Steven R. Buckley

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.

Production and characterization of doped mandrels for inertial‐confinement fusion experiments

A key feature of current inertial‐confinement fusion (ICF) experiments is the incorporation of dopant atoms into the thin polymer microshell which, in a finished ICF capsule, forms its inner wall. These dopants provide a spectroscopic signal during the implosion that can be used to diagnose the degree of mix at the capsule–fuel interface. The high‐Z dopants can also be used to directly image the fuel–pusher interface. The current status of doped mandrel development is reviewed, with a focus on the mandrel surface smoothness. With the development of unique surface mapping characterization tools which will be described, it has been discovered that mandrel surface smoothness is a function of the polymers used to form the mandrels. In this report it will be shown that Cl‐doped mandrels produced from a blend of polystyrene and poly(p‐chlorostyrene) are rough on a length scale of 10’s of microns with amplitudes of as much as a 100 nm. The origin of this roughness will be discussed, and it will be shown that thi...

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.

Temperature and moisture dependence of dielectric constant for silica aerogels

The dielectric constants of silica aerogels are among the lowest measured for any solid material. The silica aerogels also exhibit low thermal expansion and are thermally stable to temperatures exceeding 500{degrees}C. However, due to the open porosity and large surface areas for aerogels, their dielectric constants are strongly affected by moisture and temperature. This paper presents data for the dielectric constants of silica aerogels as a function of moisture content at 25{degrees}C, and as a function of temperature, for temperatures in the range from 25{degrees}C to 450{degrees}C. Dielectric constant data are also given for silica aerogels that are heat treated in dry nitrogen at 500{degrees}C, then cooled to 25{degrees}C for measurements in dry air. All measurements are made on bulk aerogel spheres at 22GHz microwave frequency, using a cavity perturbation method. The results of the dependence found here for bulk materials can be inferred to apply also to thin films of silica aerogels having similar nano-structures and densities.

Resorcinol/Formaldehyde Foam Shell Targets for ICF

Resorcinol/formaldehyde (R/F) low-density foam making processes have been adapted to microencapsulation techniques. This has been done in an effort to make low density, low Z, transparent foam shells for use as cryogenic ICF targets. It was necessary to modify the normal R/F formulation and processing to accelerate the gelation time from tens of hours to less than one hour. Proper selection of the inner and outer oil phase solvents was critical for density matching and prevention of the dehydration of the gelling preform, respectively. 12 refs., 5 figs., 1 tab.

Hollow foam microshells for liquid‐layered cryogenic inertial confinement fusion targets

Future U.S. inertial confinement fusion (ICF) targets will use capsules, 1–2 mm in diameter, with uniform 100 μm thick, cryogenic fuel layers. Research is currently underway to identify optimal methods for producing these thick, uniform layers. One method is to use a spherical polymer foam layer within a full density polymer overcoat to support the fuel. Targets of this type, 0.4–0.8 mm in diameter, with 10–30 μm walls, have been developed by the Institute of Laser Engineering at Osaka University, Japan. Reported here are the results obtained from work to extend the method to the future ICF target design. Overcoated foam shells of the proper dimensions were produced, but their optical properties precluded the use of current diagnostics to determine the amount and uniformity of the fuel fill. Briefly outlined are options for improving the optical properties.

Preparation of hollow shell ICF targets using a depolymerizing model

A new technique for producing hollow shell laser fusion capsules was 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. We have developed methods for controlling the size of the PAMS mandrel by either grinding to make smaller sizes or melt sintering to form larger mandrels. Sphericity and surface finish are improved by heating the PAMS mandrels in hot water using a surfactant to prevent aggregation. Using this technique we have made shells from 200 {mu}m to 5 mm diameter with 15 to 100 {mu}m wall thickness having sphericity better than 2 {mu}m and surface finish better than 10 nm RMS.

Progress Toward Meeting NIF Specifications for Vapor Deposited Polyimide Ablator Coatings

Abstract We are developing an evaporative coating technique for deposition of thick polyimide (PI) ablator layers on ICF targets. The PI coating technique utilizes stoichiometrically controlled fluxes from two Knudsen cell evaporators containing a dianhydride and a diamine to deposit a polyamic acid (PAA) coating. Heating the PAA coating to 300°C converts the PAA coating to a polyimide. Coated shells are rough due to particles on the substrate mandrels and from damage to the coating caused by the agitation used to achieve a uniform coating. We have developed a smoothing process that exposes an initially rough PAA coated shell to solvent vapor using gas levitation. We found that after smoothing the coatings developed a number of wide (low-mode) defects. We have identified two major contributors to low-mode roughness: surface hydrolysis, and deformation during drying/curing. By minimizing air exposure prior to vapor smoothing, avoiding excess solvent sorption during vapor smoothing, and using slow drying we are able to deposit and vapor smooth coatings 160 μm thick with a surface roughness less than 20 nm RMS.

Vapor-Deposited Polyimide Ablators for NIF: Effects of Deposition Process Parameters and Solvent Vapor Smoothing on Capsule Surface Finish

Abstract Recent progress made at LLNL on fabricating NIF scale polyimide capsules using vapor deposition techniques is detailed. Our major focus has been on improving the capsule surf ace finish through better understanding of the origin of surface roughness created during the deposition process and implementation of a post-deposition vapor smoothing procedure prior to imidization. We have determined that the most important factors during the deposition process that impact surface finish include mandrel quality, monomer mixing, selfshadowing, and abrasion. We have shown that high rate deposition (above 10 μm/h) is effective at reducing roughness, which we believe is due to the shorter total time of shell agitation in the bouncer pan. By adjusting the coating conditions, coatings up to 160 μm thick have been reproduc-My fabricated with 300 nm RMS roughness. Solvent vapor smoothing, a new technique also developed at LLNL, further improves the surface to 30 nm RMS.

Pyrolytic Removal Of The Plastic Mandrel From Sputtered Beryllium Shells

Abstract An engineering model is presented for the removal of the plastic mandrel from the inside of a sputtered Be shell. The removal is accomplished by forcing heated air in and out of the 4 to 5 μm laser drilled fill hole in the capsule wall by cycling the external pressure between 2 and 5 atm. The plastic is combusted to CO2 and H2O by this exposure, thus removing the mandrel. Calculations are presented to evaluate the various parameters in the approach. Experimental confirmation of the effectiveness of the removal is shown.