Investigation on Electric Field Uniformity in the Output Cavity of High-Power High-Efficiency Multibeam Klystrons

High-power, high-efficiency multibeam klystrons (MBKs) serve as essential RF power sources for large-scale accelerator facilities, where high-efficiency energy extraction from multiple beams without reflected electrons is critical for achieving stable and efficient operation. However, in practical MBK designs, the electric field distribution within the output cavity gap often becomes nonuniform due to the asymmetrical loading introduced by the coupling port. This field distortion leads to differentiated beamlet deceleration, resulting in reflected electrons in certain beamlets and degraded RF conversion efficiency. Based on the design of a 3-MW S-band high-efficiency MBK, this work proposes a method to improve electric field uniformity by shifting the ferrule region toward the coupling-port side and by introducing tuning rods on both the coupling-port side and the opposite side, arranged parallel to the beam tunnels. This hybrid design improves the field uniformity in the output gap from 53.8% to 92.1%, thereby enabling optimal beam–wave interactions. 3-D CST/PIC simulations demonstrate that the proposed method increases efficiency from 45.1% to 62.7% compared with a conventional reentrant output cavity. Furthermore, the fabricated new cylindrical reentrant output cavity structure was experimentally evaluated using the perturbation (Slater’s) method, confirming the improved field uniformity and validating the effectiveness of the proposed technique.