Modeling of Coupled Cell Output Structures for the Klystrons

High-frequency, high-power klystrons proved to be significant for application in compact particle accelerators, high-resolution radars, and fast data communication systems. As most of the linear beam devices, the performance of the klystron deteriorates progressively with an increasing operating frequency due to the reduction of individual RF cavity impedances. Coupled cell structures can alleviate such degradation by extending the beam-wave interaction region. The accurate simulation of the klystron with the coupled cell structure relies on the resources and the time-consuming particle-in-cell (PIC) codes. To speed up the klystron design process, the coupled-mode theory was carefully reviewed and extended to facilitate the calculation of the complex spectra of the arbitrarily coupled cavity system and its interaction with the bunched electron beam. These new methods were implemented in the large signal klystron simulation code KlyC. The design and optimization of the Ku-band, low-voltage klystron is explained as a demonstration of the new technique's effectiveness. The KlyC simulations have been thoroughly benchmarked with the CST PIC code and both codes showed a good (within 1%) agreement, while KlyC is significantly (about 100 times) faster.