Start currents in an overmoded gyrotron

High-power long-pulse millimeter-wave gyrotrons operate in high-order modes. The spectral density of these modes is very high. Therefore, self-excitation conditions can be fulfilled for several modes simultaneously. Correspondingly, in order to determine which mode will be excited first in such a device, the starting currents of competing modes should be calculated much more accurately than in gyrotrons with a rarer spectrum. In the present paper, an existing linear theory is generalized to take into account effects of magnetic field tapering, cavity profile, finite beam thickness, velocity spread and axially dependent beam coupling to the fields of competing modes. Starting currents are calculated for the operating and the most dangerous competing mode in a 140 GHz gyrotron, which is under development at Communication and Power Industries. Calculations show that the radial position of the electron beam plays a critical role in deciding which mode dominates the mode competition. (C) 2003 American Institute of Physics.