The Bates Linear Accelerator Transmitter Phase and Amplitude Monitoring System

Abstract

The Bates Linear Accelerator uses six 4MW 2.856GHz RF transmitters. Phase drifts between the transmitters 12 klystrons of greater than 0.5 degree or any amplitude drifts compromise beam delivery and are a common occurrence. Machine operators in years previous to the Phase and Amplitude Monitoring system "PAM" were unable to tell which klystron was affecting beam and resorted to shifting the phases of each klystron experimentally. The PAM system has proven itself as a viable monitor, easing phase re-adjustment. PAM is comprised of off-the-shelf RF amplitude and phase detectors, phase shifters, splitters, and VME hardware under EPICS . The detected signals are amplified and matrixed to the control room via an EPICS driven custom video switch, are sample/held, then digitized by a VME 12bit ADC. Each of the 14 channels employ phase shifters and code that can auto zero the displayed phase. Phase drift is then apparent. Amplitude setpoints can also be auto zeroed with drift displayed as power deviations. To ensure repeatability to less than 0.25 degree a continuous calibration sequencer is employed that has been shown to measure phase drift independent of amplitude shifts. Under investigation is the viability of 60Hz sourced and slow phase feedback and correction.