3-D Simulations and Design of Multistage Depressed Collectors for Sheet Beam Millimeter Wave Vacuum Electron Devices

There is an increasing interest in sheet electron beam-based vacuum electron devices from the microwave through terahertz region. As with standard pencil beam devices, it is desirable to increase overall net efficiency through the use of multistage depressed collectors. The sheet electron beam, however, brings unique challenges to this task due to the elliptical cross section of the beam. This paper focuses on the comprehensive 3-D simulation modeling of the electron beam dynamics for the ac modulated sheet beam topology from the beam-wave interaction region up to the designed energy recovery structure, i.e., the multistage depressed collector. For purposes of illustration, the design and analysis of a five-stage depressed collector for a W-band sheet beam klystron using the 3-D electromagnetic particle-in-cell (PIC) finite difference time domain code MAGIC is described herein. Numerous PIC simulations were carried out to determine the optimum operating parameters. The effects of the secondary electrons on the recovery efficiency and the electron back streaming were both investigated. To accommodate the elliptical cross section of the sheet beam, the option of adding a racetrack-like structure to decrease the back streaming was also explored. Through optimization, a recovery efficiency of 88.38% was achieved, with a minimized total electron back-streaming fraction of 0.25%.