在單通高增益自由電子雷射系統中束團位置的精密調控是非常重要的，束流位置偵測器的解析度往往需要達次微米等級。腔型束流位置偵測器自1960被提出後，在加速器領域中已廣泛使用在精確的電子束位置偵測上。經各大加速器研究中心不斷改進及應用，現在已成了許多加速器裝置中束流診斷的重要工具。與加速腔不同，腔型束流位置偵測器的工作模態為 TM_110模，此模態的特點在於共振腔軸線上電場為零並在橫向呈線性增強，對於處在不同位置的帶電粒子我們可以偵測到不同大小的電壓訊號，經過分析達到束團位置偵測的目的。
本研究中，我們以日本 SACLA X光自由電子雷射設施及韓國 KAERI THz自由電子雷射設施設計參考，依國家同步輻射研究中心提出的 VUV/THz FEL計畫需要，設計一組 2.5GHz的次微米解析度束流位置偵測器。
我們從研究偵測器物理原理和束流位置偵測器腔體結構的微波特性出發，估算出位置偵測器所需的腔體尺寸優化，接著使用 HFSS Eigenmode solver作為模擬偵測器腔體微波特性的工具，再調整腔體尺寸，使每個腔體有相同的頻率和工作於正確的模態。另外使用 CST Particle Studio模擬電子束經過偵測器的變化以及預期能獲得的訊號，從而推算出在我們的設計之下預期能夠達到2.2μm的解析度。我們進一步製作一套束流位置偵測器原型並量測其微波特性，得知量測電腦模擬結果相符。

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.

參考文獻
[1] N.Y. Huang, et al, “Preliminary injector design for VUV FEL facility at NSRRC,” 2014 FEL winter school, Hsinchu, Taiwan.
[2] L. H. Yu et al., “High-Gain Harmonic-Generation Free-Electron Laser”, Science 289,392 (2000)
[3] M. Labat et al., “High-Gain Harmonic-Generation Free-Electron Laser Seeded by Harmonics Generated in gas”, physical review letters, PRL 107, 224801 (2011)
[4] L. H. Yu et al., “First Ultraviolet High-Gain Harmonic-Generation Free-Electron Laser”, physical review letters, vol 91, NUMBER7
[5] M. Wendt, ” OVERVIEW OF RECENT TRENDS AND DEVELOPMENTS FOR BPM SYSTEMS”, Fermilab, Batavia, IL 60510, U.S.A.
[6] Sean Walston, “PerformanceofaHighResolutionCavityBeamPositionMonitor System”, Fermi National Accelerator Laboratory, Batavia, Illinois, USA
[7] SIMULATION OF A CAVITY BPM FOR HIGH RESOLUTION SINGLEPASS BEAM POSITION MEASUREMENTS A. Morgan, G. Rehm, Diamond Light Source, UK
[8] Z.D.Farkas, " PRECISION ENERGY MEASUREMENT TECHNIQUE,"
[9] T. Shintake, “DEVELOPMENT OF NANOMETER RESOLUTION RF-BPMs”, KEK
[10] T. Slaton and G. Mazaheri , T. Shintake DEVELOPMENT OF NANOMETER RESOLUTION C-BAND RADIO FREQUENCY BEAM POSITION MONITORS IN THE FINAL FOCUS TEST BEAM
[11] D. Lipka, “CAVITY BPM DESIGNS, RELATED ELECTRONICS AND MEASURED PERFORMANCES, DESY, Hamburg, Germany
[12] Ronald Lorenz., “Cavity Beam Position Monitors”, DESY Zeuthen, Platanenallee 6, D-15738
[13] H. Maesaka, “DEVELOPMENT OF THE RF CAVITY BPM OF XFEL/SPRING-8”
[14] Hirokazu Maesaka. PERFORMANCE OF THE RF CAVITY BPM AT XFEL/SPRING-8 “SACLA
[15] Stephen R. Smith. Beam Position Monitor Engineering*
[16] Robert E. Shafer., “BEAM POSITION MONITORING”
[17] S.SHIN, “Design of a Low-QS-BandCavityBeamPositionMonitor”
[18] Fabio Marcellini, Boris Keil, Martin Rohrer, Markus Stadler, Jerome Stettler, Daniel Marco Treyer, PSI,Villigen, Switzerland, “DESIGN OF CAVITY BPM PICKUPS FOR SWISSFEL ‘’
[19] P. Craievich et al., “COMMISSIONING OF THE X-BAND TRANSVERSE DEFLECTOR FOR FEMTOSECOND ELECTRON/X-RAY PULSE LENGTH MEASUREMENTS AT LCLS”, Proceedings of IPAC2013, WEOBB201
[20] P. Krejcik et al., “DESIGN OF THE CAVITY BPM FOR FERMI@ELETTRA”, ELETTRA, Trieste, Italy
[21] Seon Yeong Noh, Eun-San Kim, Ji-Gwang Hwang, A. Heo, Si won Jang, Nikolay A. Vinokurov, Young UK Jeong, Seong Hee Park, and Kyu-Ha Jang, " Development of an S-band cavity-type beam position monitor for a high power THz free-electron laser
[22] Sean Walston et al.”Performance of a high resolution cavity beam position monitor system”
[23] www.cst.de
[24] www.ansoft.com
[25] John Byrd, “RF cavity higher order mode measurements”, Microwave measurements Laboratory, USPAS and CCAST, Beijing, China, 1998.