Automated correction of phase errors in the Advanced Photon Source linac

Maintaining stable phasing in a linear accelerator is crucial for maintaining optimal performance. If phasing is incorrect, the beam will in general have an energy error and increased energy spread. While an energy error can be readily detected and corrected using position readings from beam position monitors at dispersion locations, this method is not useful for correcting energy spread in a system with many possible phase errors. While energy spread can be corrected by looking at beam size at a dispersive location, this typically involves a beam-intercepting diagnostic and is not compatible with top-up operation. Uncorrected energy spread results in poor capture efficiency in downstream accelerators, such as the Advanced Photon Source (APS) particle accumulator ring or booster synchrotron. To address this issue, APS has implemented beam-to-rf phase detectors in the linac, along with software for automatic correction of phase errors. We discuss the design, implementation, and performance of these detectors, as well as their use in feedback to automatically correct linac phase errors during top-up operation.