A transmission line, linking the gyrotron (or solid-state) source to the NMR probe is an essential piece of THz instrumentation for DNP-NMR spectrometers. While quasi-optical transmission system have been used for setups using low-power solid-state sources, far more typical is the use of circular (corrugated) waveguides to deliver the THz power to the sample. Below is a list of two articles that have been published recently covering the topic of low-loss THz transmission lines for application in DNP-NMR spectroscopy:
Nanni, E., et al., Low-loss Transmission Lines for High-power Terahertz Radiation. J. Infrared Millim. Te., 2012: p. 1-20.
http://dx.doi.org/10.1007/s10762-012-9870-5
Bogdashov, A., et al., Transmission Line for 258 GHz Gyrotron DNP Spectrometry. J. Infrared Millim. Te., 2011. 32(6): p. 823-837.