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FAQ - Diffusion Pump - What you need to know!

Posted: Thu Jan 12, 2012 4:41 pm
by Richard Hull
The "diff" or diffusion pump is almost a must have item for the amateur fusioneer. It utilizes fast, downward directed jets of oil vapor to force recalcitrant air molecules in molecular flow regime to exit the system, allowing for a 1000 fold reduction in fusor chamber pressure over that obtained by the mechanical "fore-pump" alone.
The diff pump is simplicity itself! it consists only of a heated boiler filled with a special ultra low vapor pressure oil which is forced up a column with multiple layers of jets that blow the oil back down into the boiler. Other than for the oil, there is nothing moving or mechanically operative in the entire pump. As such, it is easy to use and maintain.

Like any pump it has an inlet and outlet. The inlet is large and usually at the top of the pump utilizing a flange of some sort for connection to a throttle valve or a chamber. The outlet is smaller and usually near the base or bottom of the pump coming out the side, often at a right angle with an "up turn" in it. This may have a small flange on it, but is often just a cylindrical tube to attach to the fore-pump's vacuum hose.

For those wishing details on its operation go to google and type "diffusion pump principle"


The hose connecting the inlet of your mechanical fore-pump to the outlet of the diffusion pump must be of large diameter and as short as possible. A typical good hose is about 6 inches long and 1 inch in diameter!! (Keep it short and fat)

Buying a pump:

The fusioneer can readily obtain his needed pump, USED, for under $300.00 off e-bay and other surplus venues. Professionally rebuilt pumps of the type needed can cost as much as $800.00.

The fusioneer need never purchase a diff pump of more than 4" in diameter with 2 or 3 inches diameter being ideal.

Before buying a pump, make sure it is complete!!! Make sure that it has its main guts. This is called a "Christmas tree", but its officlal name is the Jet Stack. It is what makes the pump actually function. Without it, the pump is just a high tech flower pot!

The average fusioneer should not purchase a water cooled pump, identified by a coiled "snake" of copper tubing wrapped around the body of the pump. You can, of course, buy a water cooled pump, but you must have a regular source of flowing water nearby and a drain to dump the hot water flow. You can also design a nice closed circuit recirculating water system with a water tank, (reservoir), water pump and a radiator. Question....Do you really want the hassle?

Air cooled pumps are very desirable as no source of flowing water is needed. Air cooled pumps are easily recognized by the cooling fins located up near the throat and or a large attached blower or fan. If you purchase an air cooled pump without a fan, but just fins, you will have to make and design a simple air plenum and supply a rather high CFM blower or fan to force air past the fins.


The diff pump is traditionally a two piece item. The pump body is a hollowed out tube with an attached fan or a coiled water cooling tube wrapped around the outside. At the base of the tube is the boiler which houses a heater to which power is applied to heat a specially designed oil.

In most pumps, just turning it upside down will have the second item, the "jet stack" also called the "Christmas tree", fall out in your hands. In a few cases, it is retained by a crude wire arrangement, "spider", at the top that allows the stack it to be retained. The spider is easily removed with a bit of upward effort. The Christmas tree is a series of stacked cones with downward directed jet nozzles. A diff pump must be scrupulously clean, internally, to do its job properly. Used ones are usually filthy creatures from the black lagoon. Jets must be unclogged and the boiler gunk must be removed from years of use and, often, abuse. Just pouring oil into the open inlet and turning it on is a path to disaster.


If you have purchased a used pump, you will need to follow a routine to insure it is both clean and serviceable.

First check the boiler heater for continuity before doing anything else. if open circuit, then you will need to hunt about for a replacement heating element. Check Duniway stockroom or Kurt Lesker for a replacement.

I would never re-use unknown oil in a diff pump. If your used pump still has oil in it, toss it out now!

The jet assembly or "Christmas tree" inside the pump will need to be taken out and inspected. I would not take apart the Christmas tree unless it is filthy. Soak it in acetone, cloth wipe and clean it after a 24 hour soak and put it back in fresh acetone until you are ready to go for final assembly. The best soaking and easiest method is to just dump the oil out and leave the tree inside, filling the pump with acetone. (do all this outdoors)

If forced to take apart the tree, dismantle and lay it out on a table, in an ordered manner, all of the pieces so that re-assembly is correct.

The interior of the pump body, which is just a hollow tube, especially the boiler zone in the bottom, needs to be scrupulously cleaned. If rust or scale is there, I have used a wet soapy brillo pad attached to a long piece of drill rod in a drill to scour this zone. Ultimately, it needs to be acetone soaked and dried well before re-assembly.

No re-assembly should be done until you are ready to seal the pump off in a system, for cleanliness sake. The last thing you do before sealing it into the system is to put the fresh diff pump oil in the pump and then drop in the tree assembly. The cheapest and best solution is to use one of the modern silicone oils like 704 or 705. The amount of oil needed is never less than 35ml and rarely more than 100ml, with 50-75ml being the norm in small pumps.

With the above tasks done well and with good sealing, hooked to a decent fore-pump, you should be able to hit 10e-5 torr easily and 10e-6 torr in many cases.


All diff pumps are installed in a vertical orientation. You will need to design this into the overall system configuration well in advance.

The special diff pump oil, usually a silicone based oil made just for this purpose, is simply dumped into the throat or inlet into the bottom of the pump within the boiler and then the christmas tree is inserted prior to mounting. NOTE* all pumps have a specific amount of oil required. Most small, 2-3 inch diameter pumps have between 35 and 75 cc of oil required with most being in the 50 cc range. Larger pumps will require more oil than this. If you can't find data and your pump is 2-3 inches use 50 cc....It will pump with this OK.

The Diff pump will have to have a valve between its large mouth or "inlet" and the fusor chamber to allow it to be sealed off from the fusor and to act as a throttle to control the vacuum and deuterium pressure within the fusor. It will also have to have another valve between its exhaust port at the bottom of the pump and the mechanical "fore-pump".

The issue will be in adapting the upper valve between the diff pump's throat or "inlet" and the fusor chamber's evacuation port. The degree of difficulty will depend on the varied ports and plumbing involved. The distance between the diff pump and fusor should be little more than the valve body dimension, itself, and maybe an inch or two of tubing and adapters.

Note: It is wise to use a right angle throttle valve atop the Diff pump and then run a short right angle metal tube or metal flex hose into the chamber from the valve. In this fashion you can work on just the diff pump or the fusion chamber in future without pulling the entire system apart.

The valve inlet and the fusor evacuation port should be as large as possible to allow the diff pump to have a maximum conductance and attain a deep vacuum rapidly.

Also important: Make sure that there are separate power switches for the diffusion pump boiler power and the air plenum's cooling fan power.


The hose connecting the inlet of your mechanical fore-pump to the outlet of the diffusion pump must be of large diameter and as short as possible. A typical good hose is about 6 inches long and 1 inch in diameter!! (Keep it short and fat) repeat from above....It is that important!

Operation in a simple system:

Once installed, the operation of the pump is straight forward. See attached diagram for parts I.D. and hookup

1. All valves are normally closed to start. The fore-line gauge, (TC gauge), should also be turned on.
2. The fore line, mechanical pump is turned on and the short, gauge fore-line pumped to less than 30 microns.
3. The fore-line valve is opened and the diff pump body is pumped until it and the fore-lines are at or below 30 microns.
4. The fusor chamber valve is opened and the fusor chamber and entire system pumped until the fore-line TC gauge bottoms out. This should be at or below 30 microns as read on the TC, fore-line gauge. (The lower the better. A well sealed system and good fore-pump might go as low as 10 microns.)
5. You might use this time to glow clean the fusor by applying high voltage until the pressure increases and then starts to go back down again. (optional)
6. Now turn on the fusor chamber's high vacuum gauge. (Ion gauge, cold cathode gauge, capacitive manometer, etc.)
7. Only now can you turn ON the diff pump's electrical boiler heater and cooling fan or flowing water, if water cooled.

Note: The mechanical, fore-line pump, once turned on, is never turned off! It is always the first item that has power applied when operating a fusor and the last item to have power turned off.

As the boiler heats, any water or other volatile material in the diff pump oil will boil off and the fore-line pressure will rise a good deal and slowly settle back down. Suddenly, as the oil begins to operate the jets and the diff pump reaches running temperature, the fusor's internal pressure will plunge from the fore-line pressure to a deeper vacuum by at least a factor of 100. The fusor should be at 10e-5 or 10e-6 torr when the diff pump is working properly. The fusor is now ready to have its fusor throttle valve throttled or choked off such that the fusor's chamber pressure will rise to about 0.5 microns, (5X10e-4 torr), on the high vacuum, fusor chamber gauge.

We are now ready to admit a constant, controlled flow of deuterium gas. Slowly, crack the deuterium leak valve (critical adjustment until the pressure on the high vacuum gauge rises to between 5 and 10 microns and stabilizes there.

****note**** the fore-line pressure indicated on the TC gauge may rise a bit as the deuterium is also being leaked in, but it should not rise to above 50 microns! If it does rise to over 50 microns, then you have too much gas flow or the diff pump throttle valve is closed too much.

TIP**** I like to close the throttle valve all the way and wait for the pressure to rise in the chamber. I now barely open the throttle valve until the pressure drops back to some decent stasis point in the 10e-4 torr range. Now I crack the D2 valve until I get a fusor starting pressure of about 5 microns to start. I now run the HV power up to a stable glow. I can now adjust the voltage, gas flow and throttle valve until I get in the fusion range and the neutron counter is singing.

After doing your fusion run or operating session and you are ready to shutdown....


1. Make sure the high voltage is OFF! Close off the deuterium gas flow using the deuterium leak valve.
2. Close off the fusor throttle valve completely.
3. Turn off the diff pump's boiler heater power, but LEAVE THE COOLING FAN ON!!!!!
4. Leave the mechanical fore-pump on and running. ALWAYS!
5. You can turn off the fusor's high vacuum gauge now, if you wish.
6. Let the mechanical pump and the diff pump's cooling fan run until the boiler/oil temperature is back in the 100 degree F range! This may take as long as 30 minutes, so be patient. (Boiler is just warm to the touch)
7. Close off the fore-line valve at the bottom of the diff pump.
8. Only now can you turn off the diff pump's cooling fan.
9. You can finally stop the mechanical fore pump as you have valved off the entire system. You must now quickly let the small pre-foreline and TC gauge up to air to avoid getting mechanical pump fluid into the mechanical pump's pre-fore-line. I do this by releasing a KF fitting clamp on the top of my mechanical pump, tilting the fitting a tiny amount, hearing the air suck in and then retightening its clamp. THIS HAS TO BE DONE!

The above 9 steps might take 30-40 minutes with almost all of it being consumed in step #6

You are done.

Richard Hull

Re: FAQ - Diffusion Pump - What you need to know!

Posted: Wed Jul 09, 2014 8:38 pm
by Martin Hathaway
Why would one start pumping out just the foreline, then the diffusion pump, then finally the chamber instead of keeping all vacuum valves open and just pumping out the entire thing all at once? Does the air rush from the SATP areas to the low vacuum areas pose any significant problems when opening valves?

Martin Hathaway

Re: FAQ - Diffusion Pump - What you need to know!

Posted: Thu Jul 10, 2014 4:39 am
by Richard Hull
I like to check my pump out and only pull down the fore-line first. It would yank down quick. (under one minute) I then open the valve to the diff pump's volume this can take a few minutes as water or other foul vapors can accumulate. Once I get this as low as it will go, I open the fusor throttle valve all the way and evacuate the fusor chamber. Once the entire system is down I start the Diff pump.

There is absolutely no reason at all why you can't pull the entire mess down from the get go by opening all the valves. It is a matter of operator preference. It involves how much you want to be able to isolate your system components. All the valving, I noted above, will allow you to shut your system down in a staged manner, much as when you started it up.

The biggest advantage is that if you blow a fuse and your fore-line pump turns off the vacuum at the fore-line will disappear and you might suck oil from the forepump into the diffusion pump, causing the diff pump oil to flash boil and blow into the fusor chamber (super-ultra mess). With valving you can quickly isolate all three in an emergency.

Richard Hull

Re: FAQ - Diffusion Pump - What you need to know!

Posted: Fri Jul 11, 2014 2:26 am
by Martin Hathaway
This means that if I were to bring down my entire chamber all at once, the only valve I would actually need would be the one between the diffusion pump and the chamber, correct? I see no need for the foreline valve otherwise.

Martin Hathaway

Re: FAQ - Diffusion Pump - What you need to know!

Posted: Fri Jul 11, 2014 3:53 am
by Richard Hull
With no foreline valve you can never truly isolate your diff pump, a possible fatal error. If you lose your forepump with a running, boiling diff pump, ( Loss of power, bad pump motor, locked rotor, etc.), you will have a nasty mess as you flash the diff pump oil with loss of foreline pressure. Requiring a tear down of your whole vacuum system for cleaning and repair. Do as you wish, of course.

Richard Hull

Re: FAQ - Diffusion Pump - What you need to know!

Posted: Wed Oct 15, 2014 11:44 pm
by Charles Vorbach
Should you lose your forepump while the diff pump is still running, not that I am hoping to, what would be the procedure to prevent a mess?

Re: FAQ - Diffusion Pump - What you need to know!

Posted: Thu Oct 16, 2014 4:10 pm
by Richard Hull
Immediately isolate the diff pump from both the fore pump and the fusor chamber. Two valves here are a must have!!!

Once both valves are sealed off, kill the boiler power and pray the oil didn't flash boil and burn in the diff pump and that you valved off the fusor in time to avoid the diff pump vomitting into it.

Best out come.........You retained a low enough pressure in the diff pump with quick valve operation and nothing is messed up.

Richard Hull

Re: FAQ - Diffusion Pump - What you need to know!

Posted: Wed May 13, 2020 5:50 pm
by caden_Burkhardt
Just a note that in 2012, when this FAQ was originally posted one could find a diffusion pump for as low as $100. Because of changing times, the price has increased to around $300 in 2020, if you are reading further in the future, they might be even more

Re: FAQ - Diffusion Pump - What you need to know!

Posted: Wed May 13, 2020 8:37 pm
by Richard Hull
We post on what is current at the time of writing. When small complete functional turbos with controller and all cabling appear on e-Bay for $300, you will be able to get diff pumps of $25.00 each. Don't hold yer breath, though. Used diff pumps are the poor man's secondary pump option. Today "poor-man" is in the $300-$500 range and even then you will have to adapt to an often odd-ball throat flange!!

Richard Hull