Bunch-by-bunch position measurement with sub-micron resolution at nanoseconds separation using wideband cavity beam position monitors.

We developed a wideband RF cavity beam position monitor (CBPM) with a 217 MHz bandwidth centered at the 4.875 GHz dipole mode frequency as part of the preliminary research for a high-repetition-rate hard x-ray free electron laser project at the Chinese Academy of Engineering Physics. This paper presents new results demonstrating bunch-by-bunch position measurements on electron bunches spaced by 2.

Correction of coupled higher-order modes in the S-parameter characterization of wideband cavity beam position monitors.

Wideband RF cavity beam position monitors (CBPM) have been increasingly employed for short bunch interval operations in several linear accelerators. At a few nanoseconds of bunch spacing, the loaded quality factor of the CBPM TM dipole eigenmode is required to be sufficiently low to minimize the signal interference between consecutive bunches. Moreover, the bunched beam also couples to several unwanted higher-order modes (HOM), such as the TM and TE eigenmodes, which also may result in an error of the bunch position measurement if no precautions are taken.