提出了一种利用单个谐振腔测量直线加速器束团长度的新方法。不同于传统的双谐振腔束团长度监测器，该方法仅需要使用单个谐振腔，省去了用于辅助测量的参考腔，从而简化了装置结构，节省了空间。针对国家同步辐射实验室基于可调谐红外激光的能源化学研究大型实验装置进行了监测器的物理设计。通过设定同轴探针的插入位置，解决了双模信号互相干扰的问题，利用微扰金属改变调节工作频率，克服了双模难以同时谐振的困难。利用CST软件进行建模和仿真。仿真结果表明，该监测器可实现2～5 ps束团长度的诊断，测量误差小于5%。

This paper presents a new method to measure bunch length of linac with single cavity. Compared with the traditional way, the new method does not need reference cavity. Two eigenmodes of a rectangular cavity are utilized to measure bunch length, so that the improved device is simplified and compact. A single cavity bunch length monitor is designed for the National Synchrotron Radiation Laboratory’s Tunable Infrared Laser for Fundamentals of Energy Chemistry. The positions of coaxial probes are optimized to avoid interference modes according to the electromagnetic field distribution. To control the working frequencies of the eigenmodes, the tuning screws are introduced in the cavity, and the method of dimension parameter optimization is put forward. A series of simulations based on CST is performed to verify the feasibility. The simulation results reveal that this monitor meets the requirements of bunch length measurement, and its relative error is less than 5%.