A quasi-optical mode converter has been designed to transform theTE22, 6 mode at 110 GHz to a Gaussian beam in free space. Theconverter consists of a rippled-wall waveguide launcher and two toroidalfocussing reflectors. A full vector diffraction theory was developed tosimulate the converter operation and predict the characteristics of theoutput beam. The simulation results were used to modify and improve thereflector design. The converter was built and tested on a 3 μsecpulsed gyrotron operating in the TE22, 6 mode at 110 GHz.Beam expansion and calorimetric efficiency measurements agreed well withdiffraction theory predictions. Greater than 95% of the TE22, 6 power generated by the gyrotron was converted to a fundamentalGaussian beam and successfully coupled into a corrugated waveguide. Fouradditional reflectors were built to transform the fundamental Gaussianbeam into two similar Gaussian-like beams of approximately equal powerlevel. The vector diffraction theory analysis suggested that simplesinusoidal and toroidal shaping mirrors can achieve high efficiency beamsplitting. Experiments showed that the beam splitting mirror relaysuccessfully converted the fundamental Gaussian beam, produced by thelauncher and two mirror relay, to two Gaussian-like beams. Calorimetricmeasurements indicated that 94% of the power leaving the gyrotron wasconverted to the dual beam output with 52% of the power in the upperbeam and 42% in the lower beam. The measured beam patterns andexpansions were in good agreement with predictions of vector diffractiontheory
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