Cavity-based non-destructive diagnostics of beam quadrupole moment and energy spread

A non-destructive diagnostic system based on the TM220 mode of a rectangular cavity has been developed for beam quadrupole moment measurements, and its application to indirect beam energy spread diagnostics is demonstrated for the first time. By installing the cavity downstream of a bending magnet, variations in beam energy spread lead to changes in the transverse beam distribution and the quadrupole moment, which can be measured via the TM220 mode. Motivated by the requirements of high-repetition-rate superconducting free-electron laser facilities and high-charge, high-current linear accelerators (linacs), where screen-based diagnostics become impractical, the cavity-based approach offers a narrowband response and a significantly improved signal-to-noise ratio compared with a conventional eight strip line monitor (ESM). The cavity design, fabrication, and radio frequency (RF) characterization are described, along with the electronics architecture and the implementation of online tests. Beam experiments demonstrated the capability of monitoring quadrupole moment and energy spread. For the cavity monitor, a quadrupole moment measurement uncertainty of 0.034 mm2 was achieved at 350 pC. Based on the measured quadrupole moment uncertainty and machine optics parameters of the present experiment, an energy spread resolution better than 10-5 is estimated, significantly outperforming the ESM under identical conditions. Furthermore, by correlating the cavity-based measurements with the RF accelerating phase, the feasibility of using the cavity for phase feedback was demonstrated. The observed phase dependence indicates the potential for high-resolution RF phase monitoring and feedback under practical operating conditions.