Development and modeling of a polar-direct-drive exploding pusher platform at the National Ignition Facility

Abstract

High-intensity laser facilities, such as the National Ignition Facility (NIF), enable the experimental investigation of plasmas under extreme, high-energy-density conditions. Motivated by validating models for collisional heat-transfer processes in high-energy-density plasmas, we have developed an exploding pusher platform for use at the NIF in the polar-direct-drive configuration. The baseline design employs a 3 mm-diameter capsule, an 18 μm-thick CH ablator, and Ar-doped D2 gas to achieve several keV electron-ion temperature separations with relatively low convergence ratios. In an initial series of shots at the NIF—N160920–003, -005, and N160921–001—the ratio of the laser intensity at different polar angles was varied to optimize the symmetry of the implosion. Here we summarize experimental results from the shot series and present pre- and post-shot analysis. Although the polar-direct-drive configuration is inherently asymmetric, we successfully tuned a post-shot 1D model to a set of key implosion performance metrics. The post-shot model has proven effective for extrapolating capsule performance to higher incident laser drive. Overall, the simplicity of the platform and the efficacy of the post-shot 1D model make the polar-direct-drive exploding pusher platform attractive for a variety of applications beyond the originally targeted study of collisional processes in high-energy-density plasmas.

Simulation and flow physics of a shocked and reshocked high-energy-density mixing layer

“…Also, their cases involved a converging geometry and pressures and temperatures orders of magnitude larger than those in the present study. Hence, the impact of neglecting barodiffusion and thermodiffusion is expected to be less in the present study than in Ellison et al (2018). We stress that the assessment of simplified models of mass diffusion in HED flows remains an active area of research.…”

“…It is a considerable simplification of the physics of multispecies diffusion, and it is expected to be most accurate for mixtures of ions with similar masses and charges. Ellison et al (2018) found that it gave reasonable order-of-magnitude results for an ICF-relevant flow. Specifically, they reported that diffusive mixing between various combinations of five atomic species was larger, in a majority of cases, when using the OCP approximation instead of higher-fidelity approaches.…”

“…Generally, barodiffusion is significant when pressure gradient magnitudes are large and when the atomic weights of the diffusing species are very discrepant, and thermodiffusion is significant when temperature gradient magnitudes are large. However, in the cases considered by Ellison et al (2018), the impact of neglecting these two contributions appeared to be comparable to or less than the impact of grouping species into one or more subsets (as done in the OCP approximation). Also, their cases involved a converging geometry and pressures and temperatures orders of magnitude larger than those in the present study.…”

“…Thus, D is treated as a mixture-averaged self-diffusion coefficient; binary diffusion coefficients D ab for each a-b species pair are never calculated. Ellison et al (2018) have studied this approach, which they call the one-component plasma (OCP) approximation. It is a considerable simplification of the physics of multispecies diffusion, and it is expected to be most accurate for mixtures of ions with similar masses and charges.…”

Effect of Impurity Atoms on Thermonuclear Yield

“…Relatively large (3-mm diameter) think capsules are being developed for PDD exploding pusher studies with the aim of creating high ion temperature (>10 keV) conditions for plasma physics studies and as neutron sources. NIF experiments using plastic capsules were well-modeled by one-dimensional simulations [113]. Higher gas-fill pressures at room temperature will increase the yield of the platform and increase the design space for optimization.…”