FCI-field charge interaction program for high power klystron simulations

The particle-in-cell computer code FCI is discussed. It has been applied to high-power klystron simulations. The code simulates the electron beam dynamics in the drift tube region of the klystron, taking into account the fields of the space charge, the RF cavities, and the external focusing magnet. The RF cavities are modeled by the gap fields and the equivalent circuits, whose voltages and output power are determined by solving the circuit equations. The energy compensation for the injection beam of the self-potential and the boundary conditions were taken into account carefully. The tightly bunched beam generates a high-frequency wave (a king of transition radiation) at the right boundary of the beam dump. This radiation backs into the drift tube and modulates the beam. The traveling wave boundary is used to suppress the radiation, so that the simulation gives stable solutions. In order to make the CPU time as short as possible, several techniques were employed in the space charge calculation and cavity voltage determination. A solution for one operation point of a multicavity klystron (such as the 50-MW 5045 tube used in the Stanford Linear Collider) can be obtained within a CPU time of 10 min. or less on the HITAC M-280H computer with a maximum performance of 17 MIPS (million instructions per second). The simulation results are compared with the experimental data. The gain of the simulation results, which uses nominal cavity parameters and a magnetic field profile, is 3 dB higher than the experimental values. The difference of the peak power is about 10%. These differences show the same tendency as for the simulation carried out by K. Eppley et al. (1985)