FIREX foam cryogenic target development: residual void reduction and estimation with solid hydrogen refractive index measurements

Voidless fuel solidification within a foam material is attempted for the development of a Fast Ignition Realization EXperiment (FIREX) target. A typical target consists of a foam shell with a thin solid fuel layer, a gold cone guide and a glass fill tube. The foam layer is formed with aggregations of tiny cells. The porous foam material has the advantage of forming a uniform layer in a liquid state by the effect of capillarity. Random liquid–solid transitions of fuel, however, would cause void spaces in each cell because of their different densities. To form a voidless solid fuel layer in the FIREX target, we propose a layering method, which includes the process of controlled solidification and simultaneous liquid fuel supply to the liquid/solid interface. Normal H2 is used as a surrogate fuel for our experiments. By applying a resorcinol/formalin aerogel, which has been developed in the Institute of Laser Engineering, Osaka University, instead of a foam material, formation of a solid H2 layer with reduced void spaces is preliminarily demonstrated. Related solid H2 refractive indices are measured to estimate the void fraction. Eventually, its filling factor reaches ~99%. Furthermore, the application of the proved method is numerically simulated on the FIREX target. The method is confirmed to be applicable in it with a cone guide heating technique.