Manufacturing and Leak Check of Shell Targets for the FIREX-I Project

This article reports the latest progress in the manufacture of shell targets used for the first stage of the Fast Ignition Realization Experiment (FIREX-I). In the project, the targets are cooled to cryogenic temperature and filled with hydrogen isotope fuel through a gas feed tube. If the target has a leak or the gas feeder is blocked, the experiments cannot be carried out successfully and waste much time. The authors have developed a system to check integrity of the targets. SF6 is used instead of H2 or D2 because of its high refractive index and low diffusivity. When SF6 is loaded into the target, the optical path length that passes through a target is longer than that of an evacuated target. The difference can be measured with an interferometer. Analyzing the interference images, it is possible to know whether the gas feeder is blocked and a leak is present. The validity of the system was experimentally confirmed.