Bernard Kozioziemski (15-ERD-063)
Abstract
Very-low-density foams have been proposed for use in Inertial Confinement Fusion targets on advanced fusion-class lasers to stabilize the configuration of the deuterium–tritium fuel capsule. This project focuses on developing the underlying science needed to understand the solidification process of hydrogen (deuterium–tritium) fuel layers in foams to optimize Inertial Confinement Fusion target designs by minimizing the foam layer surface roughness for a given central vapor pressure and foam density. The foam material must have a high uniformity for ideal implosion for fusion. Confined geometries, such as nanometer-scale pores in vycor glass, have been shown to suppress solidification. A previous study examined, in part, the solidification of hydrogen in open-cell foams, but the results did not match the simple model for thermodynamic suppression of solidification. It is likely that solidification in an open-cell foam geometry proceeds very differently than in the closed-cell geometry. Therefore, we are studying the basic processes that drive solidification in foams, including the roles that the foam geometry, density, pore size distribution, surface area, and material play in the solidification process.
We expect to identify the foam characteristics necessary to control the solidification of hydrogen, which inhibits the smooth fuel layers required for implosion in a laser fusion capsule. These characteristics would be used to define the range of foam densities and materials for which uniform deuterium–tritium fuel layers would be produced. Specifically, we intend to measure hydrogen-fill fraction, vapor pressure in foams and controlled pore glasses, and temperature for solidification. We plan to design, build, and test a cryogenic hydrogen cell for study of solid hydrogen in foam, as well as perform detailed characterization of these foams by high-energy ion scattering, gas sorption, electron microscopy, and calorimetric procedures. We plan to attract a postdoctoral candidate in material sciences to support this research effort.
Mission Relevance
Suppression or enhancement of nucleation by the foam would serve to make smooth, uniformly thick deuterium–tritium fuel layers for inertial-confinement applications, including doping of the foam for uses of ignition, in support of the Laboratory's strategic focus area of inertial fusion science and technology with high-gain target design. In addition, our research aligns with the core competency in high-energy-density science.
FY15 Accomplishments and Results
For FY15 we (1) demonstrated the ability to fill, with liquid deuterium, a foam layer inside the ablator shell used to compress the laser ignition target; (2) tested rapidly freezing liquid deuterium wicked into foam; (3) determined that the layer quality from first experiments showed a poor deuterium surface, with possible voids in the solid from volume contraction upon cooling—the foam shell had also separated from the ablator during measurements; (4) established a collaboration with the Massachusetts Institute of Technology to provide patterned surfaces to test solidification of hydrogen (see figure); and (5) performed first measurements of interfacial energy of hydrogen liquid and solid using localized melting. Melting was observed close to a heated wire, but not localized enough to perform surface-energy measurements.
