PCTFE as a solution to birefringence in atom trap viewports

We have developed and characterized optical viewports with the glass-to-metal seal made by the plastic PCTFE (polychlorotrifluoroethylene). The goal is to reduce stress-induced birefringence while maintaining ultra-high vacuum compatibility. We have maintained a Stokes parameter S3 of 0.9986, and achieved <5 × 10(-11) Torr partial pressure of air. We have also measured the diffusion and permeation of helium through PCTFE and placed upper limits on nitrogen, oxygen, and argon permeation, as PCTFE has been suggested as an o-ring for transport of environmental noble gas samples, though we know of no other noble gas measurements.