Harmonic Gyrotrons Operating in High-Order Symmetric Modes

It is shown that gyrotrons operating at cyclotron harmonics can be designed for operation in symmetric TE_0,p-modes. Such operation in fundamental harmonic gyrotrons is possible only at small radial indices (⁠p ≤ 3⁠) because of the severe mode competition with TE_2,p-modes, which are equally coupled to annular beams as the symmetric modes. At cyclotron harmonics, however, this “degeneracy” of coupling is absent, and there is a region in the parameter space where harmonic gyrotrons can steadily operate in symmetric modes. This fact is especially important for sub-THz and THz-range gyrotrons where ohmic losses limit the power achievable in continuous-wave and high duty cycle regimes.