Progress toward optimization of phase-correcting mirrors for a multifrequency 1-MW gyrotron

Due to the requirement of a newly designed ultra-broadband chemical vapour deposition (CVD)-diamond window for a multifrequency 1-MW short-pulse gyrotron at Forschungszentrum Karlsruhe (FZK), the existing beam-forming mirror system inside the gyrotron tube has to be modified. This paper describes the design scheme for shaping the output beams from the launching antenna waveguide into the desired fundamental Gaussian profile by using adapted phase-correcting mirrors with nonquadratic surface contour function. Simulation results show these phase-correcting mirrors can be used for broadband operation of a frequency step-tunable gyrotron, which operates in nine modes from the TE17,6 mode at the frequency of 105 GHz to the TE23,8 mode at 143 GHz. Further calculations predict that efficiencies of more than 94% can be obtained for converting the high-order cylindrical cavity modes into the usable fundamental Gaussian mode.