Calculation of Radiation from a Helically Cut Waveguide for a Gyrotron Mode Converter in the Quasi-Optical Approximation

A gyrotron internal mode converter consists of a launcher that radiates the waveguide mode as a nearly Gaussian beam in free space, followed by a set of mirrors to focus and direct the radiation. We present a calculation of the radiation from the launcher based on a quasi-optical approach that includes diffraction. The results are in good agreement with a calculation using a numerical code based on the electric field integral equation approach. The results should be useful for designing so-called Vlasov internal mode converters and for understanding the role of diffraction in the design of internal mode converters for gyrotrons.