Improved quasi-optical launcher for coaxial cavity ITER gyrotron

A 2MW, 170GHz coaxial cavity gyrotron is under development for ITER at Forschungszentrum Karlsruhe (FZK), Germany. A quasi-optical (q.o.) mode converter integrated in the pre-prototype gyrotron is investigated, and measurements revealed that the mode converter could only provide a corrected wave beam with low fundamental Gaussian mode content in the output window plane. In order to improve the performance of the launcher, a numerical method is used to iteratively optimize the launcher wall surface to pre-shape the wave beam with high Gaussian mode content. Simulation results show that a vector power correlation coefficient of 93% of the outgoing wave beam to an ideal Gaussian-like distribution 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%.