IT will take a community of workers to bring the goals of the National Ignition Facility (NIF) to fruition. While national attention has been focused on the funding and construction of the 192-beam laser facility, scientists at Lawrence Livermore are working on myriad problems whose solutions are necessary to NIF's success. A group of materials scientists, for example, is developing techniques to produce round, hollow shells about 2 millimeters in diameter--smaller than BB-gun pellets. This work seems incongruous in a project dominated by a football-stadium-size facility. But when filled with deuterium or deuterium-tritium fuel, these shells become the targets for NIF's inertial confinement fusion (ICF) experiments. The goal of these experiments is to create fusion ignition--intense temperatures and pressures like those at the centers of stars for a small fraction of a second.
Steve Letts and Evelyn Fearon of the Laser Programs Directorate's Target Area Technology Program are among the materials scientists continuing Lawrence Livermore's more than 20 years of research and development on laser targets. Their focus now is on targets for NIF experiments. With 40 times more energy and 10 times more power than Nova (currently the world's largest operating laser), NIF will require targets about 2 millimeters in diameter, 4 times larger than those used previously, which are about half a millimeter in diameter.
The increased shell size must be achieved in tandem with making the shell very smooth and symmetrical. During an ICF experiment, extremely high laser energies are absorbed by the fuel capsule, causing the capsule wall to blow off with such tremendous force that the fuel inside is compressed to very high density. This compression, which must be as uniform as possible, is necessary for ignition. Any capsule surface or shape irregularities constitute perturbations that will grow in amplitude during implosion, because of hydrodynamic (Rayleigh-Taylor) instabilities (see Energy & Technology Review, April 1995, pp. 1-9). The perturbations cause the inner wall of the capsule to mix with the fuel, cooling it and thereby degrading efficiency.

The Progression of ICF Targets
Letts and Fearon's technique for making shells uses an entirely new approach. Previously, plastic shells were produced when droplets of polystyrene solution were dropped down a heated drop tower, where evaporation first caused a skin to form on the droplets and then further vaporization of the solvent inside the skin caused the droplets to expand into hollow shells.
Because the drop-tower technique produced shells of a limited size range, researchers tried micro-encapsulation techniques to increase shell sizes. They encapsulated droplets of water in a polymer solution suspended in an aqueous phase; the solvent containing the polymer would slowly dissipate into the aqueous phase, leaving behind a polymer shell. However, the resulting shells were uneven in thickness and had bubbles in their walls. Steve Letts explains that these techniques frequently "wouldn't produce round shells most of the time, so those that were round would have to be carefully picked out--not an easy task with such tiny things."
He came up with a new idea. While measuring mass loss in polymers when they were heated, he identified one polymer material that evolved into a gas when heated to about 300°C, disappearing cleanly without any trace or residue. He figured out a way to take advantage of the material's unique combination of characteristics.
That material was poly(alpha-methylstyrene), or PAMS. In Letts's new fabrication method, an amount of PAMS is shaped into a smooth sphere, or mandrel, which is overcoated with a thermally stable plasma polymer to a desired thickness. The overcoated mandrel is heated to about 300°C, at which temperature the PAMS depolymerizes (decomposes) into a gas, diffuses through the plasma polymer overcoat (which is thermally stable up to 400°C), and leaves behind a hollow plasma polymer shell (see the figure below).
Letts postulated that this method would be feasible for producing fuel capsules of the size needed for NIF if a suitable PAMS mandrel could be formed. In addition, because the shell is built outward from the PAMS mandrel, it might be feasible to incorporate various layers during the overcoating process, which would be useful for diagnosing shell performance. The method would be successful if good quality mandrels could be made, an even overcoat could be deposited on the mandrel, and pyrolysis (heat treatment) could be accomplished without distorting or collapsing the resulting shell.






Spherical, Smooth Mandrels
Evelyn Fearon coordinated PAMS mandrel production. She and the other fabricators ground commercial PAMS beads into smaller sizes, put them through a sieve, and suspended them in a water solution hot enough to soften them, thus taking advantage of surface tension to pull the bead into a sphere. Bead surfaces were smoothed further by exposing them to solvent vapor while dropping them down a heated column. During the drop, the bead's thin surface layer dissolved and dried, leaving a surface roughness of less than a billionth of a meter (as measured by an atomic-force microscope).
Smooth, spherical bead mandrels were fairly easy to make. However, they tended to distort from the heat generated during overcoating and become nonsymmetrical or coat unevenly. To overcome the heat effects, Fearon experimented with higher molecular weight PAMS and lowered the overcoating temperature, but the adjustments did not wholly overcome the distortion problem.
The Target Area group turned to hollow mandrels made by micro-encapsulation and supplied by General Atomics of San Diego, California, another DOE contractor. Hollow mandrels have two advantages. They contain less PAMS to depolymerize, and thus, less force is exerted on the overcoat during depolymerization. Second, higher molecular weight PAMS (96,000 versus 11,000 for beads) can be used to make them, because, unlike the beads, they do not need hot-water softening, which requires the lower molecular weight material. Because they are ultimately depolymerized, some wall unevenness and internal bubbles are tolerable, as long as the shells are spherical and their outer surface finishes are smooth. Compared with bead mandrels, the hollow mandrels have shown far less distortion during overcoating and pyrolysis.

An Even Coat of Plasma Polymer
Plasma polymer is well suited to be fuel-shell material. It is transparent, which allows fusion experimenters to diagnose the contained fuel layer. It can be used to coat the mandrels because it can withstand PAMS pyrolysis temperatures and is permeable to the gaseous, depolymerizing PAMS.
In the coating technique used by the Target Area group, the mandrels are agitated in a bouncing pan in the plasma coating chamber or, in a variation, rolled in a tilted, slowly rotating pan until they are evenly coated (see the opening figure at top). The crucial variables determining even coating are just the right amount of agitation and the correct (not-too-high) temperature.

Successful Pyrolysis
During pyrolysis, the shells can collapse, burst, deform, or shrink. While collapse is mainly caused by nonuniform coating, the other problems result from thermal effects. To avoid them, the researchers devised a temperature program that controls the rate of PAMS decomposition. It consists of raising the pyrolysis temperature by 10°C every minute until 200°C is reached, holding it there for 30 minutes to allow low-temperature volatiles to escape, and then ramping it up by 0.2°C every minute up to 300°C, where it is held for 30 hours or more, depending on the size of the shell. The plasma polymer shrinks gradually and uniformly during pyrolysis, and thus sphericity is maintained. Experimenters observe and measure the shrinkage only to predict the size of a completed shell.
An optical microscope is used to measure the wall thickness and diameter of pyrolyzed shells, a scanning electron microscope is used to determine how smooth and free of particle defects shell surfaces are, and an atomic-force microscope is used to make detailed measurements of the sphericity and roughness of the shell.

Challenges Ahead
The techniques described here have now been adopted by General Atomics as the preferred method for making 0.5-millimeter-diameter capsule targets for Nova ICF experiments at Livermore and 0.9-millimeter-diameter capsules for ICF experiments at the Omega Laser facility at the University of Rochester. The success in moving this research proof of principle to actual target production is certainly encouraging. However, significant challenges still face Livermore's laser-target scientists.
Currently the development efforts at Lawrence Livermore are focused on adapting the technology developed by Letts and Fearon to the production of 2-millimeter-diameter capsules for NIF. This effort has two parts. The first, being led by Ken Hamilton, is to develop micro-encapsulation techniques to form PAMS microshells with the required outer surface sphericity and surface finish. To meet NIF specifications, these shells must be no more than 1 micrometer, or a millionth of a meter, out of round; that is, the radius to the outer surface can vary by no more than 1 micrometer (out of 1,000) as one moves across the surface. Solving this extremely difficult problem will require significant improvements in current micro-encapsulation technology. Once it is solved, the second part will be to maintain the sphericity of the shell through the coating and thermal treatment to remove the PAMS.
Members of the Laboratory's Target Area Technology Program will continue to refine laser target technology. Beyond making targets for current ICF experiments, they must focus on developing targets for the real NIF event--ignition. The PAMS technique is being investigated for that use.

--Gloria Wilt

Key Words: laser target, National Ignition Facility (NIF), inertial confinement fusion (ICF), fuel capsule, plasma polymer, polymer shell, micro-encapsulation, poly(alpha-methylstyrene) (PAMS), hydrodynamic instability.

For further information contact Steve Letts (510) 422-0937 (letts1@llnl.gov) or Evelyn Fearon (510) 423-1817 (fearon2@llnl.gov).


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