四杆与四翼型RFQ内的二极场研究

Dipole Field in Four Vane and Four Rod RFQ

RFQ(射频四极透镜)是直线加速器中最重要的前级聚焦加速结构,其工作模式下束流孔中的电场四极对称性是保证束流品质及加速效率的关键。为了解决其各自存在的工作模式下四极场的不对称问题,分别对四杆型RFQ和四翼型RFQ的结构和模式谐振特性做了研究。针对四翼型RFQ,模拟显示双端口对称耦合的方式从抵消二极模式和提高结构对称性两方面来达到削弱工作模式下的场分布中二极量的目的,对一个1 m段四翼RFQ模型腔的测量证实了双端口对称耦合抵消腔内二极模式的有效性;针对四杆型RFQ的二极场,以SSC-Linac项目的RFQ为原型,利用三维模拟计算软件CST-MWS进行模拟,通过改进支撑板的结构并调整参数,完全地消除了工作模式下的极间二极场,且理论上此种改进方法对支撑板结构的四杆型RFQ均有效。

Radio Frequency Quadruple（RFQ） is the most important front-end acceleration structure in linear acceler- ators. It can focus and accelerate beam in longitudinal and transversal direction, simultaneously. Symmetric of electric field in acceleration aperture is very important for the beam dynamics. To an idea RFQ, there is only quadruple electric field in the acceleration aperture. The dipole field in aperture is harmful and need be as small as possible. The methods to reduce it are studied in a four-rod RFQ and a four-vane RFQ in the article independently. To a four-vane RFQ, the symmetrical double-port couplers are employed to counteract the neighboring dipole modes. The availability is confirmed by the simulation with the 3-D Finite Element tool, CST-MWS. And it is also observed by measuring a model-cavity with a symmetrical double-port couplers. To a four-rod RFQ, a new structure of stems is proposed to reduce the dipole mode. Taking the RFQ for the SSC-LINAC project as example, the dipole mode contributions to the electric filed is completely eliminated by selecting stem＇s parameters carefully. It is confirmed by the simulation of CST-MWS. Theoretically, this method is independent with the frequency of an RFQ and serves for all RFQs with 4-rod or 4-wane structure.