Improvement of quasi-optical mode converter for 170 GHz coaxial cavity gyrotron at FZK

A quasi-optical (q.o.) mode converter is under development for a 2 MW, 170 GHz coaxial cavity gyrotron for ITER at Forschungszentrum Karlsruhe (FZK), Germany. Measurements revealed that the pre-prototype q.o. mode converter could only provide a corrected wave beam with low fundamental Gaussian mode content in the output window plane. A new method is used to numerically optimize the launcher wall surface to improve the performance of the q.o. mode converter. Simulation results show that a Gaussian mode content of 95 % of the outgoing wave beam can be achieved at the aperture of the improved launcher. With the new launcher, both the Gaussian mode content and the power transmission of the outgoing wave beam in the window plane are expected to be larger than 95%.