Experiments presented in a previous paper established proof-of-principle that water, the most prevalent contaminant in
high-vacuum cryogenic systems, initially collects on the surfaces of optical components as a thin film of ice, and thus
can be detected and its thickness measured via multiple-beam thin-film interference phenomena. In those earlier
experiments, a molecular sieve zeolite in a canister external to a vacuum chamber served as a water source, while the
buildup of ice was measured using a HeNe laser beam reflected off the surface of a mirror with a quartz crystal
microbalance (QCM) used for verification of the mass accumulation. Additional experiments have improved upon the
techniques used earlier and provided further insight into the ice accumulation process. Use of a shorter wavelength (450
nm) laser in conjunction with a first-surface gold mirror produced greater depth of modulation and thus increased signal-to-
noise ratio in the light interference. Data reduction using cross-correlation analysis over single-period interference
records provided more accuracy and precision in the ice thickness measurements. Ice buildup under varying pressure
and temperature ranges established baseline conditions for transparent thin-film deposition, and the transition to ice
fracture and specular reflection. These recent experiments have demonstrated that the optical monitoring of ice
accumulation via multiple-beam interference is applicable over a wider range of mass and thicknesses than the
conventionally-used QCM method.
Standard vacuum practices mitigate the presence of water vapor and contamination inside cryogenic vacuum chambers. However, anomalies can occur in the facility that can cause the accumulation of amorphous water ice on optics and test articles. Under certain conditions, the amorphous ice on optical components shatters, which leads to a reduction in signal or failure of the component. An experiment was performed to study and measure the deposition of water (H2O) ice on optical surfaces under high-vacuum cryogenic conditions. Water was introduced into a cryogenic vacuum chamber, via a hydrated molecular sieve zeolite, through an effusion cell and impinged upon a quartz-crystal microbalance (QCM) and first-surface gold-plated mirror. A laser and photodiode setup, external to the vacuum chamber, monitored the multiple-beam interference reflectance of the ice-mirror configuration while the QCM measured the mass deposition. Data indicates that water ice, under these conditions, accumulates as a thin film on optical surfaces to thicknesses over 45 microns and can be detected and measured by nonintrusive optical methods which are based upon multiple-beam interference phenomena. The QCM validated the interference measurements. This experiment established proof-of-concept for a miniature system for monitoring ice accumulation within the chamber.
An experiment was performed to study and measure the deposition of water (H2O) ice on optical component surfaces
under high-vacuum cryogenic conditions. Water was introduced into a cryogenic vacuum chamber via a hydrated
molecular sieve zeolite housed in a valved external chamber, through an effusion cell, and impinged upon a quartz-crystal
microbalance (QCM) and first-surface gold-plated mirror. A laser and photodiode setup external to the vacuum
chamber monitored the multiple-beam interference reflectance of the ice-mirror configuration while the QCM measured
the mass deposition. Data acquired and analyzed from this experiment indicate that water ice under these conditions
accumulates on optical component surfaces as a thin film up to thicknesses over 45 microns and can be detected and
measured by nonintrusive optical methods based upon multiple-beam interference phenomena. The QCM, a well-established
measurement technique, was used to validate the interferometer.