Evaluation Of Low-Frequency Operational Limit Of Proposed Iter Low-Field-Side Reflectometer Waveguide Run Including Miter Bends

The present design concept for the ITER low-field-side reflectometer transmission line (TL) consists of an similar to 40 m long, 6.35 cm diameter helically corrugated waveguide (WG) together with ten 90 degrees miter bends. This paper presents an evaluation of the TL performance at low frequencies (33-50 GHz) where the predicted HE11 mode ohmic and mode conversion losses start to increase significantly. Quasi-optical techniques were used to form a near Gaussian beam to efficiently couple radiation in this frequency range into the WG. It was observed that the output beams from the guide remained linearly polarized with cross-polarization power levels of similar to 1.5%-3%. The polarization rotation due to the helical corrugations was in the range similar to 1 degrees-3 degrees. The radiated beam power profiles typically show excellent Gaussian propagation characteristics at distances >20 cm from the final exit aperture. The round trip propagation loss was found to be similar to 2.5 dB at 50 GHz and similar to 6.5 dB at 35 GHz, showing an inverse increase with frequency. This was consistent with updated calculations of miter bend and ohmic losses. At low frequencies (33-50 GHz), the mode purity remained very good at the exit of the waveguide, and the losses are perfectly acceptable for operation in ITER. The primary challenge may come from the future addition of a Gaussian telescope and other filter components within the corrugated guide, which will likely introduce additional perturbations to the beam profile and an increase in mode-conversion loss. Published by AIP Publishing.