Redesign and 3D-Simulation of the Quasi-Optical Mode Converter for a TE22,6 118 GHz Gyrotron

Summary form only given. A quasi-optical mode converter for the European TE_22,6 118 GHz gyrotron was designed. It consists of a launcher with wall deformation, followed by a quasi-elliptical mirror, a plane mirror, and a parabolic mirror. Measurements with the mode converter installed in the gyrotron and with low power mock-up show a double peak output field pattern which was not predicted by simulation using Kirchhoffpsilas diffraction integral. To understand this behavior a detailed analysis of the mode converter was performed. The third mirror is not only focusing the main beam but also the side lobes originating from the launcher into the center of the beam. As these different waves have different phase relations the superposition results in a field pattern with minima and maxima in the window plane. To avoid the interaction two new designs for this quasi-optical mode converter are proposed. The first modification only includes a change of the third mirror which is easy to change. This proposal gives a broader output beam because the third mirror is less focusing to avoid the side lobe interaction in the window plane. But this gives only a power transmission through the window of 93% with a Gaussian mode content of 94% (window aperture 80 mm). To minimize the stray radiation and enhance the power transmission through the window the focusing properties of the second and third mirror have to be adapted. This is the better solution but cannot be installed due to high costs. The side lobes are directed in parallel to the main beam at the output. This setup has a power transmission of 98.2% and the Gaussian mode content is 95%. We will present the 3D electric field simulation results for the two designs using the electric field integral equation and show the excellent agreement to the low power measurement carried out.