Increasing robustness of indirect drive capsule designs against short wavelength hydrodynamic instabilities

Abstract

Simulations indicate dramatically reduced growth of short wavelength hydrodynamic instabilities, resulting from two changes in the designs. First, better optimization results from systematic mapping of the ignition target performance over the parameter space of ablator and fuel thickness combinations, using techniques developed by one of us (Herrmann). After the space is mapped with one-dimensional simulations, exploration of it with two-dimensional simulations quantifies the dependence of instability growth on target dimensions. Low modes and high modes grow differently for different designs, allowing a trade-off of the two regimes of growth. Significant improvement in high-mode stability can be achieved, relative to previous designs, with only insignificant increase in low-mode growth. This procedure produces capsule designs that, in simulations, tolerate several times the surface roughness that could be tolerated by capsules optimized by older more heuristic techniques. Another significant reduction in instability growth, by another factor of several, is achieved with ablators with radially varying dopant. In this type of capsule the mid-Z dopant, which is needed in the ablator to minimize xray preheat at the ablator-ice interface, is optimally positioned within the ablator. A fabrication scenario for graded dopants already exists, using sputter coating to fabricate the ablator shell. We describe the systematics of these advances in capsule design, discuss the basis behind their improved performance, and summarize how this is affecting our plans for NIF ignition.

Convergent ablator performance measurements

“…In general a single radiography measurement on its own has only enough information to determine the 15 where the remaining specific opacity of the shell evolves as successive layers get ablated.…”

National Ignition Facility target design and fabrication

“…The basic physics required for ignition has been detailed by Lindl (Lindl, 1998;Lindl et al, 2004), and specific target designs have been documented by Haan (Haan et al, 2005(Haan et al, , 2006(Haan et al, , 2007a(Haan et al, , 2007b. These designs are for indirect-drive ignition, in which the laser energy is focused onto the inside walls of a hohlraum and converted to x-rays which drive the target capsule.…”

Capsule modeling of high foot implosion experiments on the National Ignition Facility

“…As shown in figure 1, the linear stability of these implosions is quantified by comparing their ablation front growth factors. These growth factors are defined as the ratio of the perturbation amplitude at the time of peak implosion velocity relative to the initial perturbation amplitude for an individual Legendre mode [21][22][23]. For all cases, the simulations were run using a 2D wedge initialized with a small Gaussian bump (15 μm wide by 1 nm in height) on the ablator surface at the axis of symmetry.…”