| Summary: | 碩士 === 國立清華大學 === 先進光源科技學位學程 === 104 === In a short-wavelength single-pass high-gain free electron laser system, precision control of beam position is of critical importance. Resolution of the beam position monitor (BPM) to sub-micrometer level is often required. Cavity-type BPM was invented in the 60’s. They have been widely used in accelerator systems that require precise detection of beam positions since then. Thanks to the continuous efforts by major accelerator laboratories to improve cavity BPM performance, it has now become an important beam diagnostic tool. In contrast to the cavities for particle acceleration, cavity BPM usually operates at TM110 mode in which the longitudinal electric field on the cavity axis is null and it varies linearly along one transverse direction. Hence, the beam at different positions can be detected according to the amplitude of the longitudinal electric field excited by the beam.
In this study, we try to design a 2.5 GHz, sub-micrometer resolution cavity BPM for the proposed NSRRC THz/VUV FEL facility. This design is based on the BPMs that are used in the Spring-8 SACLA X-ray FEL as well as the KAERI THz FEL facilities. We start our study with the physics of the cavity BPM and the microwave characteristics of the structure to estimate the coarse dimensions of the BPM cavity. We then use the HFSS Eigen-mode Solver to calculate the microwave properties of the cavity and optimize the BPM dimensions for sub-micrometer resolution detection. On the other hand, we simulate the electron beam passing through the BPM and obtain the time domain signal picked up from the cavity by using the computer code -- CST Particle Studio. The deduced BPM resolution according to the simulated cavity picked up signals by CST is 2.2μm. Further, a prototype 2.5 GHz cavity BPM has been fabricated for bench measurement of microwave characteristics, the results agree well with the computer simulation results.
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