SPALLATION NEUTRON SOURCE LINAC BEAM POSITION AND PHASE MONITOR SYSTEM

The SNS linac currently has over 60 beam position monitors which allow the measurement of both beam position and phase from a single pickup. The signals from the pickup lobes are down converted from either 402.5 MHz or 805 MHz to 50-MHz IF signals for processing. The IF signals are synchronously sampled at 40 MHz to generate I and Q signals from which the beam position and phase are calculated. Each BPM sampling reference frequency is locked to a phase-stable 2.5 MHz signal distributed along the linac. The system is continuously calibrated by generating and measuring rf bursts in the processor that travel to the BPM pickup, reflect off of the shorted BPM lobes and return to the processor for re- measurement. The electronics are built in a PCI card format and controlled vith LabVIEW. Details of the system design and performance are presented.