Progress on HAPL Foam Shell Overcoat Fabrication Presented by Jared Hund 1 N. Alexander 1, J. Bousquet 1, Bob Cook 1, D. Goodin 1, D. Jasion 1, R. Paguio 1, K. Saito 1 1 General Atomics, Inertial Fusion Technology, San Diego, CA 19 th HAPL Workshop University of Wisconsin Madison WI October , 2008 IFT\P

Since the last HAPL meeting we have made progress on overcoats: Improved surface defects on foam – which has resulted in fewer defects on the overcoat surface Measured parameters important for fill cycle/tritium inventory –permeation rate at elevated temp Built 2 nd high capacity GDP(roto)coater for more experiments

We are focusing on low yield steps of the foam target fabrication process Foam fabrication cumulative yield (pre-coating) ~90% Droplet survival Curing/ Drying Sphericity DVB Wall uniformity (NC) 65% RF DVB RF DVB RF DVB at 3% NC RF at 3% NC DVB at 1% NC RF at 1% NC (zero yield)

We are also focusing on the overcoat yield Capsule fabrication cumulative yield DT Vapor Foam + DT Thin High Z coating ~ 2.3 mm rad 5-10 µm CH Overcoat DT HAPL Target Foam layer: 0.18 mm thickness Divinyl Benzene (DVB) or Resorcinol-Formaldehyde (RF) 65% Foam Fabrication Gas Retention Surface Finish DVB RF DVB w/15 µm RF w/15 µm 8% DVB 0% RF 0% 11% 0% RF w/10 µm 0%

We are working to reduce the overcoating thickness to 5-10 µm We are continuing our parametric evaluation of GDP coating conditions on foam –Background pressure –Foam surface –Reactant flow rates The essential function of the overcoating is to be permeable at RT yet hold the fuel at cryo –Test permeation with mass spectrometer At 20°C At cryo (LN 2 ) temperature Surface roughness –Measured with Interferometric Profiler MS Signal of Good Shell Cycled between 20°C and LN 2 temperatures GDP “Rotocoater” 3” dia. chamber

We can successfully make gas tight overcoatings at 15 microns thickness The improvement came from tailoring the coating pressure ranges Plot of gas retention Oct 2007 Improved parameters April 2008 Green = pass cryo permeation test Black = fail cryo permeation test

We have reduced the isolated defects appearing on the overcoated shells Reducing defects on the foam reduced defects in the overcoating But - the background roughness is the same ~60 nm RMS RF Coated with GDP ~30 nm RMS RF Coated with GDP Both areas ~115 nm RMS The foam surface roughness is now below the spec, but coating parameters need to be improved

A number of items need to be optimized for good surface finish and gas retention Coating pressure –Evaluated background pressures for smoothing effect Experiments of 50 – 500 mtorr showed 250 mtorr was smoother –250 mtorr background pressure for entire run Sticking was a problem we were able to overcome Stress in the coating is a problem –Use variable pressure: 250 then 50 mtorr –Current experiments: high pressure for more of the coating thickness (but not all)

The tritium inventory required is set by the coating characteristics The temperature of the fill can affect the properties of the coating (and fill rate) –Buckle strength (can decrease with T) –Permeation rate (increases with T) High temperature fill will be necessary to keep the tritium inventory low Plot of tritium inventory as a function of temperature for various E a and reduction in buckle strength

The permeation rate through GDP was measured as function of temperature ~ 4x faster permeation at 100°C ~10x faster permeation rate at 130°C But the actual fill rate may be reduced because of a reduction in buckle strength of the shell, but that will no greatly affect tritium inventory Ea= 15 kJ/mole

Conclusion Foam yields are progressing with systematic, parametric studies We are evaluating the mechanisms to improve coatings parameters for thinner, gastight, smooth coatings We will evaluate the needed Tritium inventory due to fill cycle –measure effect of temperature on buckle strength