I am currently part of a group (University) working on a project that is focused on designing a linac to find the Higgs Boson. I have a question about the the limiting factor for the tube/tunnel. Currently we have two considerations: 1) The thickness of the walls required to maintain the vacuum. 2) Wakefields and as such the distance that the walls would need to be away from the beam in order to reduce the effect of inducing a current in the walls, emittance growth etc. I am looking for some guidance and knowledge in the area as I am struggling to make any headway. Thanks for any help that you can provide.