Generation and Stable Transmission of Miniature Electron Beams for 0.65-THz Traveling Wave Tubes

The electron optical system (EOS) plays a vital role in vacuum electronic devices (VEDs). A 19.3-mA, 19.4-kV electron gun with a multistage focus electrode is designed, and the multistage focus electrode advantage is analyzed to generate a high-quality beam that meets the requirement of traveling-wave tubes (TWTs) operating higher than 0.6 THz. The tunnel and beam waist radii are 50 and
$35~\mu \text{m}$
, respectively, and the current density in the waist reaches as high as 501 A/cm2. A novel dynamic-matched focusing magnetic field upswing from 0.35 to 0.5 T at the circuit tail, produced by an open-side asymmetrical magnetic focusing system, is proposed to balance the increasing divergence force of the rise of space charge focus. Two different asymmetrical magnet design methods are analyzed. Compared to the uniform magnet (UM) and inclined magnet (IM), the inclined dynamic-matched magnet (IDMM) performs optimally in beam transmission and output power, verified by particle-in-cell (PIC) simulation. The proposed dynamic-matched focusing magnetic field can benefit the performance of terahertz TWTs by improving the beam transmission rate and beam–wave interaction efficiency.