Prevention of Residual Gas Condensation on the Laser Entry Hole Windows on Cryogenic NIF Targets Using a Protective Warm Film

Early shots on the National Ignition Facility (NIF) were plagued by the buildup of a considerable mass of extraneous ice on the laser entry hole (LEH) windows, a consequence of condensation of the residual air. This resulted in higher than desired temperatures at the LEH, which combined with the variability of the ice thickness made this a problem that needed a robust solution. In this paper, we describe our work in designing a second thin film that shielded the LEH window from the contaminating ice. The detailed cryogenic considerations required to ensure the proper functioning of this new window were simulated and verified experimentally. The data from numerous subsequent shots showed marked improvement in performance, which made this feature an essential component for all cryogenic NIF targets.