Rationale for size and number of accelerator rings

[houlahound]

large facilities like CERN have multiple rings of different diameters to prep the beam for the main ring. I get this is something like gears in a truck.

my question is what are the broad design constraints for choosing a specific number of rings of a specific diameter?

 

[rootone]

Often a previous accelerator project, after it's work is done, it get's commissioned as a pre-accelerator for a newer grand design.
This happened at CERN, I am sure mfb will get on soon to fill you in on the details.