Investigation of the Influence of Electron Beam Quality on the Operation in 0.42-THz Second Harmonic Gyrotron

High-power terahertz gyrotrons always operate at high-order modes to avoid damage from Ohmic heating. However, the spectral densities of these modes are very high, especially when gyrotrons operate at high-order harmonics. Therefore, the self-excitation condition can be satisfied for several modes simultaneously. Correspondingly, in order to determine which mode will be excited first in the designed terahertz gyrotron, the starting currents of competing modes should be calculated more accurately to predict the operating status. In this paper, an existing linear theory is generalized to take into account the influences of the field profile, finite beam thickness, and velocity spread. Starting currents are calculated for the operating and the most dangerous competing mode in a 0.42-THz second harmonic gyrotron with a complex cavity, which is recently under development at the Terahertz Science and Technology Research Center. The calculations show that the radial and velocity spreads play a critical role in the starting current and prove that the measured output terahertz radiation in the designed device is generated from the interaction between the desired mode and the electron beam.