Influence of Gas Pressure on the Infrared Blocking Property of Ultrathin Aluminum Films on Freestanding Polyimide Substrates

In indirect-drive inertial confinement fusion, thin films on laser entrance hole (LEH) windows are needed to reflect thermal infrared radiation (IR) and to protect the deuterium-tritium (D-T) cryogenic shell. In this paper, a set of ultrathin aluminum films deposited on freestanding polyimide as LEH windows were studied on optical properties in the IR band. The influences of thickness and gas pressure on the IR transmittivity of these films were examined. It was found that gas pressure plays an important role in the IR transmission and microstructure changes of LEH films because of elastic modulus mismatch between aluminum and polymide. It is undoubted that the aluminum/polymide film can block thermal IR and protect the D-T cryogenic shell when gas pressure is less than 20 KPa.