Improvements on Permeation GDP Coatings for Resorcinol Formaldehyde Foam Shells for Cryogenic Experiments on OMEGA

Resorcinol formaldehyde (RF) foam shells are needed for direct-drive inertial confinement laser fusion experiments at the University of Rochester OMEGA laser facility. As previously reported, the addition of long-chained polymers to the fabrication process has improved shell wall uniformity, but this change has led to a lower yield (from ~40 to ~15%) of shells that are gas retentive after the application of glow discharge polymer (GDP) using the standard deposition technique. We have improved this yield by modifying the coating conditions of the GDP overcoating process by modifying the background coating pressure from the constant 75 mTorr to using a two-step coating process of a high-pressure coating at 250 mTorr followed by low-pressure coating of 75 mTorr. This modification has improved the yield of the gas retention on the styrene-butadiene-styrene RF shells from ~15 to ~60%. We have found that the surface roughness of these shells is also improved from ~45 nm root-mean-square (rms) to ~20 nm rms. This technique, however, leads to a slight shrinkage of shells, which will be described.