Quasi-optical mode converters for high order mode gyrotrons

A quasi-optical mode converter has been designed to transform the TE_22,6 cylindrical waveguide mode at 110 GHz to a Gaussian beam in free space. The converter consists of an irregular cylindrical waveguide section followed by a step-cut launching aperture and several focussing reflectors in an open mirror transmission line. The irregular waveguide section, which was designed with coupled mode theory, is used to bunch the radiation into Gaussian bundles prior to the launch. The reflectors that follow were designed with Gaussian optics and diffraction theory. A rigorous diffraction theory has been developed to analyse the converter system. The converter was built and a preliminary test on a MW power level pulsed gyrotron has been completed. A symmetric Gaussian beam output with the predicted beam waist of 0.9 cm was observed. Experimental results are compared to diffraction theory simulations and very good agreement is obtained