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A quasi-optical mode converter has been designed to transform the TEz2,6 mode at 110 GWz to a Gaussian beam in free space. The converter consists of a rippled-wall waveguide launcher and two toroidal focusing reflectors. A full vector diffraction theory was developed to simulate the converter op- eration and predict the characteristics of the output beam. The simulation results were used to modify and improve the reflector design. The converter was built and tested on a 3-ps pulsed gyrotron operating in the TE22,c mode at 110 GHz. Beam expan- sion and calorimetric efficiency measurements agreed well with diffraction theory predictions. Greater than 95% of the TE22,6 power generated by the gyrotron was converted to a fundamental Gaussian beam and coupled into a corrugated waveguide. Four additional reflectors were built to transform the fundamental Gaussian beam into two similar Gaussian-like beams of approx- imately equal power level. The vector diffraction theory analysis suggested that simple sinusoidal and toroidal shaping mirrors can achieve high-efficiency beam splitting. Experiments showed that the beam splitting mirror relay successfully converted the fundamental Gaussian beam, produced by the launcher and two mirror relay, to two Gaussian-like beams. Calorimetric measurements indicated that 94% of the total power leaving the gyrotron was converted to the dual beam output with 52% of the power in the upper beam and 42% in the lower beam. The measured beam patterns and expansions were in good agreement with predictions of vector diffraction theory.