Improved launcher/mode converter for coaxial cavity ITER gyrotron

Summary form only given as follows. A TE_34.19-mode, 2 MW, CW, 170 GHz coaxial cavity gyrotron is under development for ITER at FZK, Germany. For gyrotrons operated in such a high order mode, it is necessary to adapt a quasi-optical mode converter to transform the cavity mode into a fundamental Gaussian wave beam. A pre-prototype quasi-optical mode converter has been investigated for this gyrotron. Low power measurements revealed a low fundamental Gaussian mode content in the output window plane. Our analyses show that the outgoing wave beam cannot be well pre-shaped with a high Gaussian mode content in a helically deformed launcher with an acceptable length. The Gaussian mode content of the field distribution radiated from the launcher is low and it makes the synthesis and fabrication of the adapted phase correction mirrors very difficult. In a highly oversized waveguide, if the deformations are shallow and smooth, the field remains paraxial. This allows us to calculate the field in the launcher by taking use of the scalar diffraction integral equation. Based on the scalar diffraction integral equation, a computer code has been developed for the analysis of the field on the launcher wall surface with arbitrarily shaped deformations. As an example, the field on the helically corrugated launcher wall surface used in the pre-prototype coaxial cavity gyrotron is calculated with the new code. The simulation results reveal a very high vector power correlation coefficient of 99% of the field in comparison with the field distribution calculated using the coupled mode method. Combined with the computer code for the analysis of the field in launchers, a new method has been developed for the synthesis of the launcher wall surface to achieve a wave beam with high Gaussian mode content. The numerical results show that a vector power correlation coefficient of the field to an ideal Gaussian-like field distribution of 86% is obtained at the aperture of the launcher.