# In-situ real-time monitoring of spurious modes in HE$_{11}$ transmission lines using multi-hole couplers in miter bends

Zach, Alexander; Kasparek, Walter; Lechte, Carsten; Plaum, Burkhard; Monaco, Francesco; Schütz, Harald; Stober, Jörg; Idei, Hiroshi; Hirth, Thomas

## Abstract:

Transmission of high-power millimeter waves for ECRH is often realised with oversized corrugated circular waveguides. Coupling from the gyrotron source to the waveguide is typically done via matching mirrors in free space. Small alignment errors of the system lead to the excitation of higher-order modes inside the waveguide beside the main transmission mode HE$_{11}$. Tose modes have comparably higher losses and can in worst case result in local fields exceeding the breakdown limit of the medium inside the waveguide.
For alignment control over the whole pulse duration of the gyrotron, a set of hole-array couplers placed into a miter bend mirror probes the field inside the waveguide. The arrays are designed to detect the marker modes for beam oﬀset and tilt (LP$^{(e=0)}$$_{11}$) as well as for beam waist mismatch (LP$_{02}$). In addition, a main mode coupler sensitive mostly for the HE$_{11}$ content is used as a power monitor. By maximizing the signal of the power monitor and minimizing the content of marker modes, a first-order optimization of the coupling from free space to the waveguide can be achieved. Signal processing of the 140 GHz information is done at kHz range after downmixing, using a frequency shifted part of the power monitor signal.
... mehr

 Zugehörige Institution(en) am KIT Karlsruher Institut für Technologie (KIT) Publikationstyp Zeitschriftenaufsatz Publikationsjahr 2019 Sprache Englisch Identifikator ISSN: 2100-014X KITopen-ID: 1000122164 Erschienen in The European physical journal / Web of Conferences Verlag EDP Sciences Band 203 Seiten Art.Nr. 04018/6 S. Vorab online veröffentlicht am 25.03.2019 Nachgewiesen in Dimensions
KIT – Die Forschungsuniversität in der Helmholtz-Gemeinschaft
KITopen Landing Page