Subharmonic Prebunching

Several applications exist where the normal rf structure of the beam from an electron linac is undesirable. For example, linacs as synchrotron injectors are generally operated on the sixth or seventh harmonic; since only one (or possibly two) rf cycles are captured into the synchrotron, elimination of the excess beam is desirable. Further, it is possible to produce a single bunch (picosecond pulse) in cases where the gun pulse is less than one cycle of the subharmonic frequency. Such short pulses are of interest to radiochemists working in pulse radiolysis, for calibration of fast radiation detectors (such as scintillator response functions) and for high resolution neutron time of flight work. Additionally, in some counting schemes it is possible to use single pulses if the interpulse time is sufficiently long, which may be achieved by the bunch suppression resulting from subharmonic prebunching. This report presents the results of a study of bunching by velocity modulation employing two single gap cavities, the beam passing first through a cavity supporting the subharmonic gap voltage and, after an appropriate drift distance, through a second cavity with the basic frequency gap voltage followed by a second drift space. A ballistic analysis is presented, followed by an analysis based on the charged disc model of the beam to incorporate the effects of space charge; the kinematics are relativistic.