Fast-formed liquid surfaces for inertial confinement fusion target shells

Advanced ICF targets will have an inner layer of solid or liquid fuel. Their inner surfaces must be smooth and contamination free. All of the current means to produce such a surface have problems: Liquid surfaces sag, solid surfaces tend to facet, and polymer-foam-stabilized surfaces are contaminated by carbon from the foam. An alternative may be to generate a liquid surface immediately before a shot by rapid thermal expansion of a fuel-saturated foam-walled capsule. This approach makes use of the large coefficient of expansion of liquid hydrogen relative to its foam matrix. The shell is filled by exposure to hydrogen vapor during cooling; liquid in the foam has a lower vapor pressure than free liquid, so the shell will fill to exactly 100%. It will stay at that fill fraction as the shell cools and the density of the liquid it contains increases. The shell may be frozen and cooled to 4 K so that it can be stored and handled in vacuum. When the shell is warmed, the liquid expands; the elastic modulus of the foam will force some liquid out of the surface. A simple analysis suggests that a 1 μm thick liquid film might be generated in 1 μs; that depends on the compressibility of the foam and the flow resistance of its cell structure. Surface tension would smooth this surface layer very rapidly. It would not begin to sag for 1000 μs, so there would be sufficient time during which the layer would be satisfactory. An analysis will be presented showing the feasibility of this approach, and the constraints it puts on shell wall structure and insertion-and-shot procedures