Abstract
同步加速器的電子動力來自高頻共振腔中被外加電磁波激發的基波場,但電子束本身亦可能在共振腔中激發高次模(high order mode)干擾電子的運行,影響光源品質.近年來國際間正積極研究高次模的種種抑制方法,以期達到無高次模(HOM free)的運作境界.一般採用的抑制方法,可區分為寬頻耦合及定頻耦合兩類型.前者使用波導管,將截止頻率以上的電磁波耦合到腔外予以吸收,後者使用探針(probe)或耦合環(loop)將特定頻率的電磁波耦合引出.本論文探討如何將這兩種方法合併使用以達到互補的功用,因而有效地降低或完全抑 制電子束團通過所激勵起的高次模.首先我們對以上兩種耦合方法個別做具體的硬體設計, 數值模擬,及優化分析,再依據結果將二者組合為互補的整體抑制結構.為了進型耦合成效理論分析,我們發展一種解析外插法,配合三維的HFSS程式(High Frequency StructureSimulator)使用,可以快速並精確地找到三維結構的高Q值共振模,所以不論裝上衰減元件與否,其共振模的頻譜都可因而確定和比較.結果顯示,以寬頻耦合為主,定頻耦合為輔的合併式方法可以將高基模頻率三倍以內的所有高次模的Q值降到激發起動值之內.Broadband coupling of high order cavity modes to externalloads has been studied extensively in recent years for thepurpose of suppressing multibunch instibilities in electronstorage rings. High order mode damping is typically achieved bythe use of the several waveguide filters or a set of loop andprobe couplers. In the study, we show that a combination ofthese two methods can be mutually complementary and thus veryeffective for the damping of high order modes. For thetheoretical analysis, we develop an analyticalextrapolation method which, when implemented into the HFSSprogram (High Frequency Structure Simulator), allows rapidsearch of high Q resonant modes in the 3Dconfiguration. Mode spectrum of the RF cavity with andwithout the damping components are thus determined and compared.It is shown that the combined method can reduce the Qvalues of all the high order modes up to three times thefundamental frequency to within acceptable limits.