How Good is the Bright Ring Characterization for Uniformity of Deuterium Ice Layers within Cryogenic Nuclear Fusion Targets?

Cryogenic targets, thin plastic shells surrounding thick cryogenic deuterium layers, are used to achieve nuclear fusion. These targets are irradiated by lasers causing the targets to implode and create fusion reactions by forcing the deuterium inside to fuse together. The cryogenic targets are pressurized with deuterium gas, and then the targets are chilled to about 20K to freeze an ice layer on the inside of the target. The ice layer needs to be as uniform as possible for the targets to implode correctly and thus create a successful fusion reaction. Cryogenic targets have suffered from nonuniform deuterium ice coating. This nonu~iiformity causes irregular implosions and is the source of nuclear fusion reaction failure. The uniformity of the ice layer is measured by using the technique of shadowgraphy. A shadowgraphic target image exhibits a characteristic bright ring when viewed with a CCD camera. This bright ring can be used to estimate the ice thickness in the cryogenic targets. This method is based on the assumption that the bright ring radius uniquely determines the ice layer radius in the target. A computer program has been developed to test the shadowgraphic technique. The result is that the bright ring does not uniquely define the inner ice surface. When the experimental ring radius is compared with the radius predicted by the current model, significant inconsistencies (up to 1 micron) are observed. However, if two opposite views of ,the bright ring are averaged, then the inner ice layer can be measured to an accuracy of less than 0.2 microns. This is a five-fold increase in the accuracy of measuring the uniformity of the deuterium ice layer within a nuclear fusion target.