Vapor-deposited polyimide ablators for NIF: Effects of deposition process parameters and solvent vapor smoothing on capsule surface finish

Recent progress made at LLNL on fabricating NIF scale polyimidie capsules using vapor deposition techniques is detailed. Our major focus has been on improving the capsule surface 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 inononzer mixing, self-shadowing, and abrasion. We have shown that high rate deposition (above 10 mum/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 zip to 160 mum thick have been reproducibly fabricated with 300 nm RMS roughness. Solvent vapor smoothing, a new technique also developed at LLNL,further improves the surface to 30 nm RMS.