Finally got a diffusion pump

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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steventw
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Real name: Steven Whittam
Location: Melbourne/Australia

Finally got a diffusion pump

Post by steventw » Thu Jan 07, 2021 5:10 am

Hey

Finally got diffusion pump
All that’s left is getting kf50 flange/nipple to weld onto bottom of chamber and then after 5yrs will finally get to finish my experiment.
598310AB-F718-4300-84C1-BBAC419843C6.jpeg
Chamber and pumps

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Richard Hull
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Re: Finally got a diffusion pump

Post by Richard Hull » Fri Jan 08, 2021 7:31 am

Be hyper careful!!! Never ever turn on the diff pump unless your makeshift chamber is well sealed and your mechanically pumped vacuum is below 100 microns! If you should lose vacuum for any reason with the diff pump running, you will flash boil all you pump oil, fouling the mechanical pump oil and baking on a hard, tenacious crust of back residue in every part of the christmas tree and diff pump walls and boiler. It will take a month of Sundays to get it all cleaned up again. Then you will need fresh oil in both pumps to get back to square one. Read the vacuum FAQ on operation of a diff pump before going further.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

steventw
Posts: 62
Joined: Fri Jan 24, 2014 1:45 pm
Real name: Steven Whittam
Location: Melbourne/Australia

Re: Finally got a diffusion pump

Post by steventw » Sat Jan 09, 2021 5:01 pm

I will have a read when get closer to day of it being complete.

I was told not to turn diffusion pump on till about 15micron on pump
Got 2 gauges , one on pump next to where diffusion pump will hook into
And one on top of chamber

Using hose the difference between the chamber and base of pump is quite large about 20,000 micron difference when starts getting reading at pump vs other end of hose on chamber.

I Tryed connection to diffusion pump with seal closed, need to messure hight between connections and make it level
Used a bit of cardboard to start with and noticed leak at slight wobble
But once checked everything and fixed dropped to about 250micron Pritty quickly before i turned off pump cause will see how far will go once chamber finished with kf50/mw50 flange/nipple
And messure up a table/stand to hold it all neatly to minimise leaks


From what I remember reading vs what I understand I’d need to keep the gauge on pump lower than gauge on chamber to avoid oil vapour being sucked into chamber from diff pump
Hence the shut of valve on diff pump pipe work that will be between chamber and diff pump

Only reason taken longer than expected is chamber is aluminium and pipe work is stainless steel
So ran into trouble and took awhile to find right aluminium part
Older bro says knows someone who can weld aluminium and seal properly... hopefully

Probly won’t get finished before heading into rehab on 19th
For 4 weeks
Probly be asking a few questions when get closer to date of being able to run full test and experiment

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Maciek Szymanski
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Real name: Maciek Szymański
Location: Warsaw, Poland

Re: Finally got a diffusion pump

Post by Maciek Szymanski » Sat Jan 09, 2021 10:06 pm

There are two base pumping schemes - direct and bypass pumping:


9F6F710F-0565-44F0-8AEA-E89F4F19C4FD.jpeg
1. Rotary pump 2. Diffusion pump 3. Vacuum chamber 4. Diffusion pump isolation/throttle valve 5. Foreline vacuum gauge 6. Chamber vacuum gauge 7. Foreline cut-off valve 8. Bypass cut-off valve

With direct pumping you start (assuming everything is vented and the diffusion pump is cold) with the cutoff valve (4) open and evacuate the chamber together with the diffusion pump down to 50-100 microns. Then you start the diffusion pump heater (and cooling). After 20-30 minutes the chamber gauge (6) should drop below 1 micron, while the foreline gauge (5) will still indicate 10-50 microns. With this scheme you can’t vent and evacuate the chamber again without cooling and restarting the diffusion pump. The shutdown procedure is: Close the isolation valve (4) and turn of the diffusion pump heater. The rotary pump and cooling must be running for 20-30 minutes. With the isolation valve (4) closed the chamber may be vented immediately. The valve (4) may be opened to vent the vacuum pump and allow evacuation of the system after the diffusion pump is cooled down (20-30 minutes after switching of the heater).

The bypass system is more flexible but requires two additional valves and more caution in operation. Startup procedure is: With all valves closed the rotary pump is started, then the foreline valve (7) opened. When the foreline pressure indicated by gauge (5) drops to 50-100 microns the diffusion pump heater and coolant is turned on. After 20-30 minutes the diffusion pump is ready. Now the chamber may be pre evacuated. Close the foreline valve (7) and open the bypass valve (8). When the chamber pressure indicated by gauge (6) drops below 100 microns close the bypass valve (8), open the foreline valve (7) and the isolation valve (4) the chamber pressure should quickly drop below 1 micron. To temporarily vent the chamber it’s enough to close the isolation valve (4). To pump the chamber again the preevacuation procedure must be repeated. For permanent shut down the isolation valve (4) is closed and the heater turned off. Coolant and the rotary pump should run for 20-30 minutes. Then the foreline valve (7) is closed, the coolant and the rotary pump turned off and the rotary pump vented.
The foreline valve (7) and bypass valve (8) should never be opened together. If the foreline valve is left opened while preeavcuating the chamber the air is allowed to the diffusion pump and cause flash boiling of the oil! Also the preevacuation should not last longer than few minutes as the backing pressure may raise to high. Adding a vacuum canister on the out port of the diffusion pump makes the situation better.

Regarding the back streaming due to pressure difference - below 100 microns you really have no pressure difference - the figure indicates how much gas molecules you have in the chamber, but the interactions between them are very weak. Some oil back streaming will be there as some of the oil particles will be reflected to the chamber by collisions. To minimize this basic, cooled or cryogenic baffles on top of the diffusion pump are used. The diffusion pump (and all high vacuum pumps) despite their name are more traps than pumps. They do not pump anything from the chamber but collect the particles which fall into the pump’s throat by the pure luck.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland

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Richard Hull
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Re: Finally got a diffusion pump

Post by Richard Hull » Sun Jan 10, 2021 4:50 pm

I tend to use the bypass system of valves... minus the bypass and valve #8.

Start up

I turn on the mechanical pump and open valve #7. Diff pump is NOT powered up!
I let the foreline and diff pump reach a suitable value like 50 microns.
I then open the chamber valve #4 and let everything settle to at or well below 50 microns.
I now power up the diff pump. In time, the reactor will have its pressure plunge sub-micron.
I let in the deuterium gas and do fusion.

Shutdown

I valve off the chamber by closing valve 4 securely.
I turn off the power to the diff pump (important!!! keep the mechanical pump running!!!)
Wait for the diff pump to cool to below 50 deg C.
Now you can valve off the foreline valve #7 securely
Only now may you turn off the mechanical foreline pump.

All of this is in my FAQ on operation of a diff pump system.
Never let a diff pump, hot or cold, once placed in an operational system, see atmosphere.
Every good fusor system should have two valves, minimum!
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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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