Theory of gyrotrons with coaxial resonators

In the development of powerful gyrotrons capable of long-pulse or continuous wave operation the cardinal problem is to provide stable, single-mode highly efficient operation at the desired mode with an acceptable level of ohmic losses in the resonator walls. In the present paper the selective properties of various coaxial resonators as well as ohmic losses of the microwave power in outer and inner conductors are considered. The start-up scenario in powerful gyrotrons is discussed. The results of numerical simulations of a 1 MW, 280-GHz coaxial gyrotron are presented which demonstrate that successive excitation of a number of modes during the voltage rise may not hinder the stable operation of the desired mode on the top of the voltage pulse. An electronic efficiency of 48% is predicted