Cryodeposit Thin Film Detection Techniques in a High-Vacuum Environment

It has been well documented that the principal source of contamination for optics in cryogenic systems is water. Prior studies have been successfully performed to explore methods to detect and accurately measure ice growth on optical surfaces. A new setup to detect cryodeposit thin films in high vacuum environments is under development and has been tested at the University of Tennessee Space Institute’s (UTSI) Center for Laser Applications (CLA). This setup uses a multiple-beam laser interferometer that is incident the mirror surface at 45 degrees to the normal surface. Water vapor was introduced to the vacuum system via 3-angstrom zeolite molecular sieves, thus allowing ice growth to take place on both a quartz-crystal microbalance (QCM) and a gold-plated first surface mirror. Three experimental runs involving ice-accumulation on the mirror and QCM were performed. Each experimental run lasted for a minimum of two hours in order to allow a significant amount of ice to form on the test surfaces. Using data from both the QCM and multi-beam interferometer, we were able to effectively and non-invasively measure the accumulated cryogenic ice layer thickness. During the final two-hour and thirty-two-minute run on November 26th, 2013, we obtained ice thickness values of 3.25 micrometers and 2.88 micrometers with the interferometer and QCM, respectively. The thickness values measured by the QCM and interferometer were within 12.2% of each other.