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A quasi-optical mode converter for a W-band TE02 mode gyrotron is designed, fabricated, and tested. The system consists of a step-cut waveguide launcher (cut-in-half circular waveguide) and a doubly curved reflector (an elliptical reflector and a parabolic reflector). Based on the geometric optics principle and the vector diffraction theory, the mode converter is optimized and simulated by using numerical calculations. The simulation results show that the good Gaussian mode is converted from the circular waveguide TE02 mode, and the conversion efficiency is 77.9%. Experimental measurements are performed on a quasi-continuous wave gyrotron operating in the TE02 mode at W-band and show a good agreement with theoretical predictions.