Vacuum Compatible Gap Filling

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I have a thin gap between a disk shaped component (7.5"OD, 0.5" thick, lets call it the "mount") and a long pipe serving as a vacuum chamber. I'd like to fill this gap with something to reduce the conduction between the two volumes on either side of the mount. The mount already has a sizeable hole in its center, so we're not talking about achieving an airtight seal here, just some rough baffling. The pipe it sits in is 7.8+/-0.1", where the rough tolerancing comes from weld beads and other surface variations. I pull 5e-9 torr so the material need not be quite UHV but still pretty good. The mount must be easily installed and removed.

If it weren't in vacuum, I'd use weather stripping, but that's obviously out of the question. Even if the plastic/rubber used in weather stripping was as good as SS316 for out-gassing, the foam structure would constitute a massive virtual leak. The other obvious choice would be to build a ring-shaped piece out of metal to block the gap, but the rough tolerancing of the pipe makes this challenging. Epoxy might work, but its not removable.

Any clever ideas?
 
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What are the plans for future use? Are you intending this to be a Permanent or near so installation, temporary installation, or just a test fixture? How removable should the filler be after use?
 
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The mount will support high voltage electrodes that will probably go bad due to flashover in a few years. So it will be probably removed and reinstalled 2/3 times total during its lifetime.
 

CWatters

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Could you use epoxy with a temporary plastic film or wax to stop it sticking to one part? After epoxy is set, disassemble and remove plastic film. Not air tight but you said....

we're not talking about achieving an airtight seal here, just some rough baffling.
 
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Hmm, not a bad idea CWatters. I could probably achieve this by wrapping one-sided Kapton tape sticky side out around the circumference of the mount.
 
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For the time being, I've realized that as long as the gap is thin compared with its depth (set by the 0.5" thickness of the mount) the conductance through the gap will be low. I think I can increase the OD of the part enough to make the conductance negligible, without actually grinding up against the weld joints. I'll make a 7.7" OD plastic test piece and try seeing if it will slide through the pipe to its final position or not.
 

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