Self-interference patterns and their application to inertial-fusion target characterization.

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.