Abstract Details
HIGH-MODE RAYLEIGH-TAYLOR GROWTH IN NIF IGNITION CAPSULES
Author: Bruce A Hammel
Requested Type: Oral Only
Submitted: 2009-04-20 18:08:56
Co-authors: S. W. Haan, D. Clark, M. J. Edwards, S.H. Langer, M. Marinak, M. Patel, J. Salmonson, and H.A. Scott
Contact Info:
Lawrence Livermore National Laboratory
7000 East Ave
Livermore, CA 94550
USA
Abstract Text:
Controlling the hydrodynamic growth of high-mode perturbations is essential in the optimization of NIF ignition target designs. On the outer ablator surface, mode numbers up to ~300 (λ ~20 µm) have significant growth, and are particularly important for assessing the impact of roughness on the surface of CH capsules where the dominant unstable mode is ~170. At the internal ablator:fuel interface, where very short wavelength perturbations can grow at the steep gradients (set by thermal conductivity in the cold-dense fuel and ablator), mode numbers up to ~2000 (λ ~ 3 µm) are important. In typical designs, roughness at this interface (inner ablator surface) leads to mixing of ablator material into the outer ~10 – 30% of the cold dense fuel mass. In addition, perturbations on the inner DT fuel surface, which can include grooves at crystal boundaries, seed short wavelength growth at the ablator:fuel interface. Finally, “isolated features” on the capsule, such as the “fill-tube” (~ 5 µm scale length) and defects, can seed short wavelength growth at the ablation front and the ablator:fuel interface, leading to the injection of a small amount (~ 10’s ng) of ablator material into the central hot spot. In general, for these studies, we find that accurate models for the physical data, such as material equation of state (compressibility) and thermal conductivity, are important, as they influence the magnitude of the simulated high-mode growth.
To benchmark the simulations, we are developing methods to measure high-mode mix on NIF implosion experiments prior to the actual ignition experiments. Mix into the hot spot will result in observable x-ray emission from the ablator material (Be or CH) since it is doped with high-Z (Cu or Ge). The hot spot temperatures (~3 - 7 keV in pre-ignition targets) will ionize the dopant material to the K-shell, resulting in line emission (hυ > 8 keV) that is sufficiently energetic to escape the high areal density target.
We are also assessing spectroscopic methods for measuring mix at the ablator:fuel interface. Simulations indicate that a buried high-Z (e.g. Fe) doped layer near the interface, where the temperature is low (Te ~ 50 eV), will be visible by absorption spectroscopy when the interface perturbation grows and the doped layer is spread to larger radius into the ablation region, where the temperature is higher (Te > 200 eV). The increased temperature is sufficient to create vacancies in the L-shell, resulting in strong absorption on the screened K-shell transitions (1s22s22pm – 1s2s22pm+1).
These optimization studies are performed in 2D and 3D using massively-parallel HYDRA. Simulations of the spectroscopic signatures are performed with CRETIN
This work was completed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
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