A wide variety of laboratory experimentation requires optical viewports in UHV chambers. Some of these situations also require that the viewport and chamber be nonmagnetic. This precludes the use of common techniques using magnetic materials that are commercially available. 

In our work on the design of atomic fountain clock vacuum chambers, we have pursued two weld-in designs and a mechanically sealed design. We will discuss our designs and results in producing these viewports.

WELD-IN DESIGNS

Motivation

We wanted to develop a window system with a large ratio of optical clear aperture to total area. A weld-in window design eliminates the demountable bolt circle of the standard 2.75” UHV ConFlat [1] and replaces it with a small weld, allowing the construction of compact, monolithic vacuum chambers with good optical access.

 
Another priority was producing a robust vacuum system having many windows that could withstand repeated high-temperature bakeout cycles to condition the vacuum system. Indium or lead solder has been used for mechanically sealing window joints to vacuum chambers [2] or brazing together weld-in window cells [3], but the bakeout temperature is limited by the melting point (157 o C for Indium and ~180 o C for lead solder ) and can fail at even lower temperatures, making it difficult to achieve UHV. These considerations drove us towards designs that used high-temperature brazes in the assembly of the window cells. In addition, the use of high-temperature brazes allows reliable application of optical coatings after brazing.

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