Further results in measuring water ice buildup on optical components in cryogenic vacuum chambers

Abstract

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.