Low loss Transmission Lines for High pow

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. This paper reviews concepts for THz transmission lines with a focus on overmoded, metallic, corrugated transmission lines. It highlights the successful implementation of these transmission lines using the fundamental HE 11 mode, which have demonstrated very low-loss at high average power levels in both plasma heating and THz dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) experiments. The paper derives an analytic expression for the ohmic loss in modes of a corrugated, metallic waveguide, considering both the waveguide inner surfaces and grooves. Simulations conducted with HFSS yielded results consistent with the analytic expression. Experimental assessments were carried out to measure the loss of the HE 11 mode in a helically-tapped brass waveguide. The measured losses at 250 GHz were 0.029 ± 0.009 dB/m and 0.047 ± 0.01 dB/m using different methodologies, aligning reasonably with theoretical predictions. The results indicate that the loss of approximately 1% or less per meter is suitable for the DNP NMR application, though practical line losses may be higher due to mode conversion related to imperfections.