作为直线加速器的前级聚焦加速部分，RFQ (射频四极场)在束流动力学和运行稳定性上都应表现良好，需要保持电场平整和电磁强耦合。为实现这两个目标，提出并研究了窗耦合结构。针对工作频率为81.25 MHz的1 m长模型腔，利用三维电磁仿真软件CST MWS微波工作室对传统的四翼型、四杆型高频结构进行了仿真，并重点研究了窗耦合型结构。对开窗的对称方式、开窗的个数和大小等进行了分析，发现合适的窗耦合结构能保持较为紧凑的横向尺寸同时能耗适中，同四翼型结构相比二极模频率也远离了运行频率。为验证模拟结果，建造了一个铝模型腔，并对模型腔进行了冷模测试，实测频率为81.41 MHz，相邻模频率差为10.74 MHz，与模拟结果接近。仿真模拟和模型腔测试的结果表明，窗耦合四翼型结构可作为较低频率RFQ的一种设计。
As a focusing and acceleration element in front part of the linear accelerator RFQ, should have a high performance in both beam dynamic and operation stability. It requires the electro-magnetic field of RFQ to keep uniform and strongly coupled. In this paper the window coupled structure is proposed and investigated to meet the requirements of RFQ design. Different structures have been compared and analyzed, including four-vane type and four-rod type, and the four-vane type with windows. It was concluded that window-coupled structure is more compact in the transverse dimension with modest power loss and the dipole frequency is far from the operation frequency compare to the normal four-vane structure. A one-meter long and frequency of 81.25 MHz model-cavity of alumimum was employed as a sample and simulated by using the microwave studio of CST. The low power RF test results show that the operating frequecy is 81.41 MHz and the nearest mode frequency separation is 10.74 MHz, which is in good agreement with the simulated values. It is concluded that the window-coupled structure is a candidate for low frequency RFQ.
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