In Reply to: Backstreaming problem posted by Anneleen on 10/25/01 at 10:08 AM:
Yes and no to both.
1. If one changes the conductance between the pump throat enough
The resultant effect is the top jet sees a pressure that can prevent backstreaming.
Problem is that it also affects the speed at the chamber significantly enough so as to increase the pump down time dramatically
by using this approach.
2. Cooling will affect performance and cause backstreaming. On large pumps (16" and larger) it is possible to split up the cooling
as opposed to running the line in series from the top to the foreline (typical method). This parallel plumbing approach is usually done when one cannot deliver the proper volume of coolant
to the pump. So one ends up with one supply and return to the top, one to the body and one to the foreline arm.
If youre having backstreaming problems, a couple things to keep in mind;
a) ALL diffusion pumps backstream. Youll find the manufactures spec on the quantity in the manual and in their sales literature. The question is more one of how much can I live with.
b) ALL pumps are at their worst at the point where the system is crossed over from the roughing pump to the high vac circuit. 1st option is to cross over at a lower pressure.