Klystron simulation and design using the Field Charge Interaction (FCI) code

The field charge interaction code FCI, based on a particle-in-cell simulation, has been used in analyzing and developing high power klystrons in KEK and also in industries from 1989 onwards. Several new high power klystrons have been developed using the FCI code up to now. This paper describes some details of the code and application examples. Finally, problems in the present version and future improvements are discussed.