A terahertz backward wave oscillator (BWO) is a high-power terahertz wave source based on the Cherenkov interaction between an electron beam and a slow backward wave. Excitation of slow wave modes with different mode numbers degrades the performance of BWO due to mode competition. Analyzing the mode spectrum is a difficult challenge to confirm single mode operation. In this study, the excitation of the slow waves in a 0.4 THz cylindrical corrugated waveguide is examined. In the corrugated waveguide, azimuthal modes become slow backward waves with evanescent fields. We found the superposition of the modes with different azimuthal indices has an asymmetric field intensity and the excitation of the superposed modes produces the terahertz radiation with the asymmetric pattern. We conduct a BWO experiment using a 0.4 THz corrugated waveguide. The experimental radiation pattern is obtained and corresponds well to the calculated asymmetric pattern. The symmetry of the radiation can be regarded as an indicator of the mode control.

中文翻译：

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0.4 THz后向波振荡器多模辐射分析

太赫兹后向波振荡器（BWO）是一种基于电子束与慢后向波之间切伦科夫相互作用的高功率太赫兹波源。由于模式竞争，不同模式数的慢波模式的激励会降低 BWO 的性能。分析模式谱是确认单模操作的一项艰巨挑战。在本研究中，研究了 0.4 THz 圆柱形波纹波导中慢波的激发。在波纹波导中，方位模变成具有渐逝场的慢后向波。我们发现具有不同方位角指数的模式的叠加具有不对称的场强，并且叠加模式的激发产生具有不对称图案的太赫兹辐射。我们使用 0.4 THz 波纹波导进行 BWO 实验。获得了实验辐射方向图，并且与计算的不对称方向图很好地对应。辐射的对称性可以被视为模式控制的指标。