Development of the beam chopper timing system for multi-turn injection to the J-PARC RCS

Multi-turn injection using charge exchange is employed for the J-PARC rapid cycling synchrotron (RCS). To reduce space charge effects, the bunching factor of the beam in the ring should be increased. Therefore the momentum- offset injection scheme is used. In each turn, the bunch trains from the linac are injected into the RF buckets with a momentum offset. The bunch train is called the "intermediate pulse". The intermediate pulses are generated in the low energy section of the linac by the RF chopper and pre-chopper. Since the pulses must be synchronized to the RF voltage in the ring, the timing signals for the choppers are generated by the low-level RF (LLRF) system of the RCS. The RF chopper and the pre-chopper require different pulse widths. Thanks to the direct digital synthesis (DDS) in the LLRF system, precise zero-cross signals for the reference of the chopper pulses are generated without difficulties. The cable length from the RCS LLRF system to the linac chopper control system is more than one kilometer. Thus, the chopper pulses are sent via optical cables. We developed the chopper timing module. We describe the details of the hardware.