Analysis of reasonable diffraction loss rate in 220 GHz confocal waveguide gyro-TWT amplifier
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摘要: 共焦波导结构因其衍射损耗可降低模式密度的特性，能够有效抑制模式竞争，进而有助于回旋行波管放大器（gyro-TWT）在太赫兹（>100 GHz）频段实现稳定工作。采用理论分析与三维粒子模拟（3D-PIC）相结合的方法，针对220 GHz共焦波导gyro-TWT的衍射损耗率（DLR）展开综合分析。研究结果表明，DLR的大小对gyro-TWT性能具有显著影响。较小的DLR会激发低阶竞争模式的回旋返波振荡（GBWO）；而较大的DLR则会大幅降低共焦波导gyro-TWT的束波互作效率、增益、带宽，同时降低其对电子束速度零散的容忍度，应避免使共焦波导gyro-TWT工作在较大的DLR下。在该设计的共焦波导gyro-TWT中，HE07单模稳定工作的DLR不小于0.38 dB/cm，对应的镜面宽度角θ不大于47°。Abstract:
Background The confocal waveguide structure can effectively suppress mode competition due to its characteristic of reducing mode density through diffraction loss, thereby facilitating stable operation of gyro-traveling-wave-tube (gyro-TWT) amplifiers in the terahertz (>100 GHz) frequency range.Purpose This study aims to conduct a comprehensive analysis of the diffraction loss rate (DLR) in a 220 GHz confocal waveguide gyro-TWT, employing a combination of theoretical analysis and three-dimensional particle-in-cell (3D-PIC) simulations.Methods The research integrates field distribution theory with 3D-PIC simulations to investigate the DLR of the confocal waveguide. A non-ideal waveguide model incorporating the mirror width angle was utilized, and simulations were performed to evaluate beam-wave interaction dynamics under varying DLR conditions.Results The study reveals that a low DLR induces gyro-backward-wave oscillation (GBWO) in low-order competing modes, while a high DLR significantly reduces beam-wave interaction efficiency, gain, and bandwidth, and lowers tolerance to electron beam velocity spread.Conclusions For stable single-mode operation of the HE07 mode in the designed gyro-TWT, the DLR should not be less than 0.38 dB/cm, with the corresponding mirror-surface width angle not exceeding 47°. These findings provide crucial design guidelines for terahertz gyro-TWTs.-
Key words:
- confocal waveguide /
- diffraction loss rate /
- terahertz generation /
- gyro-TWT
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图 1 CST模拟得到共焦波导HE07模式电场分布
Figure 1. Electric field distribution of the HE07 mode in CST simulation
图 2 CST模拟得到HE06、HE07和HE08模式的DLR随频率变化（Rc = 5.06 mm，θ = 46.6°）
Figure 2. Variations of the DLRs of the HE06, HE07 and HE08 modes versus frequency obtained in CST simulation (Rc = 5.06 mm and θ = 46.6°)
图 3 CST模拟得到DLR随θ变化（HE06、HE07和HE08模式的Rc分别为4.36 mm、5.06 mm和5.76 mm）
Figure 3. Variations of the DLRs versus θ obtained in CST simulation (Rc of the HE06, HE07 and HE08 modes are 4.36 mm, 5.06 mm and 5.76 mm, respectively)
图 6 不同DLR下，HE07模式的输出功率随腔体轴向长度的变化
Figure 6. Variations of the output power versus the axial length of the cavity at different DLRs of the HE07 mode
图 7 HE07模式的DLR为0.3 dB/cm时，220 GHz共焦波导gyro-TWT在3D-PIC模拟中的输出电场频谱
Figure 7. 3D-PIC simulation results of the output electric field spectrum of the 220 GHz two sectional confocal waveguide gyro-TWT with the 0.3 dB/cm DLR of the HE07 mode
图 8 表1参数下HE06和HE07模式的色散关系
Figure 8. Dispersion relations of the HE06 and HE07 mode at the parameters in Table 1
图 9 不同DLR下，HE06模式GBWO起振所需的谐振腔长度
Figure 9. GBWO cavity length of the HE06 mode at different DLRs
图 10 不同DLR和电子束速度零散下束波互作用效率的非线性理论中计算结果
Figure 10. Variation of the beam wave interaction efficiency versus the axial position of the cavity
图 11 不同DLR和电子束速度零散下增益随频率变化的非线性理论计算结果
Figure 11. Variation of the gain versus frequency
表 1 共焦波导gyro-TWT电参数
Table 1. Electron parameters of confocal waveguide gyro-TWT
beam voltage
Ub/kVbeam current
Ib/Amagnetic field
B0/Tpitch factor
αguiding center radius
Rb/mminput power
Pin/W30 7 7.91 1.2 0.76 10 
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