Design and fabrication of compact waveguide mode control devices
Abstract
The design of compact and efficient mode converters has been of considerable interest since the development of high power gyrotrons for microwave heating of plasma in fusion processes. The conventional designs of these mode converters are based on the coupled mode theory and have a periodic perturbation profile. The efficiency of these mode converters is high but the smallest achievable length of the converter is limited by the perturbation period. This dissertation deals with the design of more compact mode converters and other mode control elements. A scattering optimization method has been proposed and implemented to design practical devices. The proposed design approach aims at achieving mode conversion by a strong field interaction using larger variation in waveguide profile as compared to a slight periodic perturbation. In the design procedure the profile of a scattering surface is optimized in such a way as to scatter required amount of energy into an output mode of interest. The scattering optimization design method is first introduced for parallel plate waveguides and then implemented for more practical applications in circular waveguides. Designs for various circular waveguide mode converters for gyrotron applications are presented. A comparison with earlier designs indicates that the scattering optimization method results in shorter mode converters with comparable efficiencies to that of the rippled wall designs. The design of a TE$\sb{11}$ to TM$\sb{11}$ mode converter for a circular waveguide at 9.94 GHz is discussed in detail. This mode converter was fabricated using easily available components and was characterized using network analyzer and far field measurements. The output of the mode converter was found to be 98.1% pure TM$\sb{11}$ mode at the design frequency which is very close to the theoretical conversion efficiency of 99.5%. Various design and fabrication issues and ideas for further improvements in the scattering optimization method are discussed. Finally a brief overview of the optical applications of the method is presented.
Degree
Ph.D.
Advisors
Gallagher, Purdue University.
Subject Area
Electrical engineering|Nuclear physics
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