Multimode short-wavelength perturbation growth studies for the National Ignition Facility double-shell ignition target designs

Abstract

Detailed multimode two-dimensional simulations of short-wavelength perturbations imposed on the material interfaces of a recently proposed indirect-drive double-shell ignition target [Amendt et al., Phys. Plasmas 9, 2221 (2002)] are presented. In this work, the effect of roughness imposed only on the surfaces of the inner shell is studied. Realistic perturbations are adopted from a measured spectrum of a glass capsule (as a surrogate for the high-Z inner shell). It is found that perturbing the inner surface of the inner shell shows minimal degradation in capsule performance. On the other hand, when roughness is imposed on the outer surface of the inner shell, the growth of large Legendre mode number perturbations (l>200) leads to shell breakup. Further analysis reveals a new pathway for the Rayleigh–Taylor (RT) instability. L-shell radiation (>8 keV) from the high-Z hohlraum wall ablates the outer surface of the high-Z inner shell, promoting large outward expansion which is reversed by the converging outer shell. The classic conditions for RT instability are met: low density material pushing onto the higher density inner shell. It is shown that this effect can be controlled by tamping the outward expansion of the inner shell with a variety of materials. Simulations with separate CH and Ti tampers demonstrate that the redesigned capsule can withstand perturbations with high mode number content without exhibiting shell breakup. Furthermore, the outstanding question of determining the cutoff mode number (lc) is addressed by performing simulations with successively larger maximum l, reaching values beyond 1000, and calculating the mix width of the pusher/tamper interface for the CH-tamped case. These numerical studies suggest that the mix width approaches a constant value close to 40% of the shell width at peak compression. While not a proof that lc has been found, this result suggests that a mix-relevant mode number may be within reach of current simulation capabilities.