Based on the current element model for magnetic ring, the magnetic field distribution along central axis was analyzed for permanent magnetic ring. Theoretical calculation and measured results show that the whole magnetic ring only exhibits an axial magnetic field without radial component. For the open magnetic ring with a gap, the axial magnetic field decreases and the radial magnetic field increases, producing a non-axis-symmetrical magnetic field along central axis. Therefore, the application of the open magnetic ring in periodic permanent magnet(PPM) focusing system is limited. The structure of the PPM focusing system with open magnetic rings was proposed and optimized, according to the distribution characteristic of the axial magnetic field and radial magnetic field, and the periodic magnetic field along central axis was achieved.%基于磁环的元电流模型,分析了永磁磁环的轴向磁场分布规律.理论计算和测试结果表明:轴向磁化磁环中心轴上的磁感应强度不存在径向分量,只有轴向分量；但磁环开口后,轴向磁场降低,而径向磁场增加,其中心轴上的合成磁感应强度体现出非轴对称性,不利于其在周期永磁(PPM)聚焦系统中的应用.根据磁场的轴向分量和径向分量的分布特点,设计和优化了带开口磁环的PPM系统,实现了轴向磁场的周期化.
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