Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: Re: Vacuum Alternatives
Date: May 12, 8:23 am
Poster: Richard Hull

On May 12, 8:23 am, Richard Hull wrote:

>I've noticed that compressors usually cost considerably less than vacuum pumps. Wouldn't a compressor placed *within* an area to be evacuated, and connected to the outside through the valve which would otherwise feed the compressed air tank work, at least to some extent? It would not be hard to imagine a set-up involving this.

The vacuum required for minimal fusor service is on the order of 1 millionth of atmospheric pressure. (1 micron - 10e-3 torr)

Ideally, we would actually like to be able to reach one ten millionth atmosphere. All regular "compressor" oils boil at these pressures and all compressor valves fail well above these pressures. Thus, the need for a special and unusually engineered pump (which costs money).

Vacuum pump oils are special and expensive and all but the $100.00/quart oils (fomblin) still vaporize to a greater or lesser degree in even the finest mechanical vacuum rated pumps that money can buy.

Yes, you have to have a real vacuum pump made for scientific purposes or at bare minimum a good, new, clean, refrigeration technician's direct drive vacuum pump. RH

>Also, why not use some kind of cryogenic means to condense the air within a vacuum chamber to the liquid state, at which point it would then be pumped out?

Cryogenics would be more expensive and hassle laden in the long run than a simple scientific vacuum pump. Also most of the cryo apps which trap the enclosed gases, leave them where they lie and do not "pump" them out of the system, but instead, just stick them to the walls of the system at the cryo piping point (usually a cryopump). RH

>Finally, one might be able to heat the air within a vacuum chamber, allowing it to escape as it expands. The air could be heated up to the thermal tolerances of the chamber's construction material. Closing the escape valve and allowing the temperature to drop to ambient would produce a degree of evacuation. Also, anyone with any type of vacuum pump might obtain a higher degree of evacuation if the gas being evacuated was already hot.

The degree of vacuum obtained in your first method of heating and valving is way higher in pressure than we need. The heated assist to a regular pump also would not be very feasible as once the pump bottoms out near 10 microns, the system is in a molecular flow regime and heat is not readily transferred molecule to molecule anymore. The normal method is to next freeze out the remaining molecules (cryogenics, blast them out kinetically (diffusion pumping or turbo pumping), combine them out (Titanium sublimation pump or getter pump), or bury them (ion pump). All of these methods take the vacuum far below that of even the most expensive mechanical scientific vacuum pumps, but at additional costs.

Richard Hull