Thinshell symmetry surrogates for the National Ignition Facility: A rocketequation analysis

Several techniques for inferring the degree of flux symmetry in indirectly driven cylindrical hohlraums have been developed over the past several year s for eventual application to the National Ignition Facility (NIF) [Paisner , Laser Focus World 30, 75 (1994)]. These methods use various ignition cap sule surrogates, including non-cryogenic imploded capsules [Hauer , Phys. P lasmas 2, 2488 (1995)], backlit aerogel foamballs [Amendt , Rev. Sci. Instr um. 66, 785 (1995)], reemission balls [Delamater, Magelssen, and Hauer, Phy s. Rev. E 53, 5240 (1996)], and backlit thinshells [Pollaine , Phys. Plasma s 8, 2357 (2001)]. Recent attention has focussed on the backlit thinshells as a promising means for detecting higher-order Legendre flux asymmetries, e.g., P6 and P8, which are predicted to be important sources of target perf ormance degradation on the NIF for levels greater than 1% [Haan , Phys. Pla smas 2, 2490 (1995)]. A key property of backlit thinshells is the strong am plification of modal flux asymmetry imprinting with shell convergence. A si mple single-parameter analytic description based on a rocket model is prese nted which explores the degree of linearity of the shell response to an imp osed flux asymmetry. Convergence and mass ablation effects introduce a mode st level of nonlinearity in the shell response. The effect of target fabric ation irregularities on shell distortion is assessed with the rocket model and particular sensitivity to shell thickness variations is shown. The mode l can be used to relate an observed or simulated backlit implosion trajecto ry to an ablation pressure asymmetry history. Ascertaining this history is an important element for readily establishing the degree of surrogacy of a symmetry target for a NIF ignition capsule. (C) 2001 American Institute of Physics.