Harmonic mode competition in a terahertz gyrotron backward-wave oscillator Phys. Plasmas 19, 103103 (2012) Effect of ripple taper on band-gap overlap in a coaxial Bragg structure operating at terahertz frequency Phys. Plasmas 19, 092105 (2012) Low-order-mode harmonic multiplying gyrotron traveling-wave amplifier in W band Phys. Plasmas 19, 093103 (2012) The Ka-band 10-kW continuous wave gyrotron with wide-band fast frequency sweep Rev. Sci. Instrum. 83, 074706 (2012) Selective mode suppression in a W-band second harmonic coaxial-waveguide gyrotron backward-wave oscillator

In this paper, the results of the efficiency study of a 670 GHz gyrotron operating at TE31,8-mode are presented. Calculations are performed by using the self-consistent nonstationary code MAGY. Three cavity configurations were examined. The effects of ohmic losses and electron velocity spread were included in the simulation. The results show that the output efficiency can reach 35% and the velocity spread in the electron beam does not degrade the operation significantly. Furthermore, we verified that the smoothing of the sharp corners for a small tapering angle would reduce mode conversion; the parasitic excitation of neighboring radial modes is less than 1% of the amplitude of the operating mode and the effect on efficiency is small. Lastly, the simulation results show that the after-cavity interaction causes only slight variations in the efficiency.

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