title: 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-W continuous-wave gyrotron for dynamic nuclear polarization (DNP) experiments. The main corrugated line (22-mm internal diameter, 2.4-m long) connects the gyrotron output to the DNP probe input. The directional coupler, inserted approximately midway, is a four-port crossed waveguide beamsplitter design. Two beamsplitters, a quartz plate and ten-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 through 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 HE(11) -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.
authors: P. P. Woskov, M. K. Hornstein, and R. J. Temkin are with the Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, MA 02139 USA (e-mail: ppwoskov@mit.edu ; wasy@mit.edu ; temkin@mit.edu ). V. S. Bajaj and R. G. Griffin are with the Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139 USA (e-mail: vikbajaj@mit.edu ; rgg@mit.edu ).
authors: Paul P
authors: PP
authors: Woskov
authors: Vikram S
authors: VS
authors: Bajaj
authors: Melissa K
authors: MK
authors: Hornstein
authors: Richard J
authors: RJ
authors: Temkin
authors: Robert G
authors: RG
authors: Griffin