A Study of Some Transversal Motions of the Electron Beam of a Linac

It is well known that when the magnetic focusing field at the input of an electron linac increases the output accelerated current does not increase regularly. Such a behaviour is shown to be connected with rotations, in the output plane of the linac, of the center of charges of the beam. These rotations have been displayed experimentally by means of a beam position monitoring system and related to the transverse momentum transferred to the beam by the spurious TM11 mode existing in the mode converter which changes at the input of the Saclay linac, the TE01 mode from the rectangular waveguide into a TM01 mode on the accelerator circular waveguide.