Vacuum System Not Performing. Why?

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
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Jeremy Adams
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Real name: Jeremy Adams
Location: Chicago Ill

Vacuum System Not Performing. Why?

Post by Jeremy Adams »

Hi Everyone,

Today I did a vacuum test and switched on the hot cathode gauge about 55 minutes in to running the system (ample time for the diffusion pump to warm up and pump down a small space where the gauge was attached). As soon as the filament flicked on it started releasing gas into the glass which I took as a bad sign so I quickly turned it off. The vapors which it released were hardly being sucked into the diffusion pump too. My roughing pump is 12CFM and confirmed to hold a good low pressure. The diffusion pump is an Edwards diffstack 63. Before the test I made sure each joint was properly sealed (only 3 joints before the pressure gauge all with copper gaskets in place so I don't think it's a leak). I also replaced the oil in the diffstack since the last time I tried to run it was several months ago. I have no idea why my vacuum system is not working - the joints are good, the diffstack is heating up the oil, I used proper water cooling system too, and the roughing pump is doing great.

Perhaps it is the pressure gauge? A while back this pressure gauge was operated at too high temperatures and developed a white coating around part of the filament. Maybe I mistook the coating burning off under a low vacuum as the filament not working right? I did not run the filament longer than a couple seconds because I was concerned the vacuum system was at too high of pressures.

Jeremy
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John Futter
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Re: Vacuum System Not Performing. Why?

Post by John Futter »

Jeremy
With no backing pirani gauge on your backing line I would suspect all of those joints done with teflon tape.
Lose all of that crap and get a plastic vacuum hose (the one with the metal spring supporting the plastic or some milking rubber as used to milk cows.
Milking rubber is cheap and probably the best.
Use it with barb connectors and hose clamps.
Diff stacks stall (ie they stop working)at 2 by ten to minus 1 Torr. A backing pressure higher than this indicates massive leaks in your backing connection to your diffstack
ian_krase
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Real name: Ian Krase

Re: Vacuum System Not Performing. Why?

Post by ian_krase »

What kind of ion gauge tube is that? I think it might be toast. An iridium one is much less likely to be destroyed by air -- I have one, they're barely more expensive -- but it can be ruined by diffpump oil which is its own problem.


But yeah, do you have a thermocouple or pirani gauge? If so, you could 1. measure your foreline pressure and 2. watch the pressure go from some tens of microns to "zero" without any risk of burning anything up. This can be a dramatic test of a high vacuum pump.

Alternatively, do you have any way to set up a glass chamber that you can then send a high voltage glow discharge through? (if your ion gauge tube is indeed destroyed you could use it.) This makes a very simple, cheap leak detector. You would leak test with the diffstack turned off. In any case it is fairly easy for a foreline pressure to be degraded from 10 microns to 200+ without noticing much wrong.
Jerry Biehler
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Re: Vacuum System Not Performing. Why?

Post by Jerry Biehler »

As long as the filament is intact the tube should be fine. Most ion gauge controllers have auto shutdown for high pressure.

You really need to ditch all those brass connections. Generally teflon can seal for roughing side of things but with all those connections you are asking a lot. You need to get a thermocouple or pirani gauge installed on the foreline in the very least, you should not even be turning on the diff pump until you know you can reach 50 mtorr.
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