非對稱性電子束團填充模式對被動式諧波共振腔功能的影響

Effects of Asymmetric Filling Pattern on the Performance of Passive Harmonic Cavity for Light Sources

劉維婷(Wei-Ting Liu)

指導教授 ： 施宙聰 ； 王兆恩

國立清華大學/電機資訊學院/光電工程研究所/碩士(2006年)

摘要

摘要

電子束生命期和亮度一直是同步輻射光源用戶的考量。對中低能量的第三代同步輻射光源而言其電子束生命期主要受限於Touschek散射，而拉長電子束的縱向長度是可以在不影響亮度下，減低電子散射的機率，進而提升電子束的生命期。實際的作法是加入被動式諧波共振腔，利用電子束在諧波共振腔感應的高頻諧波電壓降低電子束團的回復力，拉長電子束長度。然而，非對稱性的電子束團填充模式會產生電子束團同步相位的改變，造成電子束團序列裡諧波電壓的不均勻，因而顯著地影響束團拉長的效果和生命期的有效延長。在之前的文獻，諧波共振腔因非對稱性電子填充模式造成的性能低落只能利用數值方式來模擬。造成性能低落的機制被推測是因為諧波共振腔的非線性效應，並且認為只能使用數值方式分析。
本論文撰寫一個程式可以重現前人的模擬結果，根據此程式，我們在有穩定解的假設下發展了解析解，用來分析電子束團拉長效果降低的機制。解析解的結果和前人文獻的模擬數據相當吻合，無論是常溫共振腔或低溫超導共振腔。此解析解解釋了非對稱性電子束團填充模式在有諧波共振腔的系統下會放大電子束團位移量的原因。其原因是來自於電子束團產生之電壓的變動和所需之電子束團同步相位的位移兩者之間互相競爭，以達到能量平衡狀態，意即每個束團從高頻產生器獲得的能量等於所需回補的能損。當被動式諧波共振腔操作在拉長電子束團的模式，電子束團如需要往獲得較大高頻能量的相位移動，其移動的同時會感應更多的電壓，意即損失較多的能量在共振腔，如此電子束團又得往獲得較大高頻能量的相位移動，直到兩者達成平衡。電子束團在共振腔感應的電壓變化的傾向和需求相反，因此需要更多的位移量來達到能量平衡。反之當被動式諧波共振腔操作在縮短電子束團的模式，電子束團在共振腔感應的電壓變化的傾向和需求一致，需要的位移量就小的多。

關鍵字

Touschek生命期 ； 暫態間隙效應 ； 諧波共振腔 ； 束團填充模式

並列摘要

Abstract

For a low-to-medium energy 3rd generation synchrotron light source, the beam lifetime is dominated by Touschek scattering which can be improved by lengthening the bunch length. Physically, the longitudinal restoring force can be reduced by introducing a harmonic voltage generated by passive harmonic cavities. The gap transient induced by an asymmetric filling pattern of bunch train can cause a considerable shift of synchronous phase that result in a huge variation of the harmonic voltage along the bunch train. Therefore, the bunch lengthening as well as the lifetime improvement is greatly degraded owing to deviation of the harmonic voltage from its optimal target in a degree strongly dependent on the size of filling gap. In this thesis, a tracking code is developed which reproduces the simulation results of previous studies. An analytic model, under assumption of stationary circulation of bunch trains around the storage ring, is conceived benefiting from the tracking code as a test bench for probing the degradation mechanism of bunch lengthening. The analytic model explain that the amplification of synchronous phase shift by implementing the passive harmonic cavity with an asymmetric filling pattern of bunch train reflects a competition between the variation of beam induced harmonic voltage owing to transient beam loading and the required shift of synchronous beam phase to gain opposite amount of rf generator voltage, in addition for maintaining the energy balance of individual stationary bunches along the bunch train.

並列關鍵字

Touschek lifetime ； gap transient effect ； harmonic cavity ； filling pattern

參考文獻

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