Optimization criteria for standing wave transverse magnetic deflection cavities

An important linear accelerator requirement, in order to demonstrate narrow energy spectra, is the injection of electron bunches of narrow phase spread and negligible inter-bunch current. This can be achieved by r-f transverse modulation and clipping of the beam by an aperture prior to injection into the accelerator waveguide, i.e., chopper operation. By magnetically biasing the beam to one side of the centerline, it is possible to arrange for transmission into the accelerator at a time during each r-f cycle when the radial momentum imparted to the beam by the chopper cavity is passing through zero. The low efficiency of beam utilization normally associated with this type of operation, because of the high ratio of collected to transmitted current, can be considerably improved by combining the transverse chopping action with a suitably phased longitudinal velocity modulating field as obtained from a simple prebunching cavity. Transverse r-f deflection techniques also enable sub-harmonic bunch selection and injection into linear accelerators which are used as injectors for electron synchrotrons. This is achieved by driving the chopper cavity at the same frequency as the synchrotron r-f system (which is maintained at a precise sub-multiple of the linear accelerator fundamental frequency) and then prebunching the chopped beam at the fundamental frequency prior to injection into the linear accelerator