Curve concepts for oversized circular waveguides in view of a lower hybrid system for DEMO

Three candidates of waveguide bends for the transmission lines of Lower Hybrid systems in large-size tokamaks are assessed in terms of RF performance and integration issues. The three options are respectively characterized by profiled curvature, corrugated waveguide and elliptical cross-section; the unsatisfactory behavior of a standard, smooth wall, circular bend is also shown for comparison. The design of ideal curves achieving no mode conversion at their output is rather attended in literature because it has always been a critical issue for long-distance high-power transmissions at microwave frequencies. Here the problem is addressed taking advantage of modern numerical advancements and, unlike usual, focusing on small curvature radii. Analysis outcomes indicate the bend based on corrugated waveguide as the most suitable option from the viewpoint of both RF performance and integration issues. Details about adopted method and results are discussed. © 2014 AIP Publishing LLC.