熱陰極高頻電子槍是利用建立高功率微波在共振腔內之高梯度電場克服空間電荷力(Space charge)的影響，加速置於腔體內熱陰極所發射之電子以產生高亮度之電子束。若配合阿爾發-磁鐵 (Alpha-magnet)之使用，可以將電子束團長度壓縮至數十微米範圍內。以電腦程式(PARMELA)模擬電子在共振腔內的加速過程及在束團壓縮機制的射束動力學發現藉由優化電子槍腔體內之電磁場分佈，可使輸出電子束能量隨時間成線性分佈(Linear energy chirp)。再經過具有適當磁場梯度之阿爾發-磁鐵後，束團長度可以壓縮至三十微米以下(100飛秒)。本研究就是以研製2.856 GHz熱陰極高頻電子槍共振腔體為主題，並探討以該系統產生短束團電子束之物理機制。

實驗方面，電子槍的製作已在「國家同步輻射研究中心」高頻小組協助下完成。我們針對該共振腔進行冷測，證實其微波特性與電腦模擬結果相符，全腔室與半腔室內的電場強度比例已調校成二比一，該比例為產生短束團之理想值。我們並利用微擾方法測量驗證調校後沿著腔體中心軸縱向電場強度之分佈。此外，陰極燈絲測試、陰極活化、以及高功率微波測試等工作均已完成。

目前該實驗站正由國家同步輻射研究中心輻射安全防護小組進行輻射屏敝與防護工程、同時我們也正針對飛秒級電子束團之射束診斷及量測方法進行評估，預計在不久的將來即能進行電子束特性之量測。

In a thermionic RF-gun, electrons emitted from its cathode are accelerated by the high gradient accelerating field in the microwave cavity. Since the effect of space-charge force has been compensated by the high electric field on cathode surface, low emittance electron beam can be obtained. On the other hand, a beam with linear energy chirped bunches can be generated by optimizing the electric field ratio between full-cell and half-cell of the RF-gun cavity. With the alpha-magnet as bunch compressor, these linear energy chirped bunches can be compressed into bunches with very short lengths. Simulation results showed that a beam with bunch lengths at around 30 micrometers can be obtained from this scheme.

The RF-gun has been fabricated at National Synchrotron Radiation Research Center (NSRRC). Characterization of the cavity has been performed and measurement results are found to be in good agreement with those obtained from simulation. Electric field ratio between full-cell and half-cell has been optimized. Longitudinal electric field along the cavity axis is verified with bead-pull method. Cathode heater test, activation and testing of high power microwave system were completed for beam tests.

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