Study on Miniaturized Periodic Reverse Permanent Magnet Focusing for Multibeam Klystrons

An ultra-compact periodic reverse permanent magnet (PRPM) focusing system is proposed and experimentally validated for S-band high-power, high-efficiency multibeam klystrons (MBKs). Conventional PRPM designs typically use only 3–6 magnets to confine beam, requiring large transverse dimensions to suppress transverse magnetic field (TMF) and maintain full beam transmission, resulting in bulky structures and complex assembly. In contrast, the proposed approach employs ten plus magnets to form a multi-period configuration, enabling flexible TMF shaping by optimizing the dimensions of magnets and pole pieces, as well as the placement of pole piece apertures. This design generates a multi-reversal, symmetric TMF profile that cancels transverse beam deflections. Additionally, by locally trenched beam tunnel in non-cavity regions, 100% beam transmission is achieved even under strong beam–wave interactions. Compared with conventional configurations, the proposed PRPM reduces transverse dimension by 2 to 3 times, while simplifying fabrication, lowering cost, and supporting MBK miniaturization.