You can't start linear
A simple room-size cyclotron can build up more energy than a linear accelerator the size of the moon - it is residence time that matters - acceleration times time s= u 1/2 a.t^2 and e=m. C^2.
Near light-speed particles don't reside long in a linear tube...
(And you can't reflect them at the ends - that needs massless particles like photons and then you've built a laser the size of the Moon.
Hmm, I know someone who could do SERIOUS things with one of those!)