用於產生高品質電子束的雷射激發光陰極射頻電子槍在先進加速器光源系統上有許多重要的應用，其中包括低發射度同步輻射光源、X光自由電子雷射等。在國內，國家同步輻射研究中心已自行研發出一套2998 MHz光陰極射頻電子槍系統。它主要是用於光源研究，未來可望應用於極紫外光自由電子雷射的研究上。目前該射頻電子槍的加速梯度約為70 MV/m，束團重複率為10 Hz。然而，我們預期將來潛在用戶會需要更高重複率進行實驗。因此，研發能夠產生高品質電子束且在幾十甚至是幾百赫茲的重複率下操作的新電子槍是一項重要課題。
本研究中，我們希望在提高重複率的同時也將加速梯度提升至100 MV/m以上以提高電子槍的束流品質。因此，我們將電子槍的2998 MHz共振腔結構做了許多優化設計，其中包括在波導管連接共振腔的位置採用Z形耦合結構、兩腔之間耦合半徑由圓形改成橢圓形，減少在耦合孔上的表面電場防止發生射頻崩潰、波導管傾斜之設計、以及使用雙邊波導管去改進場的對稱性等並降低耦合孔附近的場強等，去改善電子槍整體性能。射頻電子槍的共振腔腔體的電磁設計將採用SUPERFISH以及HFSS這兩個模擬軟體。另外，在高重複率、高加速梯度操作下，射頻功率在腔體金屬表面消耗時所引起的散熱問題必須要等到有效解決。因此，我們使用了COMSOL去分析電子槍的熱傳現象，在只考慮空氣自然對流的情況下，腔體表面溫度將高達千度，已超過銅材料的熔點，所以水路設計是必要的。考慮當脈衝長度為兩個微秒，以及加速梯度為120 MV/m的情況下，腔體表面溫度最高上升約70℃。未來水路設計希望能最大限度帶走腔體表面的熱，使上升溫度控制在50到100℃，便可達到在高重複率高微波功率下正常運作。

In the past decades, laser-driven photo-cathode rf gun technology has been developed for production of high quality electron beams for various applications including linear collidiers, ultrafast electron diffraction system, free-electron lasers and many other novel accelerator-based light sources. A 2998 MHz photo-cathode rf gun has been built in NSRRC for light source research and it has been operational regularly at 10 Hz rf pulse repetition-rate since 2013. Based on the experience we have accumulated on high brightness electron beam technology, a design study of an EUV FEL facility has been carried out recently. We can foresee that potential users may require EUV radiation at higher pulse repetition rate and therefore to design a new gun system that delivers high quality electron beam for tens or even hundred Hertz operation is essential. At high repetition-rate, it is more likely to have rf breakdown in the cavity and higher average rf power has to be dissipated on its surface. In this study, design strategies are implemented to avoid possible rf breakdown and to improve beam quality. These improvements include adoption of z-coupling scheme, modification of coupling iris dimensions and reduction of surface field on the iris, waveguide tilt to reduce pulse heating, using dual rf feed waveguides to balance the electric field along x-axis and other minor changes to improve performance and diagnostic capabilities. RF design of the gun cavity has been performed by using the codes SUPERFISH and HFSS. By using COMSOL, a commercial multiphysics code, time-dependent analysis of heat conduction in the cavity with average rf dissipation on inner cavity wall is performed. At 100 Hz rf repetition frequency, 120 MV/m field gradient, about 2 kW rf power is dissipated on cavity wall and its temperature can goes beyond the melting point of cooper. Adequate water cooling pipe layout is therefore absolutely necessary. Simple water cooling method by soaking the cavity into a constant temperature heat sink is preliminarily studied. Guidelines for more practical cooling channel design are suggested. If the rf gun operates at higher gradient field (i.e. > 100 MV/m) for a lower emittance beam, pulse heating of cavity surface that occurs in a couple of microseconds has to be studied. Maximum temperature rises due to pulse heating on the cavity inner surface are predicted also by COMSOL at different values of field gradient.

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