Measurement of Total Condensation on a Shrouded Cryogenic Surface Using a Single Quartz Crystal Microbalance

A single quartz crystal microbalance (QCM) is cooled to 18 K to measure condensation rates inside of a retractable shroud enclosure. The shroud is designed to minimize condensate on fusion targets to be fielded at the National Ignition Facility (NIF). The shroud has a double-walled construction with an inner wall that may be cooled to 75 to 100 K.

The QCM and the shroud system were mounted in a vacuum chamber and cooled using a cryocooler. Condensation rates were measured at various vacuum levels and compositions and with the shroud open or closed. A technique for measuring total condensate during the cooldown of the system with an accuracy of >1 × 10^-6 g/cm² was also demonstrated. The technique involves a separate measurement of the condensate-free crystal frequency as a function of temperature that is compared to the measurement for the cooldown trend of interest. The shroud significantly reduces the condensation rates of all gases and effectively eliminates H₂O condensation.