During numerical simulations of the model of gyrotron with fixed field-structure, zones of stable single-mode generation of the work mode are analyzed on the plane of parameters «magnetic field – beam current». It is shown that area and sizes of the zones are strongly dependent on the spectral density of eigen-frequencies of the resonator and on number of considered parasit modes. With increase of spectral density the point of highest attainable efficiency of stable single-mode generation shifts to the region of lower currents and simultaneously this value of efficiency decreases.
