Design of correcting mirrors for a gyrotron used at Large Helical Device

We report the design of a mirror system used at the Large Helical Device (LHD) to convert the output radiation of the CPI 84-GHz gyrotron into the HE11 mode of a corrugated waveguide transmission line. The radiation pattern of the gyrotron is measured using an IR camera at LHD. The measured data is analyzed at Massachusetts Institute of Technology (MIT) to retrieve the phase distribution of the radiation. The moments of the measured amplitude arrays are calculated to improve the reliability of the IR image data. The retrieved radiation structure is treated to synthesize the correcting mirrors.