Relativistic Second-Harmonic Gyrotron With a Selective Quasi-Regular Cavity

A typical problem of gyrotrons operating at the second harmonic of the electron cyclotron frequency is the suppression of parasitic near-cutoff waves excited at the fundamental harmonic. In this paper, we discuss the possibility for the selective excitation of two different second-harmonic waves in a relativistic millimeter-wavelength gyrotron by means of the use of a method for a significant improvement of the selectivity of this system. This method is based on the use of a quasi-regular cavity with a short irregularity providing different effects on the interaction of the electron beam with the operating second-harmonic wave and the spurious fundamental-harmonic wave.