用于ns间隔电子束团位置测量的低Q腔式BPM研究

Low Q cavity BPM is a key to distinguish closely spaced electron bunches allowing precise beam handling for XFEL facilities operating in a multi-bunch mode at high repetition rate, which currently is a significant trend posed by the development of XFEL. The signal pollution issue from previous bunches is raised by such close bunch separation down to nano-second and causes the position signals to be increasingly overestimated. Extremely low Q would be required if solely shorten down the decay time of a cavity BPM, the monopole mode, however can be much more appreciable at the dipole mode frequency. As a result, the error introduced to the actual position signal would greatly degrade the resolution of a cavity BPM being able to achieve. We are proposing a method to mostly remove the signal pollution at the moment of sampling by intentionally shifting the peak of the last bunch signal quadrature phase advance with respect to that of current bunch signal, where signal sampling is normally taken for nano-second spaced bunches. This phase shift is defined by properly choosing the operational frequency of dipole model regarding to the bunch frequency. This project is going to be carried out in a combining way of simulation and experiment: a low Q cavity BPM prototype will be designed and fabricated, RF measurement will be proceeded on this prototype as a benchmark to the simulation result afterwards; finally beam test will be performed on the protope at an THz-FEL beam line in Chengdu to demonstrate the method mentioned above, and also evaluate its position resolution, analysis the impact on the resolution from the residential signal pollution.

极短束团间隔的多束团运行模式是XFEL重要的发展方向，而基于低Q腔式BPM的在线式亚微米逐束团位置监测是实现精确束流操控的关键。其中的核心问题是由于束团间隔短至ns量级时凸显的束团间信号混叠对逐束团分辨影响。单纯减小衰减时间需要极低Q值的腔式BPM，会加剧单极模泄露对位置分辨率的不利影响。本项目基于束团间信号叠加的相位关系提出约束工作频率和束流重复频率的关系，将束团间信号混叠在固定采样时刻降至最低，让采样信号最大程度接近真实位置信号，放宽了对极低Q的要求。本项目将以模拟仿真和实验测量相结合的方式的开展：研制低Q腔式BPM原型腔对模拟仿真设计进行微波特性实验研究；通过在中物院十所成都基地THz-FEL装置束线上对原型腔进行束流实验研究，从原理上验证输出信号峰值采样时刻最小叠加方法，考察原型腔的位置分辨表现，评估前面通过的束团在该时刻的剩余电磁场幅度对位置分辨率的影响。

ns量级短束团间隔的高重频运行模式是XFEL重要的发展方向，而基于低Q腔式BPM的在线式亚微米逐束团位置监测是其中的关键技术。其中的核心难点是由于极短束团时间间隔引起的束团间信号混叠的问题。单纯减小衰减时间需要极低Q值的腔式BPM，会加剧其他模式对位置测量的干扰。本项目提出采用90°正交隔离，在峰值采样时刻消除相邻束团间的信号干扰的方法。项目执行期间，完成了一套低Q腔式BPM的研制，工作模式（偶极模）频率4.875GHz，有载品质因数Q=15。加工完成的腔式BPM验证了物理设计结果。在等效电荷量0.2nC条件下，离线冷测分辨率达到0.11μm。开展了54MHz重频下的束流实验，腔式BPM的响应信号给出的模式频率和带宽结果基本和S参数测量结果符合。在此基础上开展了433MHz重频下的束流实验，初步获得了间隔2.3ns的逐束团位置信息，通过建立腔式BPM对多束团通过的响应模型，评估了束团间信号串扰的影响，表明了90度正交隔离时，输出信号峰值采样时刻叠加最小。