为减少太赫兹回旋器件模式密度和降低模式竞争问题，利用具有模式选择特点的共焦波导结构作为140 GHz回旋行波管(Gyro-TWT)的高频互作用系统。在理论分析基础上，建立注波互作用计算模型并对其进行数值计算；通过对共焦波导高频场分布、衍射损耗、耦合系数以及注波互作用效率等输出参量的分析，选择HE06作为工作模式，确定了140 GHz Gyro-TWT放大器的基本结构和工作参数，并利用注波互作用非线性理论进行分析。模拟结果表明：在注电压为35 kV，注电流2 A，速度比为0.75时，该高频结构在140 GHz频点获得12 kW峰值输出功率，17.1%电子效率和38 dB饱和增益，3 dB带宽达到6 GHz。

For reducing mode density and lowering the risk of spurious mode competition in THz gyrotron, the mode-selective confocal waveguide system is adopted for high frequency circuit of 140 GHz HE06 mode confocal waveguide Gyrotron Traveling Wave Tube(Gyro-TWT) amplifier. Based on the analysis of RF system, the theoretical analysis about electron beam and waves interaction is presented. The structure and operating parameters of the gyro-TWT are determined by analysis of field distribution, diffraction loss, coupling coefficient and interaction between electron beam and wave. Nonlinear theory is utilized to calculate the amplifier, and nonlinear analysis shows that the interaction circuit can generate a saturated peak power of 12 kW, corresponding to a saturated gain of 38 dB, an efficiency of 17.1%, and a 3 dB bandwidth of 6 GHz at 140 GHz when utilizing a 35 kV, 2 A electron beam with 0.75 of the ratio of the transverse velocity to the longitudinal velocity.