2.4: Investigations on parasitic oscillations in megawatt gyrotrons

Recently megawatt gyrotrons are found to suffer from various parasitic oscillations, particularly RF oscillations in the beam tunnel induced by the electron beam prior to the desired interaction zone (cavity). The paper describes experimental and theoretical investigations on both high and low frequency parasitic oscillations, employing a special measurement system as well as a novel analysis method to determine and characterize these undesired oscillations. The results of extensive experimental investigations on two different beam tunnel setups verify theoretical predictions on the parasitic oscillations being gyro-backward wave interactions excited in the beam tunnel. In addition, other types of parasitic oscillations were observed and explained. An improved beam tunnel designed to suppress the parasitic RF oscillations, was validated by further experiments with a conventional and a coaxial-cavity gyrotron.