Preliminary assessment of ex-vessel waveguides for the ITER Plasma Position Reflectometer

Content: This work describes the preliminary assessment of the different waveguide technologies for the ex-vessel transmission lines of the Plasma Position Reflectometer (PPR) in ITER. Initially, both oversized rectangular and circular corrugated waveguides were considered for the study; the former due to reduced costs and ease of procurement and the latter due to better performance in terms of attenuation and radiation characteristics -which have a great impact in gaps and miter bends performance for instance-. Using the preliminary layout information available at this initial stage of the PPR design and the required frequency range, several waveguide dimensions were evaluated for both technologies, namely WR137, WR187, WR284, WR340 and WR430 in case of rectangular waveguides and 31.75 mm, 63.5 mm, 76.2 mm and 88.9 mm diameters for circular corrugated waveguides. These dimensions were chosen taking into account the space reservation provided by ITER and commercially available waveguide sizes. In an initial stage, only ohmic losses and mode conversion at mitre bends were taken into account for the comparison of all the possibilities. Results from a literature survey helped evaluating analytically the several possibilities in order to narrow the selection and set up some conclusions. In a later stage of the analysis , both ohmic loss and mode conversion at mitre bends were calculated with rigorous codes only for the selected geometries, together with mode conversion losses from gaps along the transmission line. This process led to the final estimation of performance for the preliminary routing of the PPR ex-vessel transmission lines. The reference solution of choice was 88.9 mm internal diameter circular corrugated waveguide, as the best compromise between in-band performance and space constraints.