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
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
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 (firstname.lastname@example.org) or Evelyn Fearon (510) 423-1817 (email@example.com).