Quasi-Optical Mode Converter For A Coaxial Cavity Gyrotron

Megawatt gyrotrons operate in high-order volume cavity modes. Thus it is necessary to use a quasi-optical mode converter to transform the operating mode into a fundamental Gaussian beam for transmission and application of the generated high-power millimeter waves. This work concentrates on the synthesis of the quasi-optical mode converter for the 170GHz, TE_34,19-mode, 2MW, CW coaxial-cavity gyrotron at Forchungszentrum Karlsruhe (FZK). The investigations are part of an EFDA project (TW5-THHE-CCGDS2) for the International Thermonuclear Experimental Reactor (ITER).A quasi-optical mode converter is the combination of an open waveguide antenna (launcher) and a mirror system. The improvement of the general method for the design of so-call dimpled-wall launcher to provide a good Gaussian mode content is described. By means of the improved method, very shorter perturbation lengths can be obtained with matched perturbation amplitudes. This method is verified through the design of a launcher operating in the TE_22.6 mode at 118 GHz.For coaxial-cavity gyrotrons operating in very high-order cavity modes such as the TE_34,19 mode, due to the ratio of caustic to cavity radius of 0.323, the ray-representation of the TE_34,19 mode cannot form a closed or even almost closed polygon in the cross-section of the launcher. In this case, the general method fails to provide a dimpled-wall launcher to transform the high-order cavity mode into a nearly Gaussian distribution. The synthesis of the quasi-optical mode converter for this gyrotron becomes very difficult and is a great challenge to obtain a high quality RF beam with high conversion efficiency and low power losses. The setting of parameters used in the quasi-optical mode converter for this gyrotron is very critical. A phase rule is proposed as a quality criterion for monitoring the optimization and the choices of parameters of the quasi-optical mode converter.High-order harmonics introduced to the launcher wall deformations are proposed for this gyrotron. The launcher is numerically optimized, the fields on the cut edges are suppressed. The fields in the launcher are well approximated by the waveguide modes, the radiated fields are calculated using the scalar diffraction integral. It provides a simple way for the simulation of the field distribution in an opened-end waveguide antenna and for the calculation of the radiated field from the opened-end waveguide antenna with low fields on the opened-end edges.The procedure for the numerical optimization of the mirror system is improved, the tolerance conditions of the phase correcting mirrors are investigated. A conversion efficiency of 95.8% to the circular fundamental Gaussian distribution with 20mmbeam waist and power transmission of 90% are achieved in the window plane using the optimized quasi-optical mode converter. The methods to ameliorate the initial conditions of the phase correcting mirrors are explored. Based on the ameliorated initial conditions, the mirror system can be improved and is anticipated to enhance the power transmission to more than 95% in the window plane.