Ultimate choice of operating modes in high-power gyrotrons

Summary form only given: There are a number of technological and physical issues limiting the power of CW gyrotrons. One of the most important ones is the admissible density of ohmic losses in the resonator walls. To increase the output power while keeping the losses at an acceptable level, it is necessary to increase the resonator cross-section and to operate at higher and higher order modes. This trend has been known for a long time and was analyzed in numerous papers. For example, to achieve a 1 MW-power level in gyrotrons developed for ITER, the chosen operating is the TE-mode of a cylindrical cavity. Recently, it was shown that this gyrotron can also stably operate in the TE-mode. A question thus arises: how large can the cavity radius be made and how high the order of the operating mode can be to still operate stably and with high efficiency? To answer this question, one should study the role of increasing the azimuthal and radial indices of the modes in cylindrical resonators. The increase in azimuthal indices was analyzed first in gyrotrons with a fixed axial structure of the RF field in studies of sideband instabilities in which the growth of satellite modes in the presence of a saturated operating mode was considered. Later, a self consistent treatment of the same problem using a wave envelope approach for describing the superposition of modes showed that operation remains stable until the azimuthal index reaches a certain critical value. So, while the maximum azimuthal index is more or less known, the maximum possible radial index is yet to be determined. In this work, the stability of very high-order single-mode operation is analyzed by studying the interaction of selected groups of modes with the help of self-consistent time dependent code MAGY⁵. It is shown for given modes, that stable, high efficiency operation is possible only if the ratio of cavity radius to wavelength is less than about 12.5.