| Gyro-amplifier usually operates in high order modes in a cylindrical guide, special input coupler is usually needed to transform the TEâ–¡10 rectangular guide mode to the TEO<sub>On circular guide mode. So mode competition is big problem, which makes that mode selection and suppression becomes very important in order to increase the output power, efficiency, and frequency bandwidth of the amplifier and keeps its operation stability. If a circuit used in gyrotron has mode selectivity for a mode, i.e TE02 mode, andcan suppress the other modes, it will be very useful for improving the characteristics of the amplifier. The input coupler discussed in this dissertation is namely a kind of circuit with mode selectivity and conversion function in Ku band for harmonic multiplying gyro-amplifier operation, which is used to excite and propagate TE02 mode in a circular guide. It mainly includes pill-box microwave input window, coaxial sector tapering transition section and sector-slotted guide mode converter with a gradually changing cross section.At first, theoretical study is performed by solving the formulas for obtaining the key dimensions of the pill-box window, the coupling wave equations for getting the mode conversion coefficient from input mode to the other modes in the coaxial sector tapering transition section. Based on the above-mentioned results, the relations of the mode conversion coefficient with frequency, tapering transition length, and the setor angle at transition end are analyzed to understand how to realize mode selection and suppression. Themode conversion condition from the TE0,n mode to TE0,n+p mode is discussedfor learning the physical process of the mode conversion.Secondly, Ansoft HFSS software is utilized to optimize the amplitude-frequency characteristic, determine the length of sector tapering transition section, analyze the effect of competition mode on operating mode in the...
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