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A quasi-optical (q.o.) mode converter consisting of a launcher with specific surface contour, a quasi-elliptical and two toroidal mirrors developed for a 2MW, 170GHz, TE34,19 mode coaxial-cavity ITER gyrotron has been theoretically and experimentally investigated at Karlsruhe Institute of Technology (KIT). Both the theoretical and experimental results reveal that the Fundamental Gaussian Mode Content (FGMC) of the RF beam is about 96.7% at the window plane. In order to improve the FGMC of the RF beam further more, and then to decrease the stray radiation in the tube, two adapted phase correcting mirrors have been designed instead of the two toroidal mirrors. The simulation results show that with the specific phase correcting mirrors, the FGMC of the RF beam can be enhanced to 99.1% at the output window plane and the stray radiation could be decreased by 0.6% in the tube.