Yet another nice article on desorbing water in vacuum systems is found in the January issue of R&D magazine. (pp 57-59) (Odd how a lot of research rags are pushing water related vacuum issues of late). Odd, but not frivolous.
Water is a real bug bear to the nether regions below one micron in vacuum systems. I highly recommend this article as it is short and sweet.
For those having no source for this article I will synopsize as best I can what I found significant. ************************************************
"Once the pressure is in the low millitorr range, the desorbing water makes up 99% of the gas load."
"....it's nearly impossible to achieve UHV without bakeout."
"for UHV systems,250 deg C is usually the required or recommended bakeout tmeperature"
"Viton has too high a permeation rate to allow UHV pressures to be attained. This leaves metal gaskets as the only reasonable choice."
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It is important to remember that UHV usually is considered to be 10e-7 torr or better! Therefore, good viton seals can allow 10e-6 torr to be readily achieved. This will satisfy the most demanding amateur needs in a fusor or fusion environment. The article gives hints on using classic bakeout or UV light desorption methods to reduce the pumpdown time due to desorbing water from chamber surfaces. UV light is comparatively new, and will not desorb as fast or as thoroughly as a good bakeout, but is amenable to viton gasketed systems, whereas bakeout is not.
Another quoted section........
"A water molecule, once desorbed, will not necessarily be pumped away. Although it might enter the pump, it's more likely that it will impinge upon another spot in the chamber (molecular flow). When it impinges, it might resorb or it might merely bounce. This results in a chaotic condition of molecules desorbing, resorbing, and generally moving within the chamber until they finally wander into the pump."
The author notes that a short wavelength intense UV source lamp inside the chamber will really do a nice job of desorbing water from the walls without heating.
We, of course, can follow a more classic route and use glow cleaning mode where UV and ion bombardment can desorb a lot of water relatively fast. This method I use a lot and so did the original Farnsworth team.
Just run the hell out of the device once you hit 100 microns by forcing as much power into the remaining gas as possible within the limits of damaging the grid. This will act sort of like a whimpy, but effective ion pump. Glow cleaning ends with extinction near 5 microns. Dry nitrogen can be readmitted to the 100 micron level and the glow cleaning can proceed again to grab more water.
This issue is of much more importance to those advanced fusioneers looking to hit 10e-6 torr levels for a pure backfill or who wish to run a filamented ion system. Most serious fusioneers will hit this "water wall" at some point.
Richard Hull
Created on Wednesday, February 21, 2001 5:35 PM EDT by Richard Hull