Beam Interactions and Instabilities

The electron beam which IS accelerated in the SLAC two-mile accelerator exhibits both transverse and longitudinal interactions with the disk-loaded waveguide, The transverse interactions include such effects as rf steering caused by field asymmetries which exist in the couplers, and beam breakup, The latter is a growing instability which occurs above a given current threshold and deflects the beam into the walls of the machine. The longitudinal interactions include bunching, acceleration and the general phenomenon of beam loading which causes spectrum broadening. Both beam breakup and beam loading are basically detrimental to the use of the beam and must be controlled or compensated for. Beam breakup, especially, is affected by the manner in which the beam is formed in the injector, and in particular by its time structure, i.e., whether the train of bunches is continuous or chopped. 1