When we use magnets in linear accelerators?

there is a very good description on all the magnets used in linacs in the historical documentation on the SLAC at Stanford, which you can find on-line. If I can remember its address I will add it in an edit to this answer.

keeping that beam steered in the right direction and "in focus" was not as hard an undertaking as one might expect. At the (almost constant) speeds with which the electrons traversed the beam tube, its 10,000 foot length was foreshortened down to about 34 inches.