Effects of cavities RF field radial non-uniformity on multiple-beam klystron efficiency

We present the results of detailed numerical investigation of the influence of RF field radial non-uniformity in the cavities on the bunching process and efficiency of multiple-beam klystrons. 2.5-D nonlinear PIC computer code “Arsenal-MSU” [1,2] was used for detailed parallel numerical simulation of bunching process in the non-identical electron beams, passing through multi-beam cavities with different RF field distributions and R/Q values. Results of numerical investigations have been compared with experimental data for two broadband multiple-beam klystrons KL4113 and KS4116 developed and fabricated in IECAS (China) [3,4]. It was discovered strong influence of RF fields radial non-uniformity on the klystron efficiency and bandwidth for some multi-beam cavities designs. For example for KS4116 tube the R/Q difference in 20%-25% between two radial layers of the beams caused phase separation of electrons in different layers and 10%-15% efficiency reduction.