The Challenge of an IFE Foam Capsule Overcoat

Abstract

The baseline design for the laser-driven Inertial Fusion Energy (IFE) target is a 4.6 mm foam capsule with a polymer overcoat of 1 to 5 microns. The specifications for this overcoat include surface finish, permeation properties, uniform wall thickness and conformal coating of the foam shell. Many of these specifications are not unlike the full density polymer National Ignition Facility targets, but the foam shell adds to the fabrication difficulty. Since the foam surface is composed of open cells, creating the overcoat by typical vacuum deposition processes would start by replicating the foam surface making it very difficult to achieve the required surface specification. Instead an overcoat is made using interfacial polymerization at the edge of the foam surface. This is done by filling the foam shell with an organic solvent containing one reactant, then placing the shell into water containing another reactant. The reaction occurs only at the interface of the two solutions.This technique was pioneered at the Institute of Laser Engineering (Osaka University) with 0.8 mm diameter methacrylate shells. The process was later extended to 0.9 mm diameter resorcinol-formaldehyde and divinyl benzene (DVB) shells. For the High Average Power Laser Program target we need to extend the process to 4.6 mm diameter DVB foam shells. The properties of the DVB foam and the larger diameter of the shell make it more difficult to produce a gas tight shell. This report will explain how we are adapting the process and the results to date.