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

Target designs are described that are meant to achieve ignition on the National Ignition Facility. Simulations of recent indirect drive cryogenic capsule designs 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 DT-ice 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 in different regions of the space, 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 'graded dopants.' In this type of capsule the mid-Z dopant, which is needed in the ablator to minimize x-ray preheat at the ablator-ice interface, ismore » 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.« less