Design of a Sheet Beam Electron Optics System for a 140-GHz Traveling Wave Tube

An integrated sheet beam electron optics system that employs a sheet beam gun together with a uniform permanent magnetic focusing structure was thoroughly analyzed and designed, for a 140-GHz staggered double vane traveling wave tube (TWT). First, a laminar quasi-elliptical beam with a 25 kV, 0.25 A, and a cross-section of $0.75\times0.15$ mm was obtained via theoretical analysis and CST simulation, featuring a current density of $\sim $ 222 A/cm2 at the beam waist. Then, a compact uniform magnetic structure was employed, conveying the sheet beam stably over a distance of 100 mm, with no beam interception. Furthermore, simulated stability analysis showed that the electron optics system could allow enough tolerances for assembly errors, magnetization errors, and operating voltage variation.

I. Introduction

Compared with the traditional cylindrical electron beam, the sheet electron beam could provide a much higher beam current by enlarging its cross-sectional area while remaining a relatively similar or even smaller space charge intensity [1], [2]. In recent years, sheet electron beam devices, including traveling wave tubes (TWTs) [3], klystrons [4], backward wave oscillators [5], and extended interaction oscillators [6] are widely studied as they are capable of generating much higher output power in comparison to vacuum electron devices based on a cylindrical beam.