Stephan A. Letts

Fabrication of polymer shells using a depolymerizable mandrel

A new technique for producing hollow shell laser fusion fuel capsules has-been developed that starts with a depolymerizable mandrel. In this technique we use poly({alpha}-methylstyrene) (PAMS) beads or shells as mandrels which are overcoated with plasma polymer. The PAMS mandrel is thermally depolymerized to gas phase monomer. which diffuses through the permeable and thermally more stable plasma polymer coating, leaving a hollow shell. Using this technique we made shells from 200 {mu}m to 4 mm diameter with 15 to 100 {mu}m wall thickness having sphericity better than 0.5 {mu}m and surface finish better than 10 nm RMS. 13 refs., 5 figs., 1 tab.

Fabrication of low-density foam shells from resorcinol-formaldehyde aerogel

Resorcinol-formaldehyde (RF) aerogel chemistry has been used with encapsulation techniques to fabricate low-density, transparent, foam shells. To accomplish this, the gelation time was reduced from several hours to several minutes by the addition of acid following base-catalyzed RF particle growth. However, additional “annealing” of the gel for at least 20 h was needed to maximize crosslinking and minimize swelling in exchange solvents. Increasing the molar ratio of formaldehyde to resorcinol from 2 to 3 also helped to increase crosslinking. Densification of the foam shells due to dehydration during curing was greatly reduced by judicious choice of immiscible oil phases and by saturating the exterior oil phase during the annealing stage. Shells have been produced with diameters of about 2 mm, wall thicknesses ranging from 100 to 200 μm and foam densities approaching 50 mg/cc.

Infrared redistribution of D2 and HD layers for inertial confinement fusion

We describe a technique to form uniform solid D2 or HD layers for inertial confinement fusion targets. Pumping the infrared (IR) collision induced vibration–rotation band in solid D2 or HD redistributes the solid into a relatively uniform layer depending on the IR intensity profile. Measured redistribution time constants are near the calculated values. We have observed redistribution time constants in HD up to ten times smaller than the DT value.

BERYLLIUM CAPSULE COATING DEVELOPMENT FOR NIF TARGETS

Abstract Various morphologies have been observed in sputter-deposited Be ablator capsules, including nodular growth, cone growth and twisted grain growth. By devising an agitation method that includes both bouncing and rolling the spherical mandrels during deposition, and by reducing the coating rate, consistent columnar grain structure has now been obtained up to 170 mm. Low mode deformation of the shells is observed on thin CH mandrels, but is suppressed if stiffer mandrels are used. Ablator density measured by weighing and x-ray radiography is 93%–95% of bulk density of Be. Transmission electron microscopy shows 100.200 nm size voids in the film and striations inside the grains. Be shells produced with rolling agitation have met most of the NIF specifications. Some of the few remaining issues will be discussed.

Improvements to Formvar Tent Fabrication Using the Meniscus Coater

Abstract The centering of an ignition target capsule strongly depends on high-quality “tents” with closely matched mechanical properties. The relevant properties are tent stiffness and relaxation behavior. Tent stiffness is matched by choosing tents of equal thickness. Here, we describe recent advances in tent fabrication that have increased the quality and production rate of tents. The most significant improvement comes from the use of a meniscus coater to produce Formvar tents of high uniformity and with good control of tent thickness and good yield. Other improvements include a switch to silicon wafers as deposition substrate and standardized tent holders. The improvements have resulted in a sixfold increase of the production rate while increasing the yield by a factor of 2, despite tighter quality control.

Fabrication of special inertial confinement fusion targets using a depolymerizable mandrel technique

A technique was developed for fabricating spherical shell targets for implosion physics experiments with diameters up to several millimeters and with unique structural features such as thin metal layers or texture on the inside surface. We start with a spherical bead or thin shell of poly(alpha‐methylstyrene) (PAMS) of the desired size, which can be textured by laser photoablation or overcoated with a thin layer of diagnostic material. The mandrel is next overcoated with plasma polymer (CH) 2–50 μm thick. Upon heating, the PAMS depolymerizes to a gaseous monomer which diffuses through the thermally stable plasma polymer coating leaving a hollow shell. Shells produced by this technique are uniform in wall thickness, and highly spherical. If the PAMS mandrel is textured, the mandrel topology is transferred to the inner wall of the plasma polymer shell. Likewise, thermally stable coatings on the mandrel are transferred to the inner shell wall.A technique was developed for fabricating spherical shell targets for implosion physics experiments with diameters up to several millimeters and with unique structural features such as thin metal layers or texture on the inside surface. We start with a spherical bead or thin shell of poly(alpha‐methylstyrene) (PAMS) of the desired size, which can be textured by laser photoablation or overcoated with a thin layer of diagnostic material. The mandrel is next overcoated with plasma polymer (CH) 2–50 μm thick. Upon heating, the PAMS depolymerizes to a gaseous monomer which diffuses through the thermally stable plasma polymer coating leaving a hollow shell. Shells produced by this technique are uniform in wall thickness, and highly spherical. If the PAMS mandrel is textured, the mandrel topology is transferred to the inner wall of the plasma polymer shell. Likewise, thermally stable coatings on the mandrel are transferred to the inner shell wall.

Development of Polyimide Ablators for NIF: Analysis of Defects on Shells, a Novel Smoothing Technique and Upilex Coatings

Abstract Over the last three years, LLNL has developed polyimide vapor deposition technology suitable for mandrel overcoating and fabrication of polyimide capsules. Agitated mandrels were overcoated with 4,4’-oxydianiline and pyromellitic dianhydride, and the PMDA/ODA coating was thermally converted to polyimide by heating to 300°C. Shells from this process did not meet smoothness requirements specified by the target designs for the National Ignition Facility (NIF). The defects and the possible mechanism(s) for defect generation were analyzed, and it was determined that surface roughness was the result of shell-pan interaction(s). A post-processing, shell smoothing technique was also developed which simultaneously levitates the shell while exposing it to solvent vapor. Efforts to form Upilex™, a high strength polyimide, using vapor deposition will also be discussed.

Tuning the rheological properties of sols for low-density aerogel coating applications

Coating of cylindrical and spherical surfaces with thin and homogeneous low-density aerogel films requires precise control over viscosity and gel time. If the viscosity is too low, shear forces can damage the growing gel network and prevent the formation of uniform coatings. Using the example of dicyclopentadiene-based polymer gels, we demonstrate that the gelation behaviour can be manipulated by reducing the amount of cross-linking through co-polymerization with a monomer that can only form linear chains. Even small additions of a linear co-polymer (1–10 wt. %) increase the viscosity at the sol–gel transition by several orders of magnitude, and drastically improve the uniformity of gel films formed under the influence of shear. These results are discussed in the context of the classical gel theory.

IR absorptive properties of plastic materials used in ICF capsules

Abstract One approach to improving the quality of the DT ice layer on the inside of a NIF capsule target is to enhance the natural β-layering process by heating the ice with infrared light (IR) tuned to a D2 or DT excitation band. However to do this the IR must pass through the capsule wall, and absorption by the capsule material results in heat generation that is deleterious both in terms of reducing the energy input to the ice as well as increasing the difficulty of symmetrically cooling the capsule. In order to optimize the choice of wavelength we have measured the wavelength dependent transmission properties of IR through the plastic materials we are considering for capsule fabrication. We will present wavelength dependent extinction coefficient data for normal and fully deuterated plasma polymer and vapor deposited polyimide.

Polyimide capsules may hold high pressure DT fuel without cryogenic support for the National Ignition Facility indirect-drive targets

New target designs for the Omega upgrade laser and ignition targets in the National Ignition Facility (NIF) require thick (80 - 100 {micro}m) cryogenic fuel layers. The Omega upgrade target will require cryogenic handling after initial fill because of the high fill pressures and the thin capsule walls. For the NIF indirectly driven targets, a larger capsule size and new materials offer hope that they can be built, filled and stored in a manner similar to the targets used in the Nova facility without requiring cryogenic handling.