Numerical Analysis of Resonant Multipolar Instabilities in High Power Klystrons

The monopole monotron instability is a well-known phenomenon in linear beam devices. In recent development of a high-efficiency 50-MW X-band klystron, such instabilities were found and mitigated in a special second-harmonic multicell cavities triplet that is used to improve the klystron's performance. In further simulations of the klystron, using 3-D particle-in-cell computer codes, the more rare and complicated phenomenon, previously unreported in nonrelativistic devices, associated with rotating multipolar monotron instabilities was discovered. In this article, the qualitative and quantitative analysis of these oscillations is presented. Various resonant multipolar instabilities suppression strategies are introduced and discussed in detail.