Renderings of the DT ice-gas interface (a) and outer ablator surface (b) used as initial conditions for the 3-D HYDRA simulation of a NIF shot on March 21, 2012. In each, the color scale shows the deviation from the mean radius of the interface. Measured low mode and high mode surface finish data are incorporated in generating each surface. A large 3-D ice groove is apparent in (a), while the dark blue bands encircling the poles in (b) are due to the surrogate tent perturbation model.
(Download Image)New NIF Implosion Models Reduce Discrepancies
New simulations that include recently identified effects which degrade the performance of NIF implosions have significantly narrowed the gap between modeling and experimental results, LLNL researchers reported in a Physics of Plasmas paper published online on Feb. 4.
The updated suite of one-dimensional (1-D), 2-D, and 3-D simulations include the current best understanding of effects identified since the National Ignition Campaign (NIC) concluded in September 2012. The effects could explain discrepancies between previous post-shot simulations of implosion performance and experimentally measured performance, particularly in neutron yield.
The new models were used to analyze a NIC experiment conducted on March 21, 2012—the highest compression shot yet fired on NIF and an important experiment for understanding the behavior of the high-compression implosions necessary to achieve ignition at the NIF energy scale. The updated simulations reduce the simulated-to-measured neutron yield ratio from 125:1 in 1-D to 1.5:1 in 3-D, as compared to 15:1 in the best 2-D simulations published previously.
Along with the experimental campaign conducted during NIC, a concerted effort was made to model NIC experiments in as much detail as possible using the HYDRA radiation hydrodynamics code. Across the full range of pulse shapes and target geometries explored during NIC, these detailed post-shot simulations consistently overpredicted implosion yields by factors ranging from three to 10. The discrepancy in yield was consistent and applied even in the case of state-of-the-art, high-resolution, 3-D simulations.
Subsequent NIF experiments have identified several important effects—absent in the previous simulations—that have the potential to resolve at least some of the large discrepancies between simulated and experimental yields. They include larger than anticipated low-mode (long-wavelength) distortions of the imploded core, due primarily to asymmetries in the x-ray flux incident on the capsule; larger than anticipated perturbations to the implosion caused by the thin plastic membrane or "tent" used to support the capsule in the hohlraum before the shot; and the presence, in some cases, of larger than expected amounts of ablator material mixed into the hot spot.
"Once these large effects are properly accounted for," the researchers said, "the discrepancies found with previous simulations are largely resolved. While the agreement with the experimental data remains imperfect, comparison to the data is significantly improved and suggests that the largest sources for the previous discrepancies between simulation and experiment are now being included" in the models.
Lead author Dan Clark was joined on the paper by LLNL colleagues Marty Marinak, Chris Weber, Dave Eder, Steve Haan, Bruce Hammel, Denise Hinkel, Oggie Jones, Jose Milovich, Prav Patel, Harry Robey, Jay Salmonson, Scott Sepke, and Cliff Thomas.
