Gyrotron internal mode converter reflector shaping from measured field intensity

We present the formulation and experimental results of a new approach to designing internal mode converter reflectors for high-power gyrotrons. The method employs a numerical phase retrieval algorithm that reconstructs the field in the mode converter from intensity measurements, thus accounting for the true field structure in shaping the beam-forming reflectors. An outline for designing a four-reflector mode converter is presented and generalized to the case of an offset-fed shaped reflector antenna. The requisite phase retrieval and reflector shaping algorithms are also developed without reference to specific mode converter geometry. The design approach is applied to a 110 GHz internal mode converter that transforms the TE_22,6 gyrotron cavity mode into a Gaussian beam at the gyrotron window. Cold test experiment results of the mode converter show that a Gaussian beam with the desired amplitude and phase is formed at the window aperture. Subsequent high-power tests in a 1 MW gyrotron confirm the Gaussian beam observed in cold tests. The general development of the approach and its validation in a quasi-optical mode converter indicate that it is also applicable to other quasi-optical, microwave applications such as radio astronomy, free-space transmission lines, and mitre bends for overmoded waveguides