Excitation of backward waves in the beam tunnel of a high power gyrotron

Gyrotrons are an important source of high power coherent radiation in 100 GHz - 200 GHz range. As applications call for higher power, the power in the beam will increase as the danger of spurious oscillations. Competition between operating modes and competing spurious modes in the interaction cavity is a well studied phenomena, with established methods for suppressing the unwanted modes. Spurious modes can also be excited in the beam tunnel, generally in the form of gyro-backward waves. We have made a numerical analysis of the conditions for excitation of backward waves in the beam tunnel of a 170 GHz gyrotron. The analysis includes tapering of the wall, variation of the magnetic field in the beam tunnel, and the presence of losses. Our results will be presented in the form of a requirement on the amount of wall loss needed to suppress backward waves for a given beam current.