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 HE11. Those 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=o)11 )as well as for beam waist mismatch (LP02). In addition, a main mode coupler sensitive mostly for the HE11 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. As the measurement system is placed in a miter bend mirror, it can also be easily installed at various locations along the transmission line to check for possible misalignments of the waveguide connections between miter bends. Simulation and low power experimental results will be shown.
Highlights
An important problem in transmission of high-power millimeter waves for ECRH is to avoid spurious mode excitation in oversized corrugated circular waveguide (OCCWG) transmission lines. These modes have higher losses compared to the main transmission mode HE11 and can result in hot spots along the transmission line, which can have influences on structure cooling demand
One is the coupling from the gyrotron millimeter wave source to the OCCWG, which is typically done with a matching optic consisting of two or more mirrors in free space
The field in the OCCWG cross section is sampled with hole couplers, which are integrated into a miter bend mirror
Summary
An important problem in transmission of high-power millimeter waves for ECRH is to avoid spurious mode excitation in oversized corrugated circular waveguide (OCCWG) transmission lines. These modes have higher losses compared to the main transmission mode HE11 and can result in hot spots along the transmission line, which can have influences on structure cooling demand. The over 60 m long transmission lines at ASDEX Upgrade have a diameter of 87 mm each and are typically operated with up to 10 s pulses at 140 GHz and 1 MW in atmosphere They are aligned with a laser system to an accuracy better than 1 mm. The following chapters describe the used method of mode detection and show results of the low power calibration for a prototype miter bend mirror diagnostic system
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