Please cite this article in press as: J.H.C.M. Belo, et al., Performance assessment of critical waveguide bends for the ITER in-vessel plasma position reflectometry systems, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.04.125 ARTICLE IN PRESS G Model FUSION-9477; No. of Pages 5 Fusion Engineering and Design xxx (2017) xxx–xxx Contents lists available at ScienceDirect Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes Performance assessment of critical waveguide bends for the ITER in-vessel plasma position reflectometry systems Jorge H.C.M. Belo ∗ , Paulo Varela, Emanuel Ricardo, António Silva, Paulo Quental Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal h i g h l i g h t s • Critical in-vessel waveguide components of ITER’s PPR diagnostic were investigated. • Simulations of the 90 ◦ bend show excellent performance over full range (15–75 GHz). • Simulations of the 90 ◦ bend compare favorably with laboratory tests up to 43 GHz. • Results show that the 90 ◦ bend can be further optimized for O-mode only operation. • Performance of the 125 ◦ bend is optimized with an hyperbolic secant geometry. a r t i c l e i n f o Article history: Received 3 October 2016 Received in revised form 19 April 2017 Accepted 28 April 2017 Available online xxx Keywords: Microwave reflectometry Waveguide bend Oversized waveguide Hyperbolic secant function a b s t r a c t A critical issue in the design of the Plasma Position Reflectometry (PPR) diagnostic for ITER is the perfor- mance 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 labo- ratory 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. © 2017 Elsevier B.V. All rights reserved. 1. Introduction The Plasma Position Reflectometry (PPR) diagnostic will be used in ITER to measure the plasma position in order to provide a ref- erence for the magnetic diagnostics during very long (>1000 s) pulse operation, where the position deduced from the magnetics is known to be subject to substantial error. The system consists of five reflectometers distributed at four locations [1], known as gaps 3, 4, 5 and 6, operating over the frequency range 15–75 GHz in O-mode, using the TE 01 waveguide mode. The systems of gaps 4 and 6, which are considered here, are known as the PPR in-vessel systems, since the bi-static antenna system and transmission line ∗ Corresponding author. E-mail address: jbelo@ipfn.ist.utl.pt (J.H.C.M. Belo). (TL), i.e. the feeding waveguides to/from the antennas, are installed inside the ITER vacuum vessel – for gap 4 the antennas are on the low-field side, whereas for gap 6 they are on the high-field side. In addition, to minimize transmission losses over the significant dis- tance that the signal has to be transmitted, the in-vessel TLs use oversized rectangular waveguides that can support higher-order propagating modes besides the fundamental (5 modes at 15 GHz and 90 modes at 75 GHz). A critical issue in the design of these systems is the performance of the TLs that, being welded to the vessel inner-shell, must conform to an intricate and constrained path/geometry that includes a 90 ◦ bend right behind the antennas, for both gaps 4 and 6, and a 125 ◦ bend just before entering the port extension of upper port 01, exclu- sively for gap 4 (see Fig. 1). However, oversized waveguide bends are known to excite higher order modes and create resonances, which can increase the transmission losses [2,3] and significantly http://dx.doi.org/10.1016/j.fusengdes.2017.04.125 0920-3796/© 2017 Elsevier B.V. All rights reserved.