Measurement of Water Ice Accumulation on a First Surface Gold Mirror under Cryogenic, High-Vacuum Conditions

Spacecraft optical components must be tested in vacuum chambers in order to achieve “space-like” conditions on earth. To simulate the low temperatures experienced in space, optical components are often cryogenically cooled in the vacuum chamber. Outgassing of contaminants, such as water molecules, from the metal walls of the vacuum chamber occur under high vacuum conditions. These free water molecules accumulate and freeze on the cryogenic optical surfaces, which affects performance and reflectivity.

A multiple beam interference set-up was used to measure the accumulation of a water ice film on a first surface gold mirror under cryogenic, high vacuum-conditions. Zeolite molecular sieves were used to introduce water vapor into the vacuum chamber. Once inside the chamber, the water molecules were allowed to accumulate and freeze on a cryogenically cooled first surface gold mirror. The external interferometer set-up was used to measure the ice film thickness over time. Simultaneously, a quartz crystal microbalance was used to measure the ice film accumulation. Comparing the thickness results showed that the laser interferometer set-up is an accurate and reliable technique for measuring the water ice film accumulation on the first surface gold mirror.

To advance the current multiple beam interference technique, a new expanded beam interferometer was developed for measuring ice accumulation at multiple locations on the gold mirror. A beam expander was placed on the laser which increased the beam diameter by four times the original size. This expanded beam measured the ice film thickness on a larger portion of the mirror using a custom designed photodiode array. These results were again compared with those obtained from the widely used QCM technique. The comparison showed that the expanded beam interferometer set-up is an accurate and reliable technique for measuring the water ice film accumulation at multiple locations on a first surface gold mirror by agreeing with QCM results to within 2.8%.