Dreidimensionale Feldberechnung für elektrisch große Geometrien und ihre Anwendung für multifrequente Hochleistungsgyrotrons

Abstract

At the Research Center Karlsruhe (Forschungszentrum Karlsruhe) high-power millimeter wave sources, so called gyrotrons, are developed. Gyrotrons generate radio frequency (RF) in the frequency range of a few gigahertz up to a couple of hundred gigahertz at power levels up to megawatts. The RF-output coupling is done by an internal quasi-optical mode converter. It focuses the millimeter waves to a pencil like beam. This report deals with the simulation of quasi-optical mode converters, which are large compared to the wavelength. For a fast simulation inside the waveguide the method of coupled modes is used and in free space the propagation is done by the Kirchhoff diffraction integral. To avoid approximations and to enhance accuracy a three-dimensional simulation method based on the electric field integral equation (EFIE) was developed. The performance of this method will be shown with an analysis of a quasi-optical mode converter of a 118 GHz TE 22,6 -gyrotron. The second focus of this work is the optimization of a broadband quasi-optical mode converter in the frequency range of 105 GHz to 143 GHz. The optimization was done with the fast simulation methods, which were mentioned above. Beside a high Gaussian mode content of the output beam for every frequency, the mode converter was optimized for a Brewster angle window. A Brewster window has no dependency on the frequency for the transmitted wave beam, but the useable aperture of the window is much smaller than for a mono-frequent window. By optimization the stray radiation could be reduced to two to three percent depending on the cavity mode with a Gaussian mode content of more than 90%. The work is based on a dissertation which was done at the Institut fur Hochleistungs-impuls- und Mikrowellentechnik at the Forschungszentrum Karlsruhe and accepted by the University of Karlsruhe (TH), Germany.