The Super Tau Charm Facility (STCF) is a new generation electron-positron colliders at the forefront of high precision, and its high brightness requirements pose a major challenge to accelerator technology. Resonant cavity-based monitors utilize characteristic mode signals for non-intercepting, high signal-to-noise ratio measurements, hence may meet the online high-resolution measurement requirements of various high-quality linear accelerators.

This study aims to address the challenges posed by the short lifetime and the small dynamic aperture of the storage ring beams in the STCF by developing high-resolution monitoring techniques for bunch length and charge to ensure efficient injection and precise measurement of these parameters.

According to the beam parameters and measurement requirements of the STCF injector, the physical design and simulation of the resonator bunch length and charge monitor were carried out. Two Pill-Box cavities were designed by using Computer Simulation Technology (CST) modeling, and their structures were optimized. Subsequently, the beam load in the CST particle studio for simulation was conducted to analyze influences of beam tilt and lateral offset on the measurement accuracy, and the measuring resolutions of bunch length and charge were evaluated using cavity beam position monitor (CBPM).

Simulation results show that the measurement errors of bunch length and charge are 3.3% and 0.02%, respectively. According to the online test results of the same type monitor, it is estimated that the resolution of bunch length of the monitor is expected to reach 100 fs@1.5 nC, and the relative resolution of charge measurement is better than 0.07%.

The currently designed monitor meets the diagnostic requirements of bunch length and charge of STCF, it will be manufactured in future for online testing.