Applications of high-power Terahertz (THz) sources require low-loss transmission lines to minimize loss, prevent overheating and preserve the purity of the transmission mode. Concepts for THz transmission lines are reviewed with special emphasis on overmoded, metallic, corrugated transmission lines. Using the fundamental HE₁₁ mode, these transmission lines have been successfully implemented with very low-loss at high average power levels on plasma heating experiments and THz dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) experiments. Loss in these lines occurs directly, due to ohmic loss in the fundamental mode, and indirectly, due to mode conversion into high order modes whose ohmic loss increases as the square of the mode index. An analytic expression is derived for ohmic loss in the modes of a corrugated, metallic waveguide, including loss on both the waveguide inner surfaces and grooves. Simulations of loss with the numerical code HFSS are in good agreement with the analytic expression. Experimental tests were conducted to determine the loss of the HE₁₁ mode in a 19 mm diameter, helically-tapped, three meter long brass waveguide with a design frequency of 330 GHz. The measured loss at 250 GHz was 0.029 ± 0.009 dB/m using a vector network analyzer approach and 0.047 ± 0.01 dB/m using a radiometer. The experimental results are in reasonable agreement with theory. These values of loss, amounting to about 1% or less per meter, are acceptable for the DNP NMR application. Loss in a practical transmission line may be much higher than the loss calculated for the HE₁₁ mode due to mode conversion to higher order modes caused by waveguide imperfections or miter bends.

中文翻译：

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用于高功率太赫兹辐射的低损耗传输线。

高功率太赫兹 (THz) 源的应用需要低损耗传输线，以最大限度地减少损耗、防止过热并保持传输模式的纯度。回顾了太赫兹传输线的概念，特别强调过调制、金属、波纹传输线。使用基本的 HE ₁₁模式，这些传输线已在等离子体加热实验和太赫兹动态核极化 (DNP) 核磁共振 (NMR) 实验中成功实现，在高平均功率水平下具有非常低的损耗。由于基模中的欧姆损耗，这些线路中的损耗直接发生；由于模式转换为高阶模式，其欧姆损耗随着模式指数的平方而增加，因此间接发生损耗。推导了波纹金属波导模式中的欧姆损耗的解析表达式，包括波导内表面和凹槽上的损耗。使用数字代码 HFSS 进行的损耗模拟与解析表达式非常一致。进行实验测试以确定设计频率为 330 GHz 的 19 毫米直径、螺旋抽头、三米长黄铜波导中HE _{11模式的损耗。}使用矢量网络分析仪方法测得的 250 GHz 损耗为 0.029 ± 0.009 dB/m，使用辐射计测得的损耗为 0.047 ± 0.01 dB/m。实验结果与理论相当吻合。这些损失值约为每米 1% 或更少，对于 DNP NMR 应用来说是可以接受的。由于波导缺陷或斜接弯曲导致模式转换为高阶模式，实际传输线中的损耗可能远高于 HE _{11模式计算的损耗。}