We explored the design space for testing "corona fusion" targets on the National Ignition Facility (NIF) via hydrodynamic simulation. In a corona fusion target, laser beams directly ablate a layer (liner) of fusion fuel on the inner surface of a hohlraum. As the ablative flows converge, the plasma particles transition from long-range interactions to collisional stagnation, heating the ions and generating fusion reactions. These targets show promise as neutron sources and provide a unique opportunity to study the interaction of converging plasma jets as they transition from kinetic to hydrodynamic interaction.
We studied three classes of corona fusion targets: vacuum targets with deuterated-plastic liners, gas-filled targets, and gas-filled targets with a cryogenic deuterium-tritium (DT) layer. Simultaneously, three target geometries were explored: cylinders, spheres with two-sided illumination, and spheres with one-sided illumination. Our hydrodynamic simulations showed that the initial interaction of the converging jets in vacuum targets was collisionless and that adding gas to the target increased collisionality and led to hydrodynamic behavior. With DT gas, simulations predicted thermonuclear yields up to 8 × 1015 neutrons on NIF. These yields scaled linearly with laser power and were not sensitive to design details. the simulations showed that targets with a cryogenic DT liner can reach yields of 1.8 × 1016 neutrons, which is comparable to the highest yields achieved on NIF to date but much lower than predicted by other researchers. Future research will focus on detailed proposals for NIF experiments and kinetic simulations to explore physics not included in hydrodynamic design codes.
The transition of a plasma from collisionless to fluid-like interaction is an important high-energy-density (HED) physics problem in astrophysics and in inertial confinement fusion (ICF). It has been studied computationally and experimentally for decades. The new line of investigation described here began when other researchers (Ren et al. 2017) described neutron source development experiments on the ShengGuang-III prototype laser facility. This group generated 3.5 × 109 neutrons by illuminating the inside of a deuterated plastic sphere. They also developed a scaling law that predicts that as much as 3.2 × 1017 neutrons could be generated using cryogenic DT targets and 1.8 MJ of laser energy on NIF.
The fusion technique used by Ren et al. (2017), known as "spherically convergent plasma fusion," is not new; it was tried earlier and known as "inverted corona fusion" (Bessarab et al. 1992). In a corona fusion target, laser beams directly ablate a layer of fusion fuel on the inner surface of a hohlraum. As the ablative flows converge, the plasma particles transition from long-range interactions to collisional stagnation, heating the ions and generating fusion reactions. This is different from a typical ICF target, where the fusion fuel is contained in a capsule that is ablated from the outside.
The novelty of the approach described here is to use nuclear diagnostics to probe the state of the reacting ions while simultaneously using optical and x-ray diagnostics to probe the state (density and temperature) of the electrons. This is possible because the reacting plasma evolves over a timescale much greater than the time resolution of the instruments. Measurements can be compared to predictions from radiation hydrodynamic codes and to other, more sophisticated models such as collisional particle-in-cell to constrain models for stopping power, plasma interpenetration, plasma diffusion, thermal conduction, and electron-ion equilibration. This line of research has already been pursued on NIF using planar targets (Ross et al. 2017) with the goal of studying how ions interact and become isotropic in a fully collisionless regime relevant to astrophysical jet interactions. We extended this concept to converging flows, which are more relevant to ICF and to neutron source development.
The goal of our project was to explore the design space of corona fusion targets and then field them on NIF. Three types of targets were studied: vacuum targets with a deuterated polyethylene plastic liner; deuterium or DT gas-filled targets with a plastic liner; and targets with a cryogenic DT layer (necessarily gas filled). Within these target types, three geometries were explored: (1) cylindrical targets, which provide diagnostic access to the reacting fuel; (2) spherical two-sided targets; and (3) spherical one-sided corona fusion targets. Spherical targets generate neutron flows that are more convergent than cylindrical targets and generate higher density plasmas on stagnation. Spherical targets also enable single-sided illumination (i.e., targets whereby the lasers enter the hohlraum cavity through a single orifice. This geometry has obvious advantages for neutron source applications, as the target can be placed very close to the object under test. The figure depicts schematic diagrams of the three illumination geometries considered.
The deliverable of this effort is a design package: a suite of radiation hydrodynamic simulations using the radiation hydrodynamic HYDRA design code (Marinak et al. 2001). The package contains sensitivity studies to design parameters: target size and geometry, laser energy, and fuel composition. The design package includes comparisons to the Ren et al. (2017) experiments. An additional objective added to the scope of research was to investigate single-sided targets for use as neutron backlighters.
Our HYDRA-based simulations established a set of parameters for both vacuum and gas-filled targets of different geometries. To test their validity, we compared HYDRA design simulations to the data reported from the ShengGuang-III prototype experimental study (Ren et al. 2017). Comparison of the HYDRA-based simulations on all three types of targets indicates that the simulated thermonuclear yields that we achieved are plausible, but more experiments are needed to optimize the targets as neutron sources on NIF.
Our research supports the NNSA goal of advancing the science, technology, and engineering competencies that are the foundation of the NNSA mission. Our research also addresses Lawrence Livermore National Laboratory's research and development challenge in materials science, as well as its core competencies in advanced materials and manufacturing, and HED science.
The initial results described here have motivated further study by Livermore scientists and external collaborators, including proposed experiments to study converging ablation plasmas through the NIF Discovery Science program and a full-scale exploratory research project, "DT Gas-Filled Hohlraums as a High-Yield Neutron Source."
We explored the design space of corona fusion targets for NIF using HYDRA-based hydrodynamic simulations. Several key conclusions can be drawn from this feasibility study.
Now that the design space has been broadly explored, future research will focus on developing proposals for continued research on NIF. Kinetic calculations using the CHICAGO code may be conducted that include simulations of the spherical one-sided corona fusion targets conducted by other researchers. Experiments at the NIF scale are needed to address the remaining questions about the utility of using corona fusion targets as neutron sources and to provide data relevant to ICF and HED science.
Bessarab, A. V., et al. 1992. "Results of First Experiments with Fusion Targets at the Iskra-5 High-Power Laser Installation." Journal of Experimental and Theoretical Physics 75(6): 970.
Marinak, M. M., et al. 2001. "Three-Dimensional HYDRA Simulations of National Ignition Facility Targets." Physics of Plasmas 8(5): 2275–89. doi: 10.1063/1.1356740.
Ren, G., et al. 2017. "Neutron Generation by Laser-Driven Spherically Convergent Plasma Fusion." Physical Review Letters 118(16): 165001. doi: 10.1103/PhysRevLett.118.165001.
Ross, J. S., et al. 2017. "Transition from Collisional to Collisionless Regimes in Interpenetrating Plasma Flows on the National Ignition Facility." Physical Review Letters 118(18): 185003. doi: 10.1103/PhysRevLett.118.185003. LLNL-JRNL-700999.
Riedel, W. M., et al. 2018. "Collisionality and Kinetic Effects in Converging Fully-Ionized Plasma Jets." 60th Annual Meeting of the APS Division of Plasma Physics, November 2018, Portland, OR. LLNL-POST-760640.