Stability of gyrotron operation on the second harmonic

Summary form only given: The attractiveness of operation at cyclotron harmonics was well understood from the first steps of development of cyclotron resonance masers and gyrotrons. Magnetic fields required to fulfill the cyclotron resonance condition are inversely proportional to the harmonic number. Therefore, by operating at cyclotron harmonics one may significantly reduce the size and cost of superconducting magnets. For gyrotrons operating at the second harmonic the critical issue is the excitation of parasitic modes at the fundamental. This issue was first studied in the framework of the generalized theory in Ref. 1. Recently this problem was addressed by using self-consistent codes in Ref. 2. In the present work we analyze the stability of operation at the second harmonic by using a simple generalized theory and self-consistent time-dependent code MAGY3. In the framework of the generalized theory, which is based on the cold-cavity approximation of the spatial structure of resonator modes, the regions of stable operation at the second harmonic are determined in the plane of cyclotron resonance detunings of the frequencies of competing modes with respect to corresponding harmonics of the electron cyclotron frequency. Such regions allow one to identify the second harmonic operating modes for which in regions of parasitic excitation there are no competing fundamental harmonic modes. After identifying such modes, for some of them MAGY simulations are performed. Results of these simulations will be presented and discussed.