Design of an Ultra-wide Band Waveguide Transition for the Ex-vessel Transmission Line of ITER Plasma Position Reflectometry

Plasma position reflectometry for ITER requires interfaces between in-vessel and ex-vessel waveguides. An ultra broadband interface (15-75 GHz) was designed between moderately oversized rectangular waveguide (20 x 12 mm), operated in TE01 (i.e., tall waveguide mode), and circular corrugated waveguide, with 88.9-mm internal diameter, propagating HE11. The interface was designed both as a sequence of waveguide components and as a quasi-optical confocal telescope. The design and the simulated performance are described for both concepts. The latter one requires more space but has better performance, and shall be prototyped.