A four-dimensional beam-position monitor

A new type of beam-position monitor (BPM) has been developed in the KEKB accelerator. The BPM employs an I/Q (In-phase and Quadrature phase) demodulator operating at an acceleration frequency of 509 MHz without down converting. The peak value of a demodulated beam pulse is sampled every revolution period. Not only horizontal and vertical positions, but also the longitudinal position can be detected turn by turn. The performance of this four-dimensional BPM is described in detail. A monitor is indispensable for tuning a beam injected into the KEKB.