The survivability and performance of the debris shields on the National Ignition Facility (NIF) are a key factor for the successful conduct and affordable operation of the facility. The improvements required over Nova debris shields are described. Estimates of debris shield lifetimes in the presence of target emissions with 4-5 J/cm2 laser fluences (and higher) indicate lifetimes that may contribute unacceptably to operations costs for NIF. We are developing detailed guidance for target and experiment designers for NIF to assist in minimizing the damage to, and therefore the cost of, maintaining NIF debris shields. The guidance limits the target mass that is allowed to become particulate on the debris shields (300 mg). It also limits the amount of material that can become shrapnel for any given shot (10 mg). Finally, it restricts the introduction of non-volatile residue (NVR) that is a threat to the sol-gel coatings on the debris shields to ensure that the chamber loading at any time is less than 1 pg/cm2. We review the experimentation on the Nova chamber that included measuring quantities of particulate on debris shields by element and capturing shrapnel pieces in aerogel samples mounted in the chamber. We also describe computations of x-ray emissions from a likely NIF target and the associated ablation expected from this x-ray exposure on supporting target hardware. We describe progress in assessing the benefits of a pre-shield and the possible impact on the guidance for target experiments on NIF. Plans for possible experimentation on Omega and other facilities to improve our understanding of target emissions and their impacts are discussed. Our discussion of planned future work provides a forum to invite possible collaboration with the IFE community.
The late-time (t ≥ 80 ns) behavior of hohlraums designed for the National Ignition Facility (NIF) is simulated using the multiphysics radiation hydrodynamics codes LASNEX and HYDRA. The spatial distribution of x-radiation outside the hohlraum is shown as a function of time. The energy spectrum of the x-ray emission is presented for various hohlraum viewing angles. We have made refinements to the grid motion algorithms in the arbitrary Lagrangian Eulerian (ALE) hydrodynamics code HYDRA to obtain the first late-time simulations of a hohlraum that can be used to give the spatial distribution of the vaporized hohlraum wall. The importance of late-time simulations in determining the lifetimes of debris shields on NIF is discussed.
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