Optimizations in Sputtering Glass as Permeation Barrier for Laser Targets

Glass thin films appear particularly interesting as semipermeable barriers for many noncryogenic target applications. This functional layer can be sputtered from quartz targets onto CH_x microshells synthesized by glow discharge polymerization. In the present work, we investigate the influence of deposit parameters (pressure, RF power, target-holder distance, and plasma composition) on glass coating microstructure and permeation properties. The permeation properties of CH_x/SiO₂/CH_x capsules are studied by mass spectrometry using deuterium (D₂) as the filling gas. The use of a low deposition pressure and a high RF power in a background atmosphere of argon appears essential to obtain the most efficient barrier. The optimized sputtering conditions allow deuterium half-lives of 1 month on 1700-m CH_x capsules, including a 1-m-thick SiO₂ coating (corresponding to a permeation coefficient of 3 × 10^-20 molm^-1s^-1Pa^-1). These capsules could be filled to the required pressures ([approximately]3 MPa) for Laser Mégajoule (LMJ) experiments.