BEPCⅡ超导高频腔输入耦合器结构及特性研究

Study on the Performance of the Input Coupler for the BEPCⅡSuperconducting RF Cavity

在高频加速系统中,加速腔的输入耦合器是至关重要的部分之一,其性能直接影响高频功率的耦合和加速腔的特性.北京正负电子对撞机重大改造工程(BEPCⅡ)中的高频系统中,超导高频加速腔采用的是同轴天线型功率输入耦合器,这是首次在国内采用的500MHz频率的大功率耦合器．本文中对这种超导腔耦合器的二次电子倍增效应情况、传输系数、热分布情况三方面的特性进行了研究．研究结果表明,耦合器在多个功率点存在二次电子倍增效应,必需采取相应的措施来改善其对耦合器的不利影响;通过对耦合器结构尺寸的优化使其在BEPCⅡ工作频率(499．8MHz)下能更有效地传输功率;通过模拟计算,得到了耦合器内外导体的热分布图象,为校核和改进冷却设计和布局的合理性提供了重要的依据．

The input coupler for the accelerator cavity is one of the most critical parts in the RF accelerating system, its performance directly influences the RF power coupling to cavity, therefore, directly influences the cavity＇s quality. In the RF system of the BEPC Ⅱ, an antenna coaxial input coupler based on the design of KEKB superconducting （SC） cavity＇s coupler will be used. It＇s first to use this kind of high power coupler under the frequency of 500MHz in China. Some studies on this RF coupler have been done, which includes the multi-pacting （MP） effect, the transmission performance and the thermal layout simulation. The results show that （1） there are several serious MP points exist in this kind of coupler and some necessary ways must be taken to protect the coupler from destroying; （2） the transmission performance under BEPC Ⅱ operation frequency can be improved by optimizing the configuration of the coupler; （3） the heat-distributing pattern of the coupler from the simulation provides the important information for the cooling system design.