Self-interference patterns and their application to inertial-fusion target characterization.
1999
The sphericity and wall-thickness uniformity requirements of
direct-drive inertial-fusion targets are of the order of less than
1%. These shells display self-interference patterns (SIP’s)
when irradiated with a spatially incoherent, narrow-bandwidth light
source and viewed with a compound microscope. These patterns are
distinct concentric fringes when the target is uniform, whereas faint,
distorted, or discontinuous fringes indicate a nonuniform
target. We determined the wall thickness to within ±0.5 µm by counting the number of fringes in the SIP, independent
of the outside diameter. Thickness uniformity is verified to an
accuracy better than 0.05 µm. The wall thickness of
gas-filled targets can be determined to this accuracy without knowledge
of the type of gas or its pressure. The SIP fringe technique is
used to select polymer shells typically of 800- to 1000-µm
diameter and 5- to 12-µm wall thickness. The fringe
locations have been modeled by use of ray tracing and agree well with
actual measurements of well-characterized shells. Details of the
formation of the SIP fringes, a theoretical model, and the method used
for quantitative measurement of the shell-wall thickness with the SIP
are presented with validation examples.
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