Beam dynamics measurements using a gated beam-position monitor at KEKB

Effective head-on collision in finite-angle crossing beam orbit was established by installing crab cavities in KEKB, an asymmetric electron/positron collider. During the crabbing collision, bunch-by-bunch measurements for determining the transverse positions of the beam and the beam phase were carried out using a gated beam-position monitor (GBPM). The measurements made using the GBPM revealed new beam behavior. Using the monitor, displacements in the horizontal beam position and variations in the beam phase along a bunch train could be determined. The horizontal displacement was found to be caused by a phase-shifted bunch passing through the crab cavity. The monitor also detected a rapid change in the beam phase at the leading part of a bunch train. Parasitic modes with low Q values in the normal conducting (NC) cavities are one candidate considered responsible for causing the rapid changes in the beam phase. Moreover, a fast gate module, which helps in selecting a bunch with a 2-ns spacing has been developed for use in future studies.