Diffusion pump project

plazmatron, Sat Oct 10 2009, 07:46PM

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 about a month or so off of eBay, for just £5.50!





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 watercoled 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






Front view.





Rear view.





Hooked up to the Javac.

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.







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.






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.......





.....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.






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!


Les