| Summary: | Targets for low-adiabat direct-drive-implosion experiments on OMEGA must meet rigorous
specifications and tight tolerances on the diameter, wall thickness, wall-thickness
uniformity, and presence of surface features. Of these, restrictions on the size and
number of defects (bumps and depressions) on the surface are the most challenging. The
properties of targets that are made using vapor-deposition and solution-based
microencapsulation techniques are reviewed.
Targets were characterized using confocal microscopy, bright- and dark-field microscopy,
atomic force microscopy, electron microscopy, and interferometry. Each technique has
merits and limitations, and a combination of these techniques is necessary to adequately
characterize a target.
The main limitation with the glow-discharge polymerization (GDP) method for making
targets is that it produces hundreds of domes with a lateral dimension of 0.7–2 μm.
Polishing these targets reduces the size of some but not all domes, but it adds scratches
and grooves to the surface. Solution-made polystyrene shells lack the dome features of GDP
targets but have hundreds of submicrometer-size voids throughout the wall of the target; a
few of these voids can be as large as ∼12 μm at the surface.
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