Ultrafast cavity beam arrival monitor for high-repetition-rate XFELs: Design and commissioning at SHINE

The development and validation of a cavity beam arrival monitor (CBAM) system for the Shanghai HIgh repetitioN rate XFEL and Extreme light facility (SHINE) achieves sub-20 fs timing resolution at 100 pC across a 10 pC–300 pC dynamic range. Based on a 3.520 GHz TM010-mode cavity probe, the system incorporates a down-conversion scheme with low-noise amplification and high-dynamic-range conditioning. A beam diagnostic processor utilizes a 14-bit ADC at 866.7 MSPS (derived from a 1.3 GHz reference via phase-locked loop (PLL)) to sample the 54.2 MHz intermediate frequency (IF) signal. Real-time signal processing employs an optimized fixed-point Goertzel Discrete Fourier Transform (Goertzel-DFT) algorithm with windowing for precise phase extraction. Beam tests at SHINE demonstrated 4.9 fs (RMS) phase resolution and 4.3 $\times$ 10^-4 amplitude accuracy at 100 pC, while SXFEL validation confirmed 13 fs resolution at 8 pC. The key innovations of this work include a novel bunch-to-bunch overlap correction algorithm for 1 MHz operation, reducing systematic phase errors from 260 fs to below 0.3 fs. Additionally, an active temperature control system for coaxial cables demonstrates exceptional long-term stability, improving phase drift by 30.6 fs over 7-hour continuous operation.