Optimization of the cavity beam-position monitor system for the Shanghai soft X-ray free-electron laser user facility

To achieve high-efficiency operation of the high-gain free-electron laser (FEL), the electron beams and radiated photon beams need to be overlapped precisely and pass through the entire undulator section. Therefore, a high-resolution beam-position monitor (BPM) is required. A cavity BPM (CBPM) with a resonant cavity structure was developed and used in the Shanghai Soft X-ray FEL (SXFEL) test facility and can achieve a position resolution of < 1 mu m. The construction and operation of the SXFEL user facility also bring about higher requirements for beam-position measurement. In this case, the factors that affect the performance of the CBPM system were further analyzed. These included the amplitude and phase stability of the local oscillator, stability of the trigger signal, performance of the radio frequency front-end, signal processing electronics, and signal processing algorithms. Based on the upgrade and optimization of the system, a beam test platform was built at the end of the linear acceleration section of the SXFEL, and the experimental results show that the position resolution of the system can reach 177 nm at a bunch charge of 500 pC, and the dynamic range is controlled within +/- 300lm, and the relative measurement uncertainty of the bunch charge can reach 0.021%, which are significant improvements compared to the attributes of the previous system.

Automated summary

The cavity BPM (CBPM) system of the Shanghai Soft X-ray FEL (SXFEL) test facility was upgraded and optimized to achieve a position resolution of 177 nm at a bunch charge of 500 pC, a dynamic range controlled within +/- 300 nm, and a relative measurement uncertainty of the bunch charge of 0.021%, which are significant improvements compared to the attributes of the previous system.