How can I fill a thin gap in a vacuum chamber without creating a virtual leak?

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Discussion Overview

The discussion centers on finding a method to fill a thin gap in a vacuum chamber between a disk-shaped component and a pipe, aiming to reduce conduction without creating a virtual leak. The context includes considerations for materials and methods suitable for high vacuum environments, as well as the need for the component to be removable.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests using weather stripping for its effectiveness in non-vacuum conditions but acknowledges it would create a virtual leak in a vacuum.
  • Another participant proposes using a ring-shaped metal piece to block the gap, noting challenges due to the rough tolerancing of the pipe.
  • Epoxy is mentioned as a potential filler, but concerns are raised about its removability.
  • A suggestion is made to use epoxy with a temporary plastic film or wax to prevent sticking, allowing for easier removal after setting.
  • One participant considers wrapping one-sided Kapton tape around the mount as a possible solution.
  • A later reply indicates that if the gap is thin compared to its depth, the conductance through the gap will be low, and proposes increasing the outer diameter of a test piece to make conductance negligible.

Areas of Agreement / Disagreement

Participants express various ideas and suggestions, but there is no consensus on a single solution. Multiple competing views on materials and methods remain unresolved.

Contextual Notes

Participants have not fully explored the implications of each proposed solution regarding their effectiveness in high vacuum conditions or the potential for out-gassing.

dreens
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I have a thin gap between a disk shaped component (7.5"OD, 0.5" thick, let's 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?
 
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.
 
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...

dreens said:
we're not talking about achieving an airtight seal here, just some rough baffling.
 
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.
 
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.