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: Glow cleaning/Ion Pumps
Date: Dec 21, 9:30 pm
Poster: Richard Hester
On Dec 21, 9:30 pm, Richard Hester wrote:
In summary, it looks like fusor researchers are stuck for clean vacuum just (as Tom Ligon sez) "where vacuum gets interesting". The stated objective here is to get a as low a base pressure as economically feasible, to reduce the background crud.
A mechanical pump will get you down to a few microns if well maintained, and if you stand on your head and cross your fingers.
A diffusion pump will work where the mechanical pump leaves off, but may have backstreaming issues at low pressure if a cryogenic foreline trap (or extremely expensive fluid) is not used.
A turbo pump is a possibility for clean vacuum, but they are delicate and extremely expensive (unless you luck out like Richard Hull did).
Ion pumps are out....
Two possibilities left are maze and molecular sieve traps.
Both of these options are moderately affordable, but I wonder about their ability to pump hydrogen. Helium pumping might be a problem, too, but I don't expect to see much of it in the average fusor.
Another possibility is is a pair of cryosorbtion pumps. These could also be used to replace the mechanical pump in the roughing phase. Can anyone remark on the ultimate pressure obtainable by cascading a pair of cryosorbtion pumps? I also wonder about their ability to pump hydrogen. I have read that using 2 pumps in sequence allows one to pump inerts (and maybe hydrogen) better, as these tend to get displaced from the sieve material as the pump starts to load up. For the weekend warrior, the LN2 is not all that expensive, and I see dewars for sale on Ebay all the time...
>>Richard Hull's post on glow cleaning was interesting. I suspect that the glow cleaning process re-buries crap in the walls that had outgasssed during the time the fusor was idle. Perhaps another way of removing this crud would be to use a small ion pump which will tuck away the garbage into a space where it can be valved off. Afer a few operations of this sort, one might be able to sort out leaks from outgassing of the chamber walls. Can anyone with ion pump experience (Tom Ligon perhaps) comment on the feasibility of using one at fusor pressures?
>> Richard Hester
>We have a Perkin Elmer 220 L/sec 8-section ion pump that can pull 1e-9 torr. It is a DI pump, i.e. it has both titanium and tantalum plates.
>An ion pump operating at 1e-3 torr is expected to have a useful life of minutes, and may produce as much gas as they pump. They are intended for 1e-6 torr and below.
>When we added a Residual Gas Analyzer to our system we found that the ion pump is a poor choice when you are adding a gas load to the system. The fresh gas impacting the plates causes old gas to be regurgitated, and all sorts of interesting chemistry occurs.
>I would be more inclined to try some sort of regeneratable sorbtion pump trap. (The MicroMaze can work this way when freshly baked).