Cold Trap Questions

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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Tom Dressel
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Cold Trap Questions

Post by Tom Dressel »

I picked up a Neslab CC-100 immersion cooler, It uses a two stage refrigerator to generate -90 Deg. C. on an 18" long, 5/8" diameter stainless steel flex tube. I would like to use this thing to make a cold trap between the diff pump and the vacuum chamber. I am not sure what is the best way to use the probe or if there is a "best way".

One way would be to put the probe INSIDE a 18" long, 3/4" diameter copper pipe, sealed at one end, and use this "cold well" INSIDE the vacuum line running from the diff pump to the chamber.

I could also use the "cold well" in a vacuum line in a "Tee" branch off the main line from the diff pump to the chamber. This would probably not be as effective as an "in line" trap.

Another way would be to wrap the flex tube around the OUTSIDE of a short length ~4" diameter of stainless pipe, possibly with baffles, situated between the diff pump and chamber. Then cover this "pancake" with insulation.

I would also like to surround the probe with a fluid to increase thermal conductivity. Is there any commonly available liquid, with good thermal conductivity, that doesn't become concrete at -90 deg. C.

Any way I do it, I know that unless I can get good thermal insulation, I can't get anyware clost to -90 C.

Does anyone have any other suggestions?

Also is there any easy way to monitor the temp in this extreme temp range?

Tom Dressel
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Richard Hull
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Re: Cold Trap Questions

Post by Richard Hull »

It would be a real pinch off and reduce the effectiveness of the diff pump to about ZERO it you connect the chamber to the pump via a tiny diameter long pipe.

HOWEVER, it will WORK! just plan on waiting a long time to bottom out....... molecular flow molecules coming from an open fusor chamber to a 5/8 hole and then chance richocetting hundreds or thousands of times down this long corridor into the diff pump is a study in low probability events.

The cooled stainless 4" baffled throat sounds better. There is little reason to make the tube larger in diameter than your diff pump opening.

Sorry, no help on the heat conductive fluid or the measuring of ultra low temps. Remember, if you get to just -30 degrees in a 4" throat with baffles you will stop virtually all th' nasties, and see a real improvement in the bottom end.

Richard Hull



Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Tom Dressel
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Re: Cold Trap Questions

Post by Tom Dressel »

I spent the last week trying different ways to build a cold trap with a 20" long 5/8" diameter cold probe. I finally decided to go with linear inside the vacuum design vs the coiled outside the vacuum design.

The cold finger is inside a 3/4" copper pipe, ten inches of which is sticking inside the length of two 2-3/4" CF tees. The other twelve inches of the probe is outside the tees, but inside a zig zag of 1" copper pipe, inside a 1-1/2" copper pipe ( to create a vacuum seal and concentric zig zag thermal brake), everything is silver soldered and the 1-2/2" pipe is soldered to a 2-3/4" CF flange. The four open ports on the two connected tees make connections for input, output, low temp probe feed through and a view port. The whole thing sits vertically on the diff pump throat. I have had it down to -127 F with the diff pump running and just a single layer of R-50 foam pipe insulation. The zig zag thermal break works really well, the CF flange mount of the probe will condense some moisture but doesnt freeze.

Yesterday, just for kicks, I let the foreline, diff pump, and cold trap up to air with the trap at -120 F. I could see through the viewport that there was a very faint coating of frost on the probe. I then turned the pump on and everything seemed to function normally, it pumped down to 4 microns. Then I turned the refrigerator off, and as the trap warmed up to about -35 F the chamber and foreline pressures started to rise up to 300 microns! It was like a micromaze bake out. Then as the trap hit zero deg. F., the pressures rapidly dropped back to baseline of 4 microns.

So it seems that the lesson is:
1. Never let the cold trap up to air when it is cold
2. The trap holds onto everything (or at least water) as long as its colder than ~-35 F.
3. The trap gives everything back at ~0 F.

Tom Dressel
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Richard Hull
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Re: Cold Trap Questions

Post by Richard Hull »

Ideally, the cold trap is never made cold until your system is sealed and the forepump has taken the system below 10 microns at full high temp with pump running. Once the diff pump starts to work, then the cold trap is activated.

Once pumped down to UHV levels and the experiment is done, but you don't want to let it up to air, you seal off the chamber (fusor) with the main chamber valve. This isolates the fusor and only now can the rest of the system be let up to air and the cold trap turned off.

Most often, the cold trap is allowed to return to full room temp well before the mechanical pump is turned off. This way, the frost will never form on the trap and its water load is pumped out as it warms.

Cold traps, like any trap, can foul a system horribly if not operated in a rigidly adhered to manner. All that frost evaporated and went into your oil in the fore pump! However, it can be balasted out or the oil changed.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Tom Dressel
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Re: Cold Trap Questions

Post by Tom Dressel »

Thats what I thought, the ballast valve was open above 200 microns.

Tom Dressel
quinnrisch
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Re: Cold Trap Questions

Post by quinnrisch »

There is no thermal fluid that would work for a cold trap. Even if there was you wouldn't want it exposed to the system, you would want to cool pipes with it, or the whole damn chamber. You should always heat the cold trap up before exposing to atmosphere or else you could get a nasty expolsion. The trapped O2 could react violently with something. For example an industrial secondary high expolsive is charcoal soaked in liquid O2.
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