Two and One-Half Dimension Particle-in-Cell Simulation of High-Power Klystrons

A technique for modeling high-power klystrons with the fully electromagnetic particle-in-cell code MASK is reported. The interesting operating parameters (efficiency, gain, etc.) are associated with a highly resonant steady state. To reach steady state within a reasonable computing time, it is necessary to reduce the time needed for the transient filling of resonant structures. The technique introduced replaces resonant cavities with rf voltages across corresponding "ports" in the beam tube. The voltage and phase of each port can be rigorously determined from power balance between work done on the beam, wall losses, and any external input or output. Given a set of cavity parameters (¿0, R/Q, Q0), power balance gives implicit relations for the voltage and phase. As an example, we show a simulation of the 5-cavity 35 MW STAC klystron in which the computed efficiency is in good agreement with the experiment.