设计了一套0.22 THz折叠波导行波管电子光学系统，详细介绍了电子枪和周期永磁聚焦系统的设计过程，在电子枪电子束束腰与磁系统不匹配情况下，对磁场过渡区进行了优化设计，以此为基础利用磁场仿真软件对磁场进行模拟和优化，并把磁场位形代入电磁仿真软件进行电子束传输仿真，优化后的电子光学系统发射束流10 mA，阴极电压15 kV，束流通过率96%。通过实验验证，流通管束流通过率93%，高频样管束流通过率94%，与设计相符。高频样管实现连续波运行，功率大于0.4 W，3 dB带宽大于12 GHz。

This paper introduces the design method of the electron optical system for a 0.22 THz folded waveguide traveling wave tube. As the electron beam current is 10 mA and the beam channel radius of the tube is only about 0.1 mm, the PPM(period permanent magnet) focusing system needs to be carefully designed to transmit this beam current from the electron gun to the collector. A tree-electrode Pierce gun is adopted. The electrodes and PPM are optimized to obtain proper beam waist radius and high transmission efficiency. Maxwell is used to calculate the magnetic flux density on the beam line, and CST is used to simulate the beam transmission. The simulation shows that about 96% of the electron beam reaches the collector and is verified by experiments, which gets about 74% transmission efficiency without magnetic modification. After magnetic modification, the transmission efficiency is improved to 94%, and this tube is able to run in DC mode.