Development of a photoelectric hybrid phase reference line system for super tau-charm facility-beam test platform

The Super Tau-Charm Facility-Beam Test Platform (STCF-BTP) is a platform developed to validate and optimize the design and feasibility of the Super Tau-Charm Facility. It requires precise synchronization of accelerating fields to achieve the desired high beam luminosity. We propose a novel phase reference line system that incorporates a phase reference distribution system based on a photoelectric hybrid approach and a control measurement system leveraging low-level radio-frequency systems. In the distribution system, phase-compensated signals are transmitted through optical fibers for drift stability, while low-noise microwave signals are delivered via coaxial cables. The receiver modules perform phase calibration between these dual paths to output optimized reference signals. Laboratory tests over a 50-m transmission path demonstrated that the system achieves <15 fs additional jitter (10 Hz to 10 MHz) and maintains phase drift of below 20 fs RMS over 24 h across all receivers, meeting stringent requirements of the STCF-BTP. This hybrid approach offers a robust solution for precise phase reference distribution in accelerator facilities requiring femtosecond-level synchronization across multiple stations.