Position-based cavity tuning

Knowledge of the longitudinal beam parameters is important for understanding beam dynamics in any machine. In a linear accelerator (linac), to guarantee a stable motion in the longitudinal plane, the settings for the radio frequency cavities must be established by determining the phase and amplitude witnessed by the beam. In most modern hadron linacs, the requirements on phase and amplitude accuracy are 1\ifmmode^\circ\else\textdegree\fi{} and 1% respectively, which is particularly challenging. Although systematic approaches for finding the phase and amplitude of cavities are available in many facilities with operational linacs, it is also vital to have a system in place when the standard time-of-flight based functionality is down or not yet available (as is often the case in the early stages of commissioning). Such a method should be relatively fast and simple, and should not depend on time-of-flight measurements or a rigorous phase calibration of the beam position monitors. This work presents a new phase- and amplitude-scan method which only requires information from transverse positions measured at beam position monitors downstream of the cavity being scanned. The scanning then relies on transverse trajectory correction, which has been developed extensively by the community. As a by-product, this method can also provide estimates for a cavity's transverse misalignment. The method has been developed and tested for a single-cell cavity; the simulated effectiveness for a multicell cavity is also presented.