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Simaltenous Data Acquisition and Analysis of a Five-Port Mitre Bend Coupler for In-Situ Measurement of Higher-Order Wavequide Modes
KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
2012 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

In Electron Cyclotron Resonance Heating (ECRH) systems for thermonuclear fusion experiments, high-power millimetre waves have to be transmitted from the gyrotrons to the plasma. The radiation from gyrotrons is often transported long distances before being launched for plasma heating. One way of transmis-sion is via an oversized corrugated metallic waveguide system connecting the gyrotron mirror optics unit to the entrance of the ECH launcher and includes mitre bends and other components.

The HE11 mode of a corrugated waveguide is the operating mode of the TL. The TL provides extremely low ohmic loss of the HE11 mode in the straight waveguide sections. Mode conversion losses of the HE11 mode occur primarily in the mitre bends, gaps. In present research on ECH TLs, it has been assumed that the mode excited at the entrance of the line is a pure HE11 mode. However, reality shows that gyrotron beams coupled onto the TL often excite high-order modes (HOMs) in addition to the fundamental HE11 mode. The excitation of HOMs is caused by imperfections in the Gaussian-like beam from the gyrotron (phase errors, incorrect waist size, etc.) as well as coupling errors of the beam into the guide (tilt, oset).

The purpose of this thesis is the 5 mode analysis in corrugated waveguide transmission lines with regard to imperfections, especially the power detection in the main HE11mode, and the level of spurious modes caused by tilt misalignments and/or by beam diameter mismatch.

Thesis presents a method for in-situ characterization of 5 modes by using a 5-port coupler, which is integrated into a mitre bend. This method pro-vides the power detection independent on the position of the couplers along the waveguide axis. The coupler signals can be directly transformed into the mode spectrum by a matrix multiplication. A general formalism for obtaining the coe?cients of the transformation matrix and for calculation of the mode powers from the coupler signals is presented along with a method for optimiz-ing the coupler positions in order to obtain the maximum dynamic range for the diagnostics.

Experimental tests are performed to benchmark the calculations. The re-sults con rm the theory and show that such a coupler is a viable tool for the alignment of the transmission lines and in-situ power measurement as well as basic mode analysis in high-power transmission systems.

Place, publisher, year, edition, pages
EES Examensarbete / Master Thesis
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
URN: urn:nbn:se:kth:diva-102041OAI: diva2:550476
Educational program
Master of Science - Nuclear Fusion Science and Engineering Physics
Available from: 2012-09-07 Created: 2012-09-07 Last updated: 2012-09-07Bibliographically approved

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