Mode Excitation in Gyrotrons With Triode-Type Electron Guns

Gyrotrons operating in high-order modes often suffer from severe mode competition. For the selective excitation of the desired mode, triode-type electron guns are often used. In the case of triode-type guns, where the beam voltage and the mod-anode voltage can be varied independently, the mode excitation during the startup process is studied in this article by using a generalized approach. The term “generalized approach” means that the obtained results can be valid for gyrotrons operating at arbitrary voltages and in any mode. The conditions for excitation of the modes are analyzed for different types of startup scenarios. The goal of the study is to find such relations between the beam and mod-anode voltages during the gyrotron startup that only the desired mode will be excited. It is shown that by using a specific triode-type startup scenario with a proper timing for the rise of the mod-anode voltage with respect to the beam voltage, the initial excitation of the desired mode can be realized in the cases of practically any mode density. The mode interaction in such a case is studied; also, the dependences of the results on the timing relation between the two voltages and on the voltage rise speed are considered. This article also contains the numerical analysis of a specific megawatt (MW) gyrotron with a triode-type gun.