Many on this list who have never worked with vacuum systems, think it is merely a matter of obtaining the magic pump and any level of vacuum should be attainable.
Vacuum newbies are stunned when their mechanical pumps pulling against a 1 gallon plastic bell jar stops dead at 300 microns, when they need to be at 10 microns.
Right after leaks (virtual and real), Water and water vapor is the ultimate limiter of pressure in amateur and even professional systems.
Water vapor has a long "residence time" on surfaces within a system which other gases do not. Therefore, you will pump out most every gas in the chamber very rapidly except for water vapor. Once the pump moves from viscous flow into transition flow, water vapor is just not pumped much anymore. In molecular flow, pumping regimes, (after a few minutes of pump on time), only the chance release and normal desorption process will allow the water molecule any chance of getting to the pump. The net result is that water is pumped from a room temperature vessel over many hours (often as many as 16 hours are required to reach a base pressure for water in a given system with a given pump.
What this means to the neophyte is that any amateur fusor system should be designed with (1) minimum volume (2) easy access, (3)the largest possible pumping line diameter.
Reducing contamination due to water absorption on components prior to assmbly will reduce pump down time.
In an experimental environment, we want to constantly fiddle with and change things. Each takeapart of the fusor to replace burned out inner grids, damaged insulators, or make modifications, starts the water cycle virtually from scratch all over again!
The first real hassle encountered in vacuum work is real leaks. The second is virtual leaks. Water vapor desorption will look like a real leak, but actually be a virtual leak as the vapor will desorb over many hours under vacuum.
Do not be surprised when you do your first pumpdown, if after sealing up many major leaks, it looks as if you have an unsealable, slow leak. (you might, of course) Most likely, it is water vapor desorbing from the surfaces throughout your fusor chamber's volume.
This is why I stress a spherical chamber over a bell jar or a cylindrical system for doing real fusion. (minimum volume for any given size)
This is just one of the technological land mines awaiting any experimenter with vacuums. The Farnsworth team moved Fred Haak from the ITT chemistry and coatings division into their group in 1962 solely to acquire his vacuum system expertise.
Richard Hull
Created on Wednesday, January 24, 2001 11:33 AM EDT by Richard Hull