Daniel Clark | 19-FS-033
Radiation hydrodynamics modeling of hohlraum experiments on the National Ignition Facility (NIF) has shown a chronic overprediction of hohlraum performance compared to experiments. Depending on the particular experiment, up to several hundred kilojoules of laser energy must be artificially subtracted from the simulations in order to match experimentally-measured quantities such as the time of peak neutron production (bang time). One hypothesis is that fine-scale hydrodynamic mixing of the high- Z hohlraum wall with the low- Z hohlraum gas, an effect omitted in current hohlraum simulations, is a possible source of some of this degradation.
Due to the complex radiation hydrodynamics that occur in NIF hohlraums, developing a truly predictive model requires coupling multiple physics effects in high resolution simulations: laser propagation and absorption, non-local thermodynamic equilibrium atomic physics, hydrodynamics, radiation transport, and laser–plasma and hydrodynamic instabilities.We developed a preliminary computational assessment using a spherical hohlraum model. Our research demonstrated that the wall–gas interface is 1) unstable during certain phases of the experiment, and 2) has the potential for turbulent mixing of the gas and wall material, depending on the hohlraum gas fill and pulse shape. We also addressed the importance of fine-scale mixing at the hohlraum wall on the hohlraum x-ray conversion efficiency. Our simulations indicated that mixing at this interface can delay the simulated bang time by up to 160 ps with high gas fills but appears negligible for low gas fills.
Impact on Mission
Developing a predictive model for laser-driven hohlraums at Lawrence Livermore National Laboratory's NIF is a major 2020 goal specified in the Department of Energy's 2016 Inertial Confinement Fusion Program Framework.