Performance assessment of critical waveguide bends for the ITER in-vessel plasma position reflectometry systems

Abstract A critical issue in the design of the Plasma Position Reflectometry (PPR) diagnostic for ITER is the performance of the transmission lines (TLs) of the in-vessel systems (known as gaps 4 & 6) to/from the antennas, due to the use of oversized rectangular waveguides that must conform to an intricate and constrained path/geometry, besides operating in a wide frequency range (15–75 GHz). The TL includes a 90° bend (for the systems of gaps 4 and 6) and a 125° bend (exclusively for gap 4). However, oversized bends can excite higher-order modes and create resonances, and these could significantly affect the diagnostic’s performance. Here, the 90° and 125° bends are studied via 3D electromagnetic simulations. Results for the 90° bend developed for the ITER High Field Side Reflectometer system reveal excellent performance: low losses and no resonances across the whole frequency range; they compare favorably with the laboratory tests of a prototype up to 43 GHz, but are unable to account for the experimental degradation over 43–75 GHz. The 125° bend is optimized with recourse to a hyperbolic secant geometry, clearly improving the performance over the baseline (constant radius) bend across most of the frequency range (with only a small degradation over 70–74 GHz) while within the space restrictions.