Electron Optics System of a 100-MW S-Band Klystron

This paper presents the design and experimental results of the electron optics system for use in an S-band 100-MW high-peak-power klystron. The gun operating voltage is 415 kV with a beam current of 535 A. For the design of the gun, special emphasis is placed on the geometric layout of the electrodes to ensure a moderate voltage gradient, reasonable cathode loading, and accurate beam perveance. The beam-focusing system is a space-charge balanced flow type with a solenoidal magnet structure. The magnetic field at the center of the cathode is 60 G, and the longitudinal magnetic field in the interaction region is in the range of 1.0-1.38 kG. The electron optics system design was accomplished with SUPERFISH, EGUN, and a Particle-in-Cell (PIC) code. Simulation results indicate that the gun has a voltage gradient lower than 220 kV/cm and excellent beam transport characteristics with a beam-to-tunnel radial fill factor of less than 0.75. The focusing beam trajectories simulated by EGUN and the PIC code both have good laminarity. Two prototypes based on a simplified RF design (not intended to achieve full RF power) were fabricated in order to demonstrate the beam optics, and the measured beam transport is in agreement with predictions made by simulation codes. RF circuit simulation designs with the 1-D large-signal code KLY_12 and the PIC code are also presented.