Abstract: A method and apparatus for suppressing lower order cyclotron harmonics in order to permit resonance within a quasi-optical gyrotron/gyroklystron configuration of a desired higher order harmonic. In the gyrotron/gyroklystron configuration at least one open resonator defined by at least two mirrors is positioned downstream from an electron beam source for receiving therethrough the beam of electrons and for exchanging energy therewith. This method includes the steps of choosing a mirror radius size .rho. for the mirrors forming the at least one open resonator which is large enough relative to the spot size of a desired radiation cyclotron harmonic .omega..sub.n so that the harmonic .omega..sub.n oscillates within the at least one resonator, but small enough so that the spot size for the next lower cyclotron harmonic .omega..sub.m is larger than the mirror so that the harmonic .omega..sub.m does not oscillate due to diffraction losses.