Really really extreme vacuum

[sudsmaster]

Way back in the early 70's I had a work-study job at the Space Science Lab at UC Berkelely. Part of my tasks was to set up and run a small vacuum chamber to test the transmittance of various materials to extreme ultraviolet light. This was, as I recall, similar if not identical to the "Lyman Alpha" light rays that permeate the universe. In any case, any air molecules would absorb this light so a very low vacuum had to be achieved. Regular fan driven or reciprocating vacuum pumps weren't enough. In order to achieve the very low vacuum levels, a special pump called an oil diffusion vacuum pump was used. This pump could not work in our normal atmosphere, so we used a reciprocating pump to bring the chamber pressure down to about 1 thousandth of an atmosphere, at which point we'd turn on the oil diffusion vacuum pump to less than a millionth of an atmosphere. Now, that's really sucking!

This work was related to some research into the first black holes or neutron stars described, but of course I played an extremely minor part in that.

For a discussion of how an oil diffusion vacuum pump works, see the following link.

Oh, I only worked at the Lab for a summer or so. It was interesting if rather confusing work and my boss wanted me to stay on, but at the time I was convinced I was destined to be a biologist. Ironically, nowadays I machine some of the parts that go into vacuum chambers used for various industrial processes.

http://www.svc.org/EP/EP_FeatureGuide.html

 

[tolivac]

Oil diffusion vacuum pumps were also used in vacuum tube,X-ray tube and CRT manufacturing-rebuilding.These devices required the "hard" vacuum to operate.

 

[sudsmaster]

I guess I may have wondered how (and how much) the vacuum is pulled in those CRT and other vacuum tubes, but I don't recall Fluorescent tubes also seem to contain a fair amount of vacuum. Incandescent light bulbs, as I understand it, don't need a vacuum, they use an inert gas like argon instead to prevent the filament from burning up.

 

[tolivac]

Flourescent bulbs are also pumpted to a high vacuum-then argon or xenon gas in put into the tube along with a small amount of mercury.The inside of the bulb is lined with a phospor that responds to the UV light generated by the argon-mercury mixture to produce light.Same sort of thing for neon tube signs-sign shops may have a diffusion pump along with the mechanical one.for neon tubes-the tube is first pumpted to high vacuum than a small amount of xenon gas is put into the tube-then the signmaker "bombards" the tube by running it from a high voltage transformer that generates more current than a standard sign transformer.This burns the impurities out of the tube and the electrodes.The bombard process may be repeated agian-repump the tube and reintroduce the xenon gas-after the bombard process the neon gas to be used is then pumpted into the tube-it is finally sealed and the sign tube is ready to use.Its an interesting process to see.My brother worked in a sign shop so I got to see these things.

 

[tolivac]

Oh yes-early incandscent bulbs used a high vacuum to prevent oxidation of the filament.It was later found to use the inert gases-the bulb is pumpted to vacuum before the gas is introduced.

 

[fa_f3_20]

The latest thing in high-vacuum pumps these days is the "cryopump", which really isn't a pump at all -- it freezes any air that is left in the chamber.

A few years ago I was working in a building next to a thing called the "X-Ray Calibration Facility" at NASA/MSFC. This has a metal tunnel, about 1m in diameter and 1500 feet (~500m) long. NASA uses it to calibrate X-ray and gamma-ray sensors on scientific spacecraft. They place the sensor at one end of the tunnel, which is inside the building. The tunnel goes through the side of the building, down a long road outside, and at the other end there is a small building with an X-ray source in it. When they pump down the tunnel for a test, they start with conventional pumps, then they turn on the oil diffusion pumps. When the diffusion pumps are on, you can tell because they have large exhaust pipes that go outside of the build, and lots of oil smoke comes out. Once the tunnel is good and pumped down, they turn on the cryopumps. There is one about every 200 feet along the length of the tunnel, and when they are operating you can hear them -- there is a valve that opens and closes and makes clicking noises at about 1.5 Hz. There are two enormous tanks of liquid nitrogen that feed them, and when the tunnel is in use, they have to get a tanker truck full of liquid nitrogen every day. Every now and then they have to purge the system, and when they do, a vent about halfway along the tube blows out enormous quantities of cold nitrogen vapor. I was near it one day when the vent opened, and I found myself in an instant fog. Got a bit light-headed too.