einzel lens for TOF

[Cesar]

Hello everyone, I wanted to develop an einzel lens to guide the ions
into a oaTOF. I have been playing around some with Simion but had a
few questions about the "real" conditions. First, the region where
the einzel lens will be placed has a pressure of 1-5E-4 Torr. Second,
the energy of the incoming ions is 20 - 150 eV.

I am curious if there is a good method to properly space apart the
lenses (3 lens setup) taking into account the pressure conditions. It
is worthwhile to note that the "beam" includes both ions and neutral
species.

Any suggestions would be appreciated, and references on practical
aspect of building and designing einzel lenses would be great.