Evaluation of low-frequency operational limit of proposed ITER low-field-side reflectometer waveguide run including miter bends

Journal Article · Thu Oct 19 00:00:00 EDT 2017 · Review of Scientific Instruments

DOI:https://doi.org/10.1063/1.4995662· OSTI ID:1407736

Wang, Guiding ^[1]; Peebles, W. A. ^[1]; Doyle, E. J. ^[1]; Crocker, N. A. ^[1]; Wannberg, C. ^[1]; Lau, Cornwall H.  ^[2]; Hanson, Gregory R. ^[2]; Doane, John L. ^[3]

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1. Univ. of California, Los Angeles, CA (United States). Dept. of Physics and Astronomy and Plasma Science and Technology Inst. (PSTI)
2. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
3. General Atomics, San Diego, CA (United States)

The present design concept for the ITER low-field-side reflectometer transmission line (TL) consists of an ~40 m long, 6.35 cm diameter helically corrugated waveguide (WG) together with ten 90° 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. We observed that the output beams from the guide remained linearly polarized with cross-polarization power levels of ~1.5%-3%. The polarization rotation due to the helical corrugations was in the range ~1°-3°. 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 ~2.5 dB at 50 GHz and ~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. Finally, 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.

• Research Organization: Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• Grant/Contract Number: AC05-00OR22725
• OSTI ID: 1407736
• Journal Information: Review of Scientific Instruments, Vol. 88, Issue 10; ISSN 0034-6748
• Publisher: American Institute of Physics (AIP)
• Country of Publication: United States
• Language: English

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