low-foot pulse at 20.6 ns, which corresponds to a radius of ∼613 μm. (b) Horizontal line-out in optical depth of the slit image shown in (a). (c) Also shown is a slit image radiograph at 14.8 ns, driven with the high-foot pulse, which corresponds to a radius of ∼600 μm for the same side-by-side modes. (d) Horizontal line-out in optical depth of the slit image shown in (c).

Figure 4

(a) Optical depth (OD) modulation amplitude of the fundamental harmonic as a function of wavelength (the top axis shows the equivalent radius) for $\ell$ = 60 for both the low-foot (black) and high-foot (red) drives observed with a 3.4 μm deep (peak-to-valley) modulation initial amplitude. The curves are hydra simulations of OD amplitude, while the data are represented by symbols. The shaded bands are included on simulations (solid curves) to illustrate the sensitivity to uncertainties in the drive and the simulation methodology. (b) OD amplitude for $\ell$ = 60 with 0.5 μm initial amplitudes. (c) OD amplitude for $\ell$ = 90 with 0.6 μm initial amplitudes.

Figure 5

(a) Measure and simulated OD growth factors for low-foot (black) and high-foot (red) drives, plotted as a function of mode number at an imploded radius of ∼630 μm. (b) Density (solid curves) and pressure (dashed curves) profiles simulated for layered DT implosions driven by the low-foot (black curve) and the high-foot (red curve) drives using the code hydra [34]. The ablation front is near the point of maximum pressure. The simulated profiles clearly show a longer ablation-front scale length, a lower peak density, and a lower in-flight aspect ratio for the high-foot drive, all properties that enhance stability.