Corrugated Waveguide and Directional Coupler for CW 250 GHz Gyrotron DNP Experiments

Abstract—A 250 GHz corrugated transmission line with a directional coupler for forward and backward power monitoring has been constructed and tested for use with a 25-Watt CW gyrotron for dynamic nuclear polarization (DNP) experiments. The main corrugated line (22 mm internal diameter (i.d.), 2.4 m long) connects the gyrotron output to the DNP probe input. The directional coupler, inserted approximately midway, is a 4-port crossed waveguide beamsplitter design. Two beamsplitters, a quartz plate and 10-wire array, were tested with output coupling of 2.5 % (-16 dB) at 250.6 GHz and 1.6 % (-18 dB), respectively. A pair of mirrors in the DNP probe transferred the gyrotron beam from the 22 mm waveguide to an 8 mm helically corrugated waveguide for transmission the final 0.58 m distance inside the NMR magnet to the sample. The transmission line components were all cold tested with a 248 ± 4 GHz radiometer. A total insertion loss of 0.8 dB was achieved for HE11 mode propagation from the gyrotron to the sample with only 1% insertion loss for the 22 mm diameter waveguide. A clean Gaussian gyrotron beam at the waveguide output and reliable forward power monitoring were achieved for many hours of continuous operation. Index Terms — Corrugated waveguides, Millimeter wav