電子在加速器儲存環中運轉的同時會因輻射而損失能量，需要藉高頻共振腔中的電場加速，以補充能量。但當電子密度高時，本身會在腔中激發出高次模（Higher Order Mode）而造成尾場（wake field）效應，影響電子穩定度。為了將共振腔中的高次模去除，清大高頻電磁實驗室和德國BESSY II加速器實驗室正密切地合作，共同進行高次模耦合衰減器的設計及研究。本論文中除了討論耦合器結構之對稱性對高次模的影響外，並優化其中的波導同軸模式轉換器（Circular Waveguide to Coaxial Transformer），最後討論新設計之高次模耦合器長度對電子加速基模的影響，以對高次模耦合衰減器的原理及設計做一完整的探討。

We present studies of a higher-order-mode coupling device, which employs a circular waveguide to coaxial transition (CWCT). The waveguide is double-ridged. A properly shaped back cavity at the far end allows broadband coupling to a coaxial window. Configurations of CWCTs with either symmetric or asymmetric ridge depth are optimized with the HFSS code. HFSS calculations have also been performed to examine CWCT damping effects on a DORIS box cavity. The anti-symmetric modes are shown to be much more effectively damped by the asymmetric CWCT, however, relative merits of symmetric and asymmetric CWCTs depend on other factors such as the cavity configuration. We also make optimizations of CWCT structure and discuss the fundamental mode damping effect of our newly designed symmetric coupler. A simulation study of the vacuum window for 7/8” coaxial is also included as an appendix.

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