Computer-Controlled Test System for the Excitation of Very High-Order Modes in Highly Oversized Waveguides

The generation of a specific high-order mode with excellent mode purity in a highly oversized cylindrical waveguide is mandatorily required for the verification of high-power components at sub-THz frequencies. An example is the verification of quasi-optical mode conversion and output systems for fusion gyrotrons. A rotating high-order mode can be excited by taking a low-power RF source (e.g. RF network analyser) and by injecting the RF power via a horn antenna into a specific adjustable quasi-optical setup, the so-called mode generator. The manual adjustment of the mode generator is typically very time-consuming. An automatized adjustment using intelligent algorithms can solve this problem. In the present work, the intelligent algorithms consist of five different mode evaluation techniques to determine the azimuthal and radial mode indices, the quality factor, the scalar mode content and the amount of the counter-rotating mode. Here, the implemented algorithms, the design of the computer-controlled mechanical adjustment and test results are presented. The new system is benchmarked using an existing TE28,8 mode cavity operating at 140 GHz. In addition, the repeatability of the algorithms has been proven by measuring a newly designed TE28,10 mode generator cavity. Using the described advanced mode generator system, the quality of the excited modes has been significantly improved and the time for the proper adjustment has been reduced by at least a factor of 10.