A High Vacuum system.

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I have wanted to get into high vacuum for some time, so I can make homemade x-ray tubes etc.
However I was always under the impression it was expensive.
I picked up this small vintage Edwards EO1 diffusion pump off of eBay, for just £5.50!

Above: The EO1 diffusion pump.

It is a water cooled affair, and had to have manifolds made for it, plus I wanted a closed loop cooling system for it, since I didn't want to be tied to the sink.

I made all the manifolds out of soldered copper and brass, and built a cooling system based around a 120mm PC radiator, the kind enjoyed by the watercooled PC crowd.

Water is pumped round the system by a small 12v window washer pump, and the whole lot is mounted on a 5mm thick Aluminium plate.

Above: The completed pump.

Above: Rear view.

Above: The Diffusion pump connected to Javac rotay vane pump.

The backing pump is a Javac CC-141 rotary vane pump, that by itself, is quoted as pumping down to 15 microns, though I estimate 20μ.


In the above photo, note there are two vacuum lines to the backing connection.

The reason is that the diffusion pump requires a backing pump speed of at least 30 l/min, and whilst the Javac is rated for 140 l/min, the small diameter of the provided inlets chokes the pump somewhat, so two of the three inlets were used, so I would have reasonable pumping speed.

Above:

A discharge tube (made from a Pyrex rolling pin!) was hooked up to the assembly, to which 30kV DC was applied The cathode is the filament from a 12v lamp, and the anode is a copper rod. The above picture was taken with just the Javac running, and shows the striated positive column.

Above:

Once the Javac has reached its base pressure, the positive column is much shorter, and the luminous disks, have become thicker, and more diffuse.


Once the Diffusion pump gets upto operating temperature, the column fluctuates in length, then shrinks until.......

Above:

.....we get into the soft x-ray region shown above. The walls fluoresce a pale blue from the impact of electrons. Note the luminous vertical mark. This is some diffusion pump oil that ran down the inside of the tube, after I applied too much to the top bung, it is brilliantly fluorescent under the impact of electrons.


Seconds after this photo was taken, the vacuum had become so hard, that the tube became dark, and ceased x-ray emission. So now a hot cathode is needed.

Above:

Once again HV is applied, except the filament is now lit. You can't tell in the photo as the light from the filament has washed it out, but the glass surface of the tube is glowing pale blue again, and the tube is once again emitting x-rays.


I spent a lot of time and effort before finally acquiring a diffusion pump, trying to squeeze an extra micron out of the Javac, by using molecular sieves and cold traps, and can honestly say that this project was much easier, in terms of cost time and effort, in reaching a repeatable high vacuum.

I highly recommend that anyone wanting to build a high vacuum system should go down this route, its not that expensive.


The Javac as I recall cost £120 new, the Diffusion pump was £5.50, and the Radiator was about £6 new (in a "brown box") everything else was out of the junk pile.


The diffusion pump is tiny (only 1 inch inside diameter) I also have a 3 inch pump (these show up on eBay all the time, out of Spectron leak detectors), and this will be radiator cooled as well. This pump was also cheap at just 50p!


I'm using Dow corning DC-704 as the pump fluid.
DC-704 is a superior fluid, but also requires a lower backing pressure. DC-704 is claimed to be a hardier oil too, being able to resist the pump being opened up to atmosphere whilst the oil is boiling, and other accidents!

Further improvements

As mentioned above, I originally used two thin backing lines to the Javac. The reason was, that the fittings on the pump manifold (1/4" SAE) were much quite small. The original refrigeration service fittings on the pump along with using such thin tubing were doing a marvelous job of choking what could be a good pump! For this reason I was using two of the available ports, however I wasn't really happy with using such narrow diameter tubes, I disassembled the pump manifold, and modified it a little.

There are three fittings on the javac inlet, a 1/4" SAE, a 3/8" SAE, and 1/2" acme. The acme is on top of the inlet, providing the path of least resistance for the pumped gases, and is also has the largest internal diameter, so I modified this port. The internal diameter of the 1/2" acme is just over 6mm, and it turns out after removing it from the pump body, that the internal diameter of the inlet manifold is 10mm, so I bored out the center of the acme to 10mm for maximum possible flow rate. I soldered on a 1/2" speedivac coupling I had in my box of junk. A more modern flange might have been better, but this is quite satisfactory for now.

Above: the original manifold.

Above: Bored out to 10mm with a coupling soldered on.

Above: The complete coupling on the manifold.

Above: hooked back up to the Javac with 1/2" reinfoced hose.

The results:
Just to test the pump, to the other half of the coupling I soldered a 6" length of standard 15mm copper plumbing pipe, that just happened to fit quite snugly in the 1/2" speedivac fitting.
To this I attached one of my glass discharge tubes, shown further up the post, connected up the EHT supply, and fired up the pump. The flow rate and base pressure now is simply amazing! It will pump down to a "soft x-ray" vacuum (i.e. the luminous discharge in the tube will cease, and the tube will produce x-rays) in about 5 seconds, all by itself!
Before, it would take around 15 minutes to reach base pressure, and this was quite a way away from the vacuum it pulls now.

A final improvement!

Since unchoking the forepump proved so successful, I modified the manifold I made for the low side of the diffusion pump. Instead of the original 15mm copper pipe, I opened it up to 25mm with a brass KF/NW-25 half nipple I had found in a box of scrap vacuum parts I bought.

Above:NW/KF-25 fitting soldered on to the Diffusion pump manifold.

Not only does this allow for the maximum pumping speed, it also allows me to connect standard sized fittings to the vacuum pump.

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